Indian Military Aviation - 21 Sept 2015

[Kersi D]

Exhaust should not be an issue. Typical exhaust temperatures for Diesel engines are around 1350F (~740C) with turbo charging. Assume Turbo consumes some of the energy (~20-30 %). Assume aviation exhaust temperatures similar to diesel. But about a foot away from the exhaust stack, the temperature drastically drops to around 400-500 F (260C). This is due to looses from mixing and radiation.
With landing gear that far away (looks like several feet from pic), temperature from exhaust should not be a problem at all, even for slow speed taxi. At take-off speeds, the mixing is significant.

I think land based diesel engines exhaust temperature range form 400 - 450 Deg C for small engines say < 1 MW to 350 Deg C for laregr engines say 2 - 3MW and above. I gues petrol engines would be in the same range.

But diesel engines are hardly ever used in aircraft. To the best to my knowledge the only success story of diesel engines used on aircraft is JU 88. Please confirm

[JayS]

I have a question about the HTT 40s exhaust pipes and this is a thermodynamics type of question for which I have no answer - only guesswork. The issue is minor.

If you look at the enlarged area of the HTT 40 image below you find that the exhaust pipes are cut "obliquely" and not orthogonally to the axis of the pipe. Many other aircraft including the image of the An-2 I have linked above have the exhaust pipe cut orthogonally. The oblique cut appears to be a design choice rather than a cosmetic variant.

From the rudimentary school level thermodynamics I can recall there are some design advantages in cutting the pipe obliquely in some situations. In this case the area of the outlet is larger and would lead to early cooling of gases and mixing with ambient air, apart from redirecting the exhaust flow away from the axis of the pipe - just like those AK-47s whose muzzle is cut obliquely to create an up-blast to counter the bucking up tendency of the AK-47 whose barrel is located above the center of mass.

Also - the engine is after all a jet engine and the mass of air flowing out is what would be expected from a jet engine and must be producing some degree of thrust - however minor. I wonder how much the exhaust outlet configuration is designed to utilize (or minimize) that thrust?

Image search yielded various types of exhaust pipes for An-2. There might be more in use. First one is I guess original as its most common. Others look like after-market fittings. Have a look:

http://img.planespotters.net/photo/1090 ... 109314.jpg
https://upload.wikimedia.org/wikipedia/ ... ig.arp.jpg
http://www.militaryaircraft.de/pictures ... 61_800.jpg
http://avia-simply.ru/wp-content/upload ... -2-ms6.jpg


BTW these engines produce as much thrust as a car/bike engine might - essentially Zero. These turboprops are not Jet engines strictly speaking. The whole point of the exhaust pipe is to eject the exhaust in such a manner that it will not have any adverse effect on the airflow over the airframe. Temperatures are not so much of an issue. I will try to come up with better answer later.

[shiv]

Nilesh the first 2 images are of the old (original) radial internal combustion engine powered An-2. Only the last one is the newer turbofan version. 3rd image is missing

[Indranil]

Hakim,

5-8% of the thrust can be derived from the exhaust of a turboprop. Pilots actually use it for taxiing. When the engines are mounted on a wing, more often than not, you will see a single exhaust stack pointed straight back. However, this is not possible in a single engine plane. Designers typically use split exhaust system to dump the exhaust symmetrically in a general backward direction pointed slightly away from the fuselage. However, in few cases, like in the Cessna grand caravan 208A/Bs, you will see a single exhaust. I do not know why they went for this setup, but I remember reading that they had problems with it. Originally, they had a round stack which deposited the exhaust to the right of a composite off-center pod. But, they had problems in the spin test. The recovery was not ideal in one direction. Then they changed it to an oval exhaust stack around a wider, but less deeper pod. This is now the standard stack on all 208s.

The oblique cut is more to do with reduction in drag. One does not need to carry any more metal and provide any more frontal area than required to dump the exhaust in the general direction desired. Everything else is secondary.

[JayS]

Nilesh the first 2 images are of the old (original) radial internal combustion engine powered An-2. Only the last one is the newer turbofan version. 3rd image is missing

I don't know why the link got broken. Was working fine on my office machine. Anyway - this another link has the same pic with some others showing more variations in the exhaust pipe. The link mentions at least two reason for which people modify the exhausts - to avoid the blackening of airframe, cutting down noise.

http://www.reaa.ru/cgi-bin/yabb/YaBB.pl ... 336844/378

Thanks for pointing out the difference in the engine. I totally missed it. But I just wanted to highlight that there could be multiple ways to have the exhaust pipes.

Indranil has already answered in best possible manner. I cross-checked. I used to think they extract all the energy before throwing out the gases, but apparently the exhaust still is hot enough and has good velocity. Thus a lot of turboprops do produce small fraction of jet thrust. Generally they calculate equivalent shaft horse power of the same and add into total shaft power output. A handful of engines have significant jet component like 15% or more. This pdf below gives thrust and other data for a lot of PT6A models. In general they have 5% as thrust component of total rated power.

http://www2.anac.gov.br/certificacao/Pr ... 05-10i.pdf

I am little confused with the shape as well. The design of these exhaust pipes in a some of the single engine aircrafts are diffuser shape (Beechcraft Super King Air 200 which uses PT6A-42 engine). Why would someone use a diffuser pipe for exhaust if they want to extract any thrust from it?? That flies in the face of it. They should be using a nozzle. OTOH I can understand use of a diffuser pipe when one is only dumping the exhaust safely (it would help creating lower than atmosphere pressure at the turbine exit, effectively increasing power extraction by increasing PR).

As a side note - This is TPE331 booklet for pilots. It mentions max Exhaust temperatures of the order of 650-770 degree Celsius (Pg 27). The exhaust out of the exhaust pipe must be around 500 deg C at least.
http://eastaire.us/files/TPE331pilotnotes.pdf

[Indranil]

You will see the diffuser shapes more commonly on the PT6s. This makes as sense as the primary goal is to dump the exhaust far away from the fuselage to minimize material degradation and soot deposition. With most 335s, the stacks are more nozzle shaped and directed towards the rear.

Another important consideration for the PT6s is the carbon monoxide contamination in the cabin as exhaust streams flows over the cabin area. With their exhausts generally placed underneath the fuselage, this is less of a concern for the 335s.

[shiv]

At least in pressurized cabins, isn't cabin air bleed air from a cool part of the turbine?

[shiv]

As a side note - This is TPE331 booklet for pilots. It mentions max Exhaust temperatures of the order of 650-770 degree Celsius (Pg 27). The exhaust out of the exhaust pipe must be around 500 deg C at least.
http://eastaire.us/files/TPE331pilotnotes.pdf

There must be studies about air temperature at increasing distances behind the outlet to and the "spread" of hot air to give a sense of how far away structures like undercarriage and tyres need to be to be safe from overheating. I read something to the effect that metallic structures heated up like this show higher temperatures and non metallic structures lower temps when exposed to the same air flow - so (wild guessing here!) if metal undercarriage structures get to 120 deg - tyres may get to a (dangerous?) 80 degrees or something?

Does anyone know at what is the maximum "safe" temperature for aircraft tyres that get heated? Sure it might vary depending on tyre wear - but there must be some cutoff never exceed temperature

[NRao]

At least in pressurized cabins, isn't cabin air bleed air from a cool part of the turbine?

You are right, just that most air is recirculated and the "bleed air" comprises of a small component of it.

[Bhaskar_T]

HAL to keep testing flights at HAL Airport. Court dismisses the petition which challenged HAL.

All measures in place for safety of citizens, HAL tells court, June 7, 2016 05:31 IST

http://m.thehindu.com/news/cities/bang ... 98342.ece

Hindustan Aeronautics Ltd. (HAL) on Monday told the High Court of Karnataka that it has already taken all possible measures under the Aircraft Act and other laws for the safety and security of citizens of densely populated areas surrounding the HAL airport in the city.

There is no danger for population in and around the airport, counsel for HAL submitted before a Division Bench comprising Chief Justice Subhro Kamal Mukherjee and Justice Ravi Malimath. The Bench disposed of a PIL petition, which had sought various directions to HAL, including shifting of experimental and test flights to other locations.

The petition had also questioned certain conditions imposed on construction of high-rise buildings around the airport as a single judge, in connection with a petition related to construction of a multi-storied building, had made an observation that “there is a humungous risk to all inhabitants surrounding the unique defence aerodrome.”

[shiv]

To get back to where this all started:

Anyone who buys an engine will get a manual and instructions from the engine maker about safety precautions to be taken regarding the exhaust - temperatures, CO levels etc. Factory technicians will be in attendance because no engine manufacturer wants an accident that might get blamed on his engine We have ourselves seen hundreds upon hundreds of aircraft with all manner of designs flying safely with exhaust pipes placed in pefectly safe places.

Why is it that someone takes one look at the first high res pic of the HTT 40 and points out that the designers may not have thought of what the exhaust gases will do? Why is it always the Indian who is accused of stupidity/incompetence based on one image? Is it the Indian designer who is stupid or is it the person who makes the comment?

[Indranil]

At least in pressurized cabins, isn't cabin air bleed air from a cool part of the turbine?

None of that air has gone through a combustion chamber and hence this example is not the same as the exhaust from the core of the jet engine.

[Bob V]

At least in pressurized cabins, isn't cabin air bleed air from a cool part of the turbine?

The bleed air is derived from the HP compressor section which is just ahead of the combustion chamber.

[JayS]

You will see the diffuser shapes more commonly on the PT6s. This makes as sense as the primary goal is to dump the exhaust far away from the fuselage to minimize material degradation and soot deposition. With most 335s, the stacks are more nozzle shaped and directed towards the rear.

Another important consideration for the PT6s is the carbon monoxide contamination in the cabin as exhaust streams flows over the cabin area. With their exhausts generally placed underneath the fuselage, this is less of a concern for the 335s.

I also read about CO issue yesterday somewhere. The exhaust pipes for PT6A looks more for quick dispersal of the exhaust air. The TPE331 engines have mostly constant dia pipes. I failed to find any nozzle type exhaust. My first thought was perhaps TPE331 has more residual jet thrust as compared to the PT6A and thus it gives more insentive to have properly directed exhaust. But apparently both engines have about 5% jet thrust components. In that case the CO issue becomes important criteria. Whereas on the wing mounted engines, where this 'CO getting into cabin' issue is irrelevant, OR when the designers are confident of not having this issue, they do use exhaust pipes which turn the exhaust fully backwards. But even in that case I noticed divergent pipes on some aircrafts (e.g. Beechcraft super king air b200).

Overall I saw all tipes of combinations and its difficult to say anything as universal rule. But for sure the exhaust pipes which have "oblique" cut which Shiv pointed out are meant to quickly disperse the exhaust in the airflow. While the ones which turn the exhaust full, are for taking maximum benefit of the jet thrust component.

There must be studies about air temperature at increasing distances behind the outlet to and the "spread" of hot air to give a sense of how far away structures like undercarriage and tyres need to be to be safe from overheating. I read something to the effect that metallic structures heated up like this show higher temperatures and non metallic structures lower temps when exposed to the same air flow - so (wild guessing here!) if metal undercarriage structures get to 120 deg - tyres may get to a (dangerous?) 80 degrees or something?

Does anyone know at what is the maximum "safe" temperature for aircraft tyres that get heated? Sure it might vary depending on tyre wear - but there must be some cutoff never exceed temperature

I couldn't find direct specs of aircraft tires stating max operating temperature, its not uncommon for the tire surfaces to get heated up till 180-200 deg C in some conditions. Interestingly even longer taxiing can overheat the tires dangerously. See this discussion: http://www.airliners.net/aviation-forum ... ain/90515/

So while it seems the tire overheating is a real sensitive issue, I am pretty sure HAL design team has considered it while doing the exhaust design. As such there are many aircrafts out there which have exactly the same kind of arrangement and if those can fly safely why shouldn't HTT-40??

Interesting factoid for you - apparently the largest size of truck one can build is limited by - guess what - maximum tire temperature current tire technology can have. It was told in one of the documentaries on those huge mining trucks which I saw many months ago.

I would expect that a non-metallic material should have higher surface temperature for given heat influx on it as compared to a metallic component. Since non-metallic materials generally are bad conductors of heat, the dissipation of heat is not as good as that in metals. And thus the heat gets concentrated on surface raising its temperature. In metals, the heat would be quickly flowing to inner layers and other parts. OTOH the inner parts on the non-metals would be cooler. There is another component to it - convection heat transfer. I am not sure about this one. This would tell us how much heat will actually be transferred from an airflow to the surface. Can you quote the source where you read that particular statement about heating.

EDIT: This book mentions that tire surface temperatures can go upto 150deg C in severe breaking conditions. Typically limits for normal operations are about 110 degC. The is for General Aviation. For bigger airliners 200 deg C can be max limit under severe breaking, I would say.
https://books.google.co.in/books?id=XtU ... re&f=false

[shiv]

At least in pressurized cabins, isn't cabin air bleed air from a cool part of the turbine?

The bleed air is derived from the HP compressor section which is just ahead of the combustion chamber.

Of course. But with the cabin itself being sealed to outside air exhaust airflow over the cabin in a turboprop will not enter a pressurized cabin no matter how the exhaust is oriented

[JayS]

^^ And a lot of aircrafts dump the exhaust right past the cabin. I think the new aircrafts with nice airtight cabin do not have the issue of CO ingestion in Cabin and thus do not hesitate to have exhaust near the cabin. One example is the French bird TBM-850


But in the past thta issue was there for sure.


Also interestingly the Cessna model mentioned by IR above - 208B has an exhaust which directs the hot gases very near to the LG, if not over it directly.

[Indranil]

Even in a pressurized cabin there are a variety of air exchanges with the outside for environment control. If there was none, the passengers/pilots will suffocate. For example, on the Super Tucano, it is scoop right behind the cockpit. So even in modern pressurized cabins carbon monoxide contamination is a real danger to design around.

Nilesh, from what I have read, the 331 has more jet exhaust thrust than the PT6. I also want to know why the exhaust of turboprops are essentially diffusers. It must be to attain the right exhaust discharge velocity (optimized for cruise speeds?!!). Anyways, I will share what I find.

P.S. I found the answer from none other than the designer of the diffuser of the 12B
Read from page 176 onwards

Exhaust diffusers, in general, are of primary impact on the total-to-static efficiency of the turbine/exhaust diffuser system assembly because they raise the static pressure up to the ambient magnitude, making use of existing kinetic energy in the flow stream. Note that the exit pressure value of the last turbine stage is normally less than the ambient pressure.

.

So it is designed for maximum static-pressure recovery.

[nirav]

Why is it that someone takes one look at the first high res pic of the HTT 40 and points out that the designers may not have thought of what the exhaust gases will do? Why is it always the Indian who is accused of stupidity/incompetence based on one image? Is it the Indian designer who is stupid or is it the person who makes the comment?

I find the suggestion that the exhaust will melt the tires bafffling.

I mean I get it that we are supposed to be SDREs, but this is just too much.
That suggestion/observation is more apt for Yahoo answers.

[Kartik]

As a side note - This is TPE331 booklet for pilots. It mentions max Exhaust temperatures of the order of 650-770 degree Celsius (Pg 27). The exhaust out of the exhaust pipe must be around 500 deg C at least.
http://eastaire.us/files/TPE331pilotnotes.pdf

There must be studies about air temperature at increasing distances behind the outlet to and the "spread" of hot air to give a sense of how far away structures like undercarriage and tyres need to be to be safe from overheating. I read something to the effect that metallic structures heated up like this show higher temperatures and non metallic structures lower temps when exposed to the same air flow - so (wild guessing here!) if metal undercarriage structures get to 120 deg - tyres may get to a (dangerous?) 80 degrees or something?

Does anyone know at what is the maximum "safe" temperature for aircraft tyres that get heated? Sure it might vary depending on tyre wear - but there must be some cutoff never exceed temperature

In the case that I was involved in, CFD studies were carried out to check whether hot exhaust from NGS system would affect panels or not and cause degradation in their allowables. And it was found that in fact they would be affected..even if they were a few meters away.

However, in flight this will most likely be a non-issue due to mixing with cold air. But on the ground, with the engine on for extended periods, it might lead to heating the structures in its vicinity. By how much and whether it affects the tires or the wheel rims, I guess they'll find out during testing.

[Kartik]

I find the suggestion that the exhaust will melt the tires bafffling.

I mean I get it that we are supposed to be SDREs, but this is just too much.
That suggestion/observation is more apt for Yahoo answers.

Melting tires and exploding tires are very different. A tire that explodes due to overheating, while an airplane is taking off could lead to a catastrophic accident. Stop using this "Arre, we are ridiculous SDREs so everyone will question us onlee" mentality in every question related to a design element. A genuine question was asked, if you have an answer give it, else shut up.

There's a reason for having twin nose wheels too on the HTT-40, apart from the smaller size of individual nose wheels..if one deflates for any reason, the other can still take some of the load while the airplane comes to a halt. A dual redundancy kind of feature.

[nirav]

Oh lord.
I repeat, such "engineering" questions are apt for Yahoo answers only.
Telling me to shut up won't change anything..

[Kartik]

Oh because you are the one who decides what is actually a real world issue and what isn't?

[JayS]

@Indranil

From Honeywell Brochures:

Model SHP ESHP
----------------------------------------------------
331-12JR 1000 1050
331-14GR 1759 1833
331-10 1000 1044

Others should be of the same order. From this the jet thrust component is about 5%. Same as PT6A. I posted a doc of specs of a lot of PT6A models previously.

Initially I also thought that may be the TPE 331 must have larger jet thrust component, that would have solved a lot of puzzles. But it looks like it doesnt. It baffles me more.

EDIT: I saw your edit later. In the reference he is talking about the annular diffuser following the turbine immediately after. However same principle can be extended to the Exhaust pipe or stub as they call it. Thats precisely what I hypothesized in one of my previous posts. That makes sense to me. But at the same time it necessarily reduced the jet thrust component as well since the diffuser is converting some of the kinetic energy into pressure energy. Note that the turbine outlet pressure is not less than ambient by design, that is in isolation. The diffuser outlet section allows designers to go below ambient, because the following diffuser section raises the pressure again to the ambient pressure. This is exactly the same principle used in reaction type water turbines e.g. Francis turbines used in big dams. The diffuser draft tube allows the exit pressure to go below ambient pressure increasing overall pressure ratio across the turbine stage thereby increasing the work output.

Now my thoughts are clear on this. The turbine exhaust is having lot of kinetic energy. So some of it is converted to pressure energy reaching the ambient pressure - this allows the turbine to allow some extra work extraction. The exhaust still has enough high temperature and velocity that it can still give out some jet thrust. The aircraft designer can play with the design of exhaust system such that they can either increase the work output of the turbine by having the entire exhaust path to be diffuser and reducing the jet thrust component extractable or they can have a part of the exhaust path to be diffuser and rest as a nozzle to maximize the jet thrust component. The turbine output will be slightly less in the later case. So you can have work output from the residual energy of the exhaust of the stock engine either as shaft power or as jet thrust.

Now I am thinking why the turboprop designers don't go a step further and extract all the energy possible in the turbine itself, similar to Turbo-shaft where the exhaust has no meaningful thrust. Why in case of turboprop a usable residual component is left to be used as jet thrust??

[nirav]

Oh because you are the one who decides what is actually a real world issue and what isn't?

I certainly will not and can not decide whats a real world issue and what isn't.

But Im baffled by the question and the insinuation that the kind folks at HAL who made the plane dont know that exhaust gases could lead to a tyre burst.

I trust that you can let me be baffled in my reply to Shiv saar and not ask me to shut up please.
Baffling out.

[JayS]

However, in flight this will most likely be a non-issue due to mixing with cold air. But on the ground, with the engine on for extended periods, it might lead to heating the structures in its vicinity. By how much and whether it affects the tires or the wheel rims, I guess they'll find out during testing.

I guess this is a valid point Kartik. For TPE-331 the exhaust temperature is higher on the ground than in the flight when the propeller is running at full power (expected since energy extraction at turbine is nil or small). This is confirmed by the Honeywell's Pilots Tip booklet for TPE-331 (link to which is posted in one of my posts above). At start EGT = 770C and at TO, EGT=650. In the flight, the EGT would be lower, the airflow would be having a strong mixing/cooling effect and the exhaust would be flowing nearer to the airframe due to the dominant airflow around aircraft. But on the ground while starting/slow taxiing all these factors will be absent. Only helping factor is a much lower mass flow rate due to engine running on close to idle rpm. Also the additional delta T increase is there due to taxiing which could be as high as 50C (this is delta T over the ambient T). So overall increase in the tire temperature could be alarming. There is a possibility prima facie.

But the config that HTT-40 uses is not a new thing. There are aircrafts out there which dump the exhaust in the same way. For example see this Tucano T1, which has exact same engine as HTT-40 has:


I am pretty sure HAL designers must have considered this point. Even if their analysis was inadequate and this issue still is unresolved I think they would have noticed it already during HSTT or they will notice is during further testing, as you too said already. Its not a big deal to change the exhausts by few degrees. Lets wait and watch.

[shiv]

Here are a series of images of 4 trainers including the HTT 40 that show the placement of the undercarriage versus the egjaust nojjals
PC-7
http://im.rediff.com/news/2012/may/04pilatus1.jpg

PC-9
http://image.shutterstock.com/z/stock-p ... 976383.jpg

The PC 7 and 9 appear to have an extended "lip" for the exhaust nozzle to delay/deflect the hot gases from fuselage contact. That is extra metal - not metal cut off to save weight

The Shorts Tucano has nozzles below the fuselage pointing back well inboard of the extended main undercarriage
https://upload.wikimedia.org/wikipedia/ ... 060%29.jpg

The HTT 40 in Sanjay Simha's image shows nozzles pointing backwards and if I extend the line of the nozzle back I get a direction that goes below the fuselage over each mainwheel well. This too is well inboard of the extended undercarriage.
http://defence-blog.com/wp-content/uplo ... AAn5tD.jpg

The Tucano mainwheels are apparently further apart but the exhaust pipe itself is close to the leading edge of the mainwing compared to the HTT 40 where the wheels are apparently close together but the exhaust nozzle is further forward at a greater apparent distance from the leading edge of the mainwing.

Maybe I am hypersensitive but I can't see what is so different and special about the HTT 40 design that makes someone say from a photograph that the undercarriage might get too hot. The comment could possible be in the "Why paratroops wear scooter helmets?' category. Aircraft design could show up a thousand faults including serious ones - but the whole business of looking at one image and being judgemental about something is either from a great expert in aircraft design or someone deliberately looking for a fault.

[shiv]

Now I am thinking why the turboprop designers don't go a step further and extract all the energy possible in the turbine itself, similar to Turbo-shaft where the exhaust has no meaningful thrust. Why in case of turboprop a usable residual component is left to be used as jet thrust??

My guess is that "maximum extraction of energy in the turbine" is dependent on maximum difference in pressure between turbine and outlet. Having a narrow outlet would raise outlet pressure and lower the pressure differential.

[Kartik]

However, in flight this will most likely be a non-issue due to mixing with cold air. But on the ground, with the engine on for extended periods, it might lead to heating the structures in its vicinity. By how much and whether it affects the tires or the wheel rims, I guess they'll find out during testing.

I guess this is a valid point Kartik. For TPE-331 the exhaust temperature is higher on the ground than in the flight when the propeller is running at full power (expected since energy extraction at turbine is nil or small). This is confirmed by the Honeywell's Pilots Tip booklet for TPE-331 (link to which is posted in one of my posts above). At start EGT = 770C and at TO, EGT=650. In the flight, the EGT would be lower, the airflow would be having a strong mixing/cooling effect and the exhaust would be flowing nearer to the airframe due to the dominant airflow around aircraft. But on the ground while starting/slow taxiing all these factors will be absent. Only helping factor is a much lower mass flow rate due to engine running on close to idle rpm. Also the additional delta T increase is there due to taxiing which could be as high as 50C (this is delta T over the ambient T). So overall increase in the tire temperature could be alarming. There is a possibility prima facie.

But the config that HTT-40 uses is not a new thing. There are aircrafts out there which dump the exhaust in the same way. For example see this Tucano T1, which has exact same engine as HTT-40 has:


I am pretty sure HAL designers must have considered this point. Even if their analysis was inadequate and this issue still is unresolved I think they would have noticed it already during HSTT or they will notice is during further testing, as you too said already. Its not a big deal to change the exhausts by few degrees. Lets wait and watch.

thanks for the great answer Nilesh. I too felt, after Indranil pointed out other aircraft with similar exhaust directional layouts, that if this was an in-service issue, it might have been flagged by existing users or designers of those aircraft.

Anyway, there is a tire burst plume analysis that OEMs need to carry out for certification purposes and it does mention that one of the primary causes of tire burst is over heating. There does exist the possibility that having such high temperature exhaust in close proximity, when the airplane is on the ground, may lead to overheating of the wheel rims or the tire itself.

tire failure characteristics

1) Tire failures are caused by foreign object damage, skid, low pressure, over load (shifting the load from one tire to the remaining tire(s) on the same truck), over pressure usually caused by over temperature

This manual by Goodyear for aviation tires, pgs 43-46 gives some good information on the temperature characteristics of tires and the effects of temperature on tire degradation..as well as some interesting charts on time it takes for a tire to cool down after being heated.

and this paper mentions that

The problem of tire strength differs from that encountered in the strength of normal metallic structures. The reasons for this lie in the complexity of the material characteristics of the constituents in an aircraft tire. In an aircraft tire almost all loads are carried directly by the textile cord reinforcement, commonly nylon as is presently practiced, in the form of a laminated composite shell. Nylon is known to be both non-linear and temperature sensitive.

In addition to the strength problem, aircraft tires generate substantial temperatures as they roll, both during takeoff and landing. The temperatures are not high in the usual sense of metallic materials, being on the order of hundreds of degrees Farenheit or less, but they are high enough to seriously degrade the strength of the load-carrying textile components of the tires. Excessive temperatures can easily cause a tire to fail even if the stress levels are not overly high.

So one might be dealing with not just the exhaust heat which might heat the tire and its rim, but the heat generated internally within the tire as it rolls during taxiing and takeoff which would add to it.

What I do find a little amusing is how some people on this forum assume that THEY can eye-ball something and tell that it surely won't be an issue..or when if someone does point out something that may be valid, they jump at it from a "oh we're SDRE onlee, so others don't trust us onlee" or call them idiots or whatever..

[nirav]

took a while to find a front on pic.



I think line of sight/perspective might be in play here.

@ Deejay ji, could you ask your photographer friend to post a front view of the HTT 40 ?
It would put to rest the 'debate' if its tyres could eggsplode of overheating due to exhaust gases !
And it would be clear where exactly those exhausts point.

The MLG of the HTT 40 appears to be placed closer to the fuselage than the Tucano.
But we dont even know yet how exactly the exhausts are canted on the HTT 40. The only sideways pic doesnt give proper perspective.

Still, eye ball analysis and debate on the exhaust causing the tyres to melt is equivalent of throwing muck on designers of HTT 40.

@ Kartik : Tyres heating up isnt a thing which started with the HTT 40. The manufacturer would take numerous factors into consideration before deciding on a recommendation on what is the optimal tyre pressure.

An issue that i have with eyeball engineering is that it tends to be in absolutes. You explained very patiently why the front nose wheel of the HTT 40 has 2 Tyres and why its so important.

There's a reason for having twin nose wheels too on the HTT-40, apart from the smaller size of individual nose wheels..if one deflates for any reason, the other can still take some of the load while the airplane comes to a halt. A dual redundancy kind of feature.

IF one were to take it as an absolute thumb rule, one can make some pretty damaging remarks about this.

Rafale, F18, Mig29 K, all have dual front wheels.

[Kakarat]

There seems to be an flight testing taking place over the Bangalore region. this unidentified aircraft has been doing loops for a long time, could be a DRDO EMB-145 AEW&C

https://www.flightradar24.com/TSTR46/9f4f123

[sum]

^^ Is there any update on the AEW&C? Is it inducted and were any follow on orders given or does it stop at the 3?

[JayS]

@Kartik

I backtrack my statement that Tucano T1 has same type of arrangement as HTT-40. The pics posted by shiv and nirav above clearly show the exhaust is directed way inboard of the tires. I couldn't find a good front view and couldn't think of looking for view from below, and whatever available pics were there gave an impression that the exhaust is going to hit the LG somewhere. Though I still think some other aircrafts have similar to HTT-40 layout (until I see some better pics to prove me wrong).

I have seen the Goodyear doc, from those graphs only I got the figure of delta T 40-50 degC. Rubber generates a lot (relatively speaking) of internal heat due to the hysteresis loop for stress-strain that it has as material property. The energy from the loop is dissipated as heat. So apart from friction at the surface, merely putting a tire through stress cycles will make it get heated to some extent.

Heating due to taxiing and TO/Landing is definitely going to be there. We don't know how much heating exhaust can cause, but whatever it may be, if it may be, it will be over and above this existing heating. Now we also do not know what are the typical max temperatures the tire reach due to taxiing/TO/Landing and how much margin they have over this till the max limit temperature allowed.

If the ambient temperature in India is taken at 50degC as max, then a 50deg delta due to taxiing/TO will give around 100degC temperature. This agrees well with the reference I quoted above which says typical max temperatures for tires under normal use for general aviation are about 110degC. According to the same reference max limit temperatures under severe breaking can reach 150degC. The tires already must be designed for this value. So we have a margin of ~50degC at least for exhaust related heating. But at the same time during emergency situation the overall heating might exceed 150C due to added heating by exhaust. Also even in day to day life any increase in normal operating temperatures can severely reduce life of the tires.

Too many unknown variables. Let's wait and watch what happens. If we see any change in config we would know if this was an issue or not. Till then we can try to get some chaiwala/panwalla gapshap on this issue.

There are many many design specific decisions in any aircraft. Sometimes rules of thumbs are followed sometimes they are not because they may be unnecessary or many actually causing some trouble. Its always difficult to judge based on limited info. A very good example is Corsair aircraft. When I first saw it, I was baffled by the shape of its wing and couldn't fathom the aerodynamic reason behind it for long time. Once in some documentary they mentioned the crank in the wing is to keep the LG height small, with straight wing it was becoming too long. It has nothing to do with aerodynamics at all..!!

Anyways I learned a lot through all this discussion.

[shiv]

What I do find a little amusing is how some people on this forum assume that THEY can eye-ball something and tell that it surely won't be an issue..or when if someone does point out something that may be valid, they jump at it from a "oh we're SDRE onlee, so others don't trust us onlee" or call them idiots or whatever..

I believe I owe you an explanation of my comments on the issue. Your only role has been to mention that someone on some other forum said that overheating of tyres from the HTT 40 exhaust was a possibility, given the exhaust configuration.

The issues I have with the comment made by that unknown (to me) person are as follows:

I have been unable to find any reference to tyre heating from exhaust gases as a cause of tyre burst among a number of reasons normally discovered for tyre bursting from overheating. That does not mean that it cannot occur or will not occur. It may be a known issue in closed specialist circles. It may be an ancillary cause.

However this raises two problems specifically related to the comment made by the unknown anonymous person about the HTT 40 on some other forum
1. If the problem is well known in specialist circles, is there any reason to believe that it is unknown to or ignored by the HTT 40 design team?
2. If, on the other hand, the problem has never occurred in the past, how was a prediction made that it might occur in the HTT 40 as opposed to all the other well documented things that can go wrong?

Either way the comment is redundant, and probably unnecessary. I believe that the person who made that comment has painted himself into a corner, and if it was in my power I would ask that person to explain why he made the comment.

[shiv]

I backtrack my statement that Tucano T1 has same type of arrangement as HTT-40. The pics posted by shiv and nirav above clearly show the exhaust is directed way inboard of the tires. I couldn't find a good front view and couldn't think of looking for view from below, and whatever available pics were there gave an impression that the exhaust is going to hit the LG somewhere. Though I still think some other aircrafts have similar to HTT-40 layout (until I see some better pics to prove me wrong).

If you see the distance from the Tucano exhaust to the fuselage skin and wing undersurface - they are both much closer than the distance to the tyres. Why is there no paint peeling or cracking in these areas? Paint should start blistering by 2-300 deg C.

Here is the image again: See the part where Qyoon Elijabeth's crown is painted?
https://upload.wikimedia.org/wikipedia/ ... 060%29.jpg
Also see
http://www.airpics.net/UserFiles/pics/l ... /6643l.jpg

This is so basic that you don't need to be a specialist to know that exhaust gases are hot. Is there any reason to believe that the HTT 40 team did not think of that?

Anyhow here is something to help you with some different viewing angles. The exhaust nozzles are a healthy distance from the wheels - compared to the Tucano the nozzles are placed relatively much further forward

[JayS]

Shiv,

I do not know why the paint does not peel there. For sure there are a million such birds flying around the world where exhaust goes directly on some metal panel. Soot deposition is real deal. A logical conclusion is then, that perhaps the exhaust gases cool quickly enough to have no significant heating effect, or perhaps the paint is heat resistant. But I couldn't say which for sure.

But I did frankly thought that the exhausts in Tucano T1 directly blow over the tires, which is not so. I am seeing this thing as an academic exercise since I do not know about it and I am learning from the discussion. Please note that I am not saying HAL did make an error or its OK to assume that HAL makes such error by default. I expect HAL designers must have done CFD simulations with the exhaust gases to study its mixing and whether the exhaust disrupts the flow in a bad way or not. Even without specifically thinking about the heating issue they would be knowing how far the danger zone of the exhaust gases extend, if at all.Frankly speaking, I don't think its a big deal. Even if HAL realises they made a boo boo it can be fixed just like that. Even in that case one shouldn't make a big deal of it that how can HAL be so stupid and all.

Now regarding the Tucano - there is a rule of thumb in aircraft design about jet mixing. It says the jet expands in a cone of 10deg. Assuming same principle but considering that the exhaust jets of turboprops would have considerable less mass and energy than a turbojet exhaust and is being swept away by incoming flow, the mixing will be much more vigorous, we can say the exhaust in this case might spread in a 30-40deg cone. Even then the exhaust would not reach the tires for the Tucano T1.

I tried finding some paper on jet mixing length, but I didn't find anything meaningful in a casual search and was wary of searching more. Else I could have said something concrete on how far the exhaust would go before it mixes completely in the surrounding flow. I have done such experiments in labs but I can't remember anything now. I have terrible memory.

[JayS]

How to resize image while posting here BTW??

[Zynda]

As expected, HAL has selected design from a phoren OEM for its regional aircraft under Make in India category. I some how, am unhappy with this Make in India project. Like it was pointed out, it seems similar or at best a notch above the previous ToT/Screwdriver giri arrangement.

Ukraine’s Antonov is the frontrunner for HAL’s military aircraft programme

Duo to manufacture 50-80 seater aircraft in India

NEW DELHI, MAY 24:
Ukrainian state-run Antonov has emerged as the frontrunner for Hindustan Aeronautics Ltd.’s (HAL) medium military transport aircraft programme.

HAL had received five proposals on its proposal to select a partner to manufacture a 50-80 seater aircraft in India. Out of the five bids, Antonov emerged as a clear winner as it met all the requirements set by HAL, sources told BusinessLine.

The Ukrainian firm, which had signed an agreement with Reliance Defence in April for joint production of their platforms in India, makes aircraft that can play both civil and military roles. (What happens to this JV with Reliance now? Will Reliance be a vendor to HAL?)

Apart from Antonov, GKN Aerospace, IAI, Ilyushin and ATR had also placed bids for the project. What worked in Antonov’s favour is that compared to all the other entrants, its series of aircraft are of recent design with state-of-the-art aviation systems like fly-by-wire, high-efficiency engines and all-weather operations. (Looks like this will be a variant of An-148/158)

According to estimates, the demand for 50-80 seater aircraft is expected to increase owing to the focus on tier II and III cities under the new Civil Aviation Policy. “HAL intends to leverage this opportunity through a partnering arrangement with an OEM under the Make in India programme,” officials said.

Antonov or AN class of aircraft have been part of the Indian Air Force (IAF) for over five decades. The IAF has more than 100 AN-32 aircraft recently upgraded on its inventory.

HAL is also looking at Antonov for additional military functions such as troop carrying and cargo, maritime patrol and reconnaissance, search and rescue operations.

[shiv]

As expected, HAL has selected design from a phoren OEM for its regional aircraft under Make in India category. I some how, am unhappy with this Make in India project. Like it was pointed out, it seems similar or at best a notch above the previous ToT/Screwdriver giri arrangement.

Ukraine’s Antonov is the frontrunner for HAL’s military aircraft programme

I can understand your feeling but in my view we will never get a passenger plane off the ground unless we do "risk sharing" with someone else. In this case Antonov is sharing the risk with us. They gain a market in an area where they can be squashed by bigger players and we get screwdriver.

[shiv]

Now regarding the Tucano - there is a rule of thumb in aircraft design about jet mixing. It says the jet expands in a cone of 10deg. Assuming same principle but considering that the exhaust jets of turboprops would have considerable less mass and energy than a turbojet exhaust and is being swept away by incoming flow, the mixing will be much more vigorous, we can say the exhaust in this case might spread in a 30-40deg cone. Even then the exhaust would not reach the tires for the Tucano T1.

I tried finding some paper on jet mixing length, but I didn't find anything meaningful in a casual search and was wary of searching more. Else I could have said something concrete on how far the exhaust would go before it mixes completely in the surrounding flow. I have done such experiments in labs but I can't remember anything now. I have terrible memory.

There is very little information that appears "easily". I am also digging for more information. There will be one stage immediately after ignition when the turbine is spooling up but the propeller is not engaged when prop backwash is absent. I am guessing that that will be when the gases are hottest. I wonder if there are any restrictions about how long the turbine can be kept spinning on "idle" without engaging the propeller. That is some stuff that I can go searching for..

[Vivek K]

I am with Hakim ji on this. While we research on the web for equations and or similar solutions, HAL has the tools and a flying prototype to solve this if it is a problem. I would only like to see a single piece canopy in the front. The forward visibility seems a little restricted. Are their plans to change the Canopy later to more like the Shorts Tucano styled one?