Stall Speed at Altitude

M

meritflyer

Well-Known Member
Had a disussion with my student today - We know that Indicated stall remains constant with altitude however, at high altitudes air density decreases and a higher pitch attitude must be maintained putting the aircraft closer to the critical AoA. So why then does IAS stall stay the same at high altitudes (the aircraft will stall at a higher true airspeed.. right?) even though the aircraft is flying at a higher AoA. I would think that the aircraft would stall at a higher IAS due to the need for a higher AoA.
 
meritflyer said:
Had a disussion with my student today - We know that Indicated stall remains constant with altitude however, at high altitudes air density decreases and a higher pitch attitude must be maintained putting the aircraft closer to the critical AoA. So why then does IAS stall stay the same at high altitudes (the aircraft will stall at a higher true airspeed.. right?) even though the aircraft is flying at a higher AoA. I would think that the aircraft would stall at a higher IAS due to the need for a higher AoA.
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Just because air density decreases does not mean you automatically need a higher pitch attitude. If you are flying at 1000' MSL at 100 KIAS you will have the same pitch attitude as you would flying 100 KIAS at 10,000' MSL. The only difference is that at 1000' your true airspeed will be around 102 and at 10,000' it will be about 120.

The same goes for stall speed, if your airplane stalls at 44 KIAS at sea level, then it will stall at 44 KIAS at 10,000' MSL. The only difference the extra altitude makes is that your TAS will be about 20% faster when you stall. IAS will not change.

The reason IAS does not change is because it measures dynamic pressure, in simplest terms the amount of air molecules flowing over the wing at any given time. Since the air is 'thinner' at higher altitudes, you have to go faster (TAS) to get the same amount of air flow.

You can use this to your advantage. Airspeed is also proportional to your drag. The higher your IAS, the more drag is created. The rule of thumb is that drag increases with the square of your speed. You want to go twice as fast, better bring 4x the power! The Cirrus SR20 and 22 are a perfect example. Even with 110 more horsepower, the SR22 only goes about 20 KTAS faster than the SR20. Sure climbs better though.

Since you can't really get more power, you can climb higher into thinner air. It seems 75% power is usually used for max cruise power in piston airplanes. If your airplane cruises at 110 KIAS at 1000' MSL your TAS will be around 112. Climb to 7-8,000' (the highest altitude you can maintain 75% in a normally aspirated airplane) and you can cruise at the same 110 KIAS, but now your TAS is around 126. So for the same fuel burn and power setting, you can pick up 14 kts, give or take a few.

This is really apparent in the jets, if you are cruising in the high 30's, if the IAS is around 220 the TAS will be around 440.
 
meritflyer said:
We know that Indicated stall remains constant with altitude however, at high altitudes air density decreases and a higher pitch attitude must be maintained putting the aircraft closer to the critical AoA.
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We do?
 
That is one question that I have had. I understand how you can get mach buffet below Mmo by pulling G's at higher speeds, but I am not real sure about how mach effects stall speeds.

I can see how even at fairly low IAS at very high altitude, as you approach stall, the TAS would be high enough that increasing the AOA might lead to shock wave formation on the top of the wing. I have been trying to find information on this, but have not been very successful. So the question would be, does mach effect the AoA where stall occurs? Is it possible to get shock wave formation as the airplane is slowed down and AoA is increased, causing the wing to stall at a lower AoA that at low altitudes? I'm thinking that this may be the case.

I first starting thinking about this while in training for the Beechjet. It has a stall system test that swings the needle on the AoA indicator and gives a shaker and stall identification. It does the test twice, once for low altitude and once for high altitude. The shaker and stall identification occur at different AoA indicator readings for each test, with the high altitude stall indication at a gauge reading significantly lower than normal. I never did get any clarification on this, and the manual had nothing to say either.

The only thing I could think of is to take the buffet margin chart for the 800XP and see what I can come up with. It would be a little bit of work though, as I would have to change Mach to TAS, to IAS, then do some more math since the chart gives low speed buffet for 1.5 G's on the low end. I would like to see how stalling IAS in the high 30's compares to lower altitudes.
 
I think you answered your own question here. You are correct that an aircraft at high altitude, where shock wave formation begins, will stall at a lower AoA. I think you actually are intuitively on the right track here. Just remember that an aircraft in transonic flight already has SOME shock wave formation, generally in normal cruise. This is normal, and part of the design (in fact, super critical wings are designed so the wing is supersonic, essentially, which is the way all the newer models are designed, but also designed not to have an abrupt shock wave, but rather "spread out"). This means there is already a shock wave (usually not a very strong one at normal cruise), and you can imagine the effect on the energy of the air aft of that shock wave and what happens as AoA is increased. It's not necessarily that you are creating the shock wave as you increase AoA, although that essentially happens too, as the increase of AoA can somewhat "sharpen" the shock wave as it rounds the leading edge (with apologies for trying to describe this in very general terms so the language is a bit sloppy!).

Look here too http://adg.stanford.edu/aa241/highlift/clmaxest.html

I'll probably read this later and not like the way I put this, but I have too many other things to do to fix it right now!

ananoman said:
That is one question that I have had. I understand how you can get mach buffet below Mmo by pulling G's at higher speeds, but I am not real sure about how mach effects stall speeds.

I can see how even at fairly low IAS at very high altitude, as you approach stall, the TAS would be high enough that increasing the AOA might lead to shock wave formation on the top of the wing. I have been trying to find information on this, but have not been very successful. So the question would be, does mach effect the AoA where stall occurs? Is it possible to get shock wave formation as the airplane is slowed down and AoA is increased, causing the wing to stall at a lower AoA that at low altitudes? I'm thinking that this may be the case.

The shaker and stall identification occur at different AoA indicator readings for each test, with the high altitude stall indication at a gauge reading significantly lower than normal. I never did get any clarification on this, and the manual had nothing to say either.
Click to expand...
 
ananoman said:
Just because air density decreases does not mean you automatically need a higher pitch attitude. If you are flying at 1000' MSL at 100 KIAS you will have the same pitch attitude as you would flying 100 KIAS at 10,000' MSL. The only difference is that at 1000' your true airspeed will be around 102 and at 10,000' it will be about 120.

The same goes for stall speed, if your airplane stalls at 44 KIAS at sea level, then it will stall at 44 KIAS at 10,000' MSL. The only difference the extra altitude makes is that your TAS will be about 20% faster when you stall. IAS will not change.

The reason IAS does not change is because it measures dynamic pressure, in simplest terms the amount of air molecules flowing over the wing at any given time. Since the air is 'thinner' at higher altitudes, you have to go faster (TAS) to get the same amount of air flow.

You can use this to your advantage. Airspeed is also proportional to your drag. The higher your IAS, the more drag is created. The rule of thumb is that drag increases with the square of your speed. You want to go twice as fast, better bring 4x the power! The Cirrus SR20 and 22 are a perfect example. Even with 110 more horsepower, the SR22 only goes about 20 KTAS faster than the SR20. Sure climbs better though.

Since you can't really get more power, you can climb higher into thinner air. It seems 75% power is usually used for max cruise power in piston airplanes. If your airplane cruises at 110 KIAS at 1000' MSL your TAS will be around 112. Climb to 7-8,000' (the highest altitude you can maintain 75% in a normally aspirated airplane) and you can cruise at the same 110 KIAS, but now your TAS is around 126. So for the same fuel burn and power setting, you can pick up 14 kts, give or take a few.

This is really apparent in the jets, if you are cruising in the high 30's, if the IAS is around 220 the TAS will be around 440.
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Awesome explanation.

In jets we always stay as high as possible for as long as possible before having to descend. Then, ideally, a power off descent to the terminal area. Most people think this is for fuel savings...as the engine burns less fuel up high...but another important factor is the increase in TAS.

If given a pilot's discretion descent from FL370 to 14,000...stay up high for two reasons...1) lower fuel burn and 2) higher TAS. The TAS at FL370 might be 450 kts...and at 14,000 350 kts. Even if you can keep the higher TAS for 5 minutes or so...that might equal 10 to 15 miles for the same time in the descent.

This is assuming no wind.
 
Well, not entirely. The higher altitude yields the best NAMs/fuel, but not necessarily the highest TAS. The highest TAS is obtained, in simple terms, at the lowest point you can attain your Mmo (or desired cruise mach), before the Vmo starts reducing the max mach. That is usually about FL 280 or so. We use this to make up time fairly often. Burn is higher, but M0.86 at FL 280 will make up a lot of time, and can be worth it in certain circumstances depending on the situation. Obviously, winds are a factor also, so often an Eastbound flight will gain more by the stronger winds up higher, or it's break even, time wise, so the fuel used isn't worth it. I run the numbers in our laptop performance computer, and have our flight dispatchers look at the scenario to see what is the most cost effective, overall.
 
seagull said:
. That is usually about FL 280 or so. We use this to make up time fairly often. .
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Hmm, that seems low for max TAS...I'll have to dig out our charts and see how they compare. I would've guessed more towards the trop...in the mid 30's.
 
Would be higher, but the constant mach number as you go above that pulls the IAS down faster than the TAS increases, in simplified terms.
 
Back to the original question, are the 'coffin corner' depictions we usually see in books truthful or not? On the low end they always show a line that represents a stall speed at a constant IAS, but the line slants to the right because TAS increases with altitude. Should this line have a constant slope, or should it have a 'kink' where the slope increases when mach effects come into play at high altitudes, causing a stall at a lower AoA and a higher IAS than at low altitudes?
 
seagull said:
Well, not entirely. The higher altitude yields the best NAMs/fuel, but not necessarily the highest TAS. The highest TAS is obtained, in simple terms, at the lowest point you can attain your Mmo (or desired cruise mach), before the Vmo starts reducing the max mach. That is usually about FL 280 or so. We use this to make up time fairly often. Burn is higher, but M0.86 at FL 280 will make up a lot of time, and can be worth it in certain circumstances depending on the situation. Obviously, winds are a factor also, so often an Eastbound flight will gain more by the stronger winds up higher, or it's break even, time wise, so the fuel used isn't worth it. I run the numbers in our laptop performance computer, and have our flight dispatchers look at the scenario to see what is the most cost effective, overall.
Click to expand...


Well, this got me thinking...so I looked at some charts and took some notes during my last trip. My understanding was that TAS would increase all the way to the trop (after all TAS increases with altitude right?) just as fuel economy increases all the way to the trop.

What I found left me a bit confused.

1. As per my expectations, the performance charts for LRC...showed TAS increasing all the way up to FL330 before starting to drop above FL350. Fuel flow, without question, decreased steadily up to the serice ceiling.

2. I watched the airspeed and took notes descending from FL330 toFL210. Using a constant Mach .74 down to 300 KTS at the mach changeover point in the descent (FL280)...I observed what Seagull pointed out. The TAS increased in the descent to FL280.

At FL330 TAS was 444kts. The TAS increased to 448 kts at FL280 before dropping off rapidly to 434 kts at FL210.

My question now is...why do the LRC charts show an increase in TAS to a much higher altitude? I forgot to look at the charted F/F...but I'm going to theorize that for LRC...the power setting is higher to obtain the higher TAS and is therefore not a linear correlation to the observed TAS in the descent.

Otherwise I understand a temperature dependent Mmo decreasing rapidly above FL280...that affects the TAS. The LRC chart has me confused.
 
You got it, the issue is the factors that determine LRC introduce other variables which are dependent on the fuel consumption (NAMs/1000). That will be impacted by many things, including items specific to the type of powerplant you're using.
 
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