Will a constant power climb result in a constant IAS?

[peter-h]

There should be a fairly simple answer to this - assuming slow subsonic flight.

Putting it another way, will the same thrust produce the same IAS, for various altitudes?

 

[Dr. O]

NOPE...

 

[JimNtexas]

I think your airspeed will drop to near zero at about 300,000 feet.

 

[peter-h]

Yes but once outside the atmosphere your TAS will eventually approach the speed of light if constant thrust can be maintained

The reason I asked is because I don't see any obvious difference in the IAS between say 2000ft and 6000ft, if the engine is running under identical conditions e.g. 23" / 2400 / 11.2 USG/hr which is peak EGT.

Maybe there is an effect which is being compensated for by a prop efficiency variation?

Obviously above a certain altitude, say 8000ft, I cannot achieve 23" anymore and then the engine power starts to drop off.

 

[poadeleted20]

No good answer -- too many other variables. You could certainly make a constant IAS climb with constant power, but TAS will be increasing as you go, so power required for level flight will go up as you go, and climb rate will decrease. Eventually, power required for the increased TAS will equal power produced, and the climb will stop.

 

[Sac Arrow]

Yes but once outside the atmosphere your TAS will eventually approach the speed of light if constant thrust can be maintained

The reason I asked is because I don't see any obvious difference in the IAS between say 2000ft and 6000ft, if the engine is running under identical conditions e.g. 23" / 2400 / 11.2 USG/hr which is peak EGT.

Maybe there is an effect which is being compensated for by a prop efficiency variation?

Obviously above a certain altitude, say 8000ft, I cannot achieve 23" anymore and then the engine power starts to drop off.

It should be noted that 23" at 6,000 feet results in more power than 23" at 2,000 feet, due to the temperature difference resulting in increased density. Probably not a significant difference, but theoretically it is there.

 

[peter-h]

You could certainly make a constant IAS climb with constant power, but TAS will be increasing as you go

That is what I would think should happen.

Assuming constant thrust, IAS should be constant over altitude.

TAS will of course be higher at higher altitudes.

But let's forget the "climb" bit. Let's just take level flight. Should say 200HP going into the prop (assume constant prop efficiency) produce the same level flight IAS at 2000ft as at 6000ft? I think it should, and it does in my airplane. But someone I know says this is impossible.

 

[poadeleted20]

That is what I would think should happen.

Assuming constant thrust, IAS should be constant over altitude.

Only if the airplane is trimmed for that IAS and no pressure applied in pitch to disturb trimmed speed.

But let's forget the "climb" bit. Let's just take level flight. Should say 200HP going into the prop (assume constant prop efficiency) produce the same level flight IAS at 2000ft as at 6000ft? I think it should, and it does in my airplane. But someone I know says this is impossible.

Assuming temperature is the same, so close to "yes" as makes no significant difference.

 

[Sac Arrow]

I think it should decrease. I think it shows more or less constant for the reason I already mentioned - you are producing more power than you think at higher altitude.

I see the same thing though in my Turbo Arrow.

 

[poadeleted20]

I think it should decrease. I think it shows more or less constant for the reason I already mentioned - you are producing more power than you think at higher altitude.

I see the same thing though in my Turbo Arrow.

What the IAS does will depend on what you do with pitch and trim, and that is true regardless of what you do with power.

 

[oldShar]

I think that propeller efficiency will also change, with more shaft power being converted to torque and less to thrust with increase in altitude (especially assuming a constant speed prop)

 

[poadeleted20]

I think that propeller efficiency will also change, with more shaft power being converted to torque and less to thrust with increase in altitude (especially assuming a constant speed prop)

Again that's hard to predict, as it depends on where you are in the efficiency curve for that prop at the RPM you're running.

 

[peter-h]

Assuming temperature is the same, so close to "yes" as makes no significant difference.

That's my view, too. I just wish I could find some slightly more rigorous reasoning for it...

The reason I think that a constant thrust must produce a near-constant IAS over a range of altitudes (i.e. air densities) is simply because as you go higher and the air gets thinner, where else is that constant thrust going to go? It must translate into a corresponding aerodynamic behaviour along the line of movement, and that behaviour is related principally to IAS.

I am talking of a first order approximation here.

 

[gismo]

Constant thrust will produce the same IAS if other parameters such as AoA, climb rate, etc are held constant. However powermust increase if the altitude increases an the IAS is constant because power is equal to thrust times velocity and in this case the velocity is TAS not IAS and of course TAS will be increasing under the stated conditions.

Edit: The above ignores the slight decrease in thrust required due to the very slowly decreasing weight of the aircraft as fuel is consumed (and you get marginally further from Earth's mass).

 

[peter-h]

I don't follow that.

Let's say you have two airplanes, one flying at 1000ft and another flying at 5000ft.

Both have engines running at the samake peak-EGT or LOP setting, say 23" 2300rpm, and the same fuel flow.

Same prop RPM, so let's assume same prop efficiency.

I think their powerplants are producing the same thrust, because that can come only from burning fuel, and their IAS will be the same too.

Their AoA will be different because the 5000ft one will have a greater AoA needed to support the same weight, but I don't think that matters.

The reason I don't think power needs to increase with altitude (to maintain the increasing TAS) is because the air is thinner higher up and it takes less work to push it out of the way. This is why fuel burn in general improves with altitude.

 

[Sac Arrow]

I don't follow that.

Let's say you have two airplanes, one flying at 1000ft and another flying at 5000ft.

Both have engines running at the samake peak-EGT or LOP setting, say 23" 2300rpm, and the same fuel flow.

I think their powerplants are producing the same thrust, because that can come only from burning fuel, and their IAS will be the same too.

I'm not sure you can have the same peak EGT or LOP setting, and the same fuel flow, at different altitudes. I think that as altitude increases the density (in the manifold, at equal manifold pressures, not outside density) which will also increase necessitating an increase in fuel to maintain the same EGT. It just might not be noticeable within the accuracy of the fuel flow meter. Also the EGT itself would decrease in temperature proportionate to altitude, further necessitating an adjustment in fuel flow if you were to maintain the same EGT you held at a lower altitude.

 

[skidoo]

Looking at the POH numbers in a T182T, it has a chart for full power (i.e. 32", 2400 rpm, and 24 GPH, standard temp) max rate of climb. At sea level, the IAS is 84 kts, at 20kft, it is 80 kts for best rate. But, at 20kft, the climb rate has also been reduced. So, this demonstrates that IAS will vary.

The OP did not state whether the climb was constant rate or not. Obviously, the IAS will vary with pitch attitude for different rates at the same power level.

Now, assuming a constant rate of climb (say something significantly less than max) and a constant power level, it would stand to reason that IAS would vary because to maintain the same rate of climb as altitude is gained, pitch would need to change.

On the other hand, assuming you maintained constant IAS and constant power, the rate of climb/descent would vary as altitude changes.

 

[peter-h]

Let's forget climbing.

Just look at steady state level flight.

To suggest that the required power increases as TAS increases is to suggest that flying higher is no more economical, which is obviously not the case.

I think the power required for level flight is according to IAS, not TAS, and to a first order approximation this is independent of altitude.

 

[flyingmoose]

It will not stay the same. Assuming constant rate then there is simply not the same amount of air to force into the pitot tube at higher altitudes. Just because you cannot see a difference between 4000-6000 msl just means you havent gone high enough to see a difference.

That's my view, too. I just wish I could find some slightly more rigorous reasoning for it...

The reason I think that a constant thrust must produce a near-constant IAS over a range of altitudes (i.e. air densities) is simply because as you go higher and the air gets thinner, where else is that constant thrust going to go? It must translate into a corresponding aerodynamic behaviour along the line of movement, and that behaviour is related principally to IAS.

I am talking of a first order approximation here.

It goes into the TAS

Looking at the POH numbers in a T182T, it has a chart for full power (i.e. 32", 2400 rpm, and 24 GPH, standard temp) max rate of climb. At sea level, the IAS is 84 kts, at 20kft, it is 80 kts for best rate. But, at 20kft, the climb rate has also been reduced. So, this demonstrates that IAS will vary.

The OP did not state whether the climb was constant rate or not. Obviously, the IAS will vary with pitch attitude for different rates at the same power level.

Now, assuming a constant rate of climb (say something significantly less than max) and a constant power level, it would stand to reason that IAS would vary because to maintain the same rate of climb as altitude is gained, pitch would need to change.

On the other hand, assuming you maintained constant IAS and constant power, the rate of climb/descent would vary as altitude changes.

Very well said

 

[gismo]

I don't follow that.

I suspect that's because it's counter-intuitive.

Let's say you have two airplanes, one flying at 1000ft and another flying at 5000ft.

Both have engines running at the samake peak-EGT or LOP setting, say 23" 2300rpm, and the same fuel flow.

Same prop RPM, so let's assume same prop efficiency.

I think their powerplants are producing the same thrust, because that can come only from burning fuel, and their IAS will be the same too.

First of all ignoring all the little variables affecting the engine and propeller efficiency, fuel consumption rate on a propeller driven airplane is proportional to power not thrust (with jets fuel flow == thrust, therefore on those HP varies with TAS for constant FF). And power delivered is always equal to thrust times velocity (that's a well proven law of physics). I assume you can understand that thrust must equal drag in constant speed level flight (it gets a little more complex if climbing but the same underlying principles apply).

Their AoA will be different because the 5000ft one will have a greater AoA needed to support the same weight, but I don't think that matters.

Actually the AoA should be the same in both cases as long as the IAS is the same. There is a fixed relationship between AoA, IAS, and weight for a given wing. This is why so many V speeds (e.g. Vs, Vbg, Vmaxrange, Va) are constant with altitude, each of those occurs at the same AoA regardless of altitude.

The reason I don't think power needs to increase with altitude (to maintain the increasing TAS) is because the air is thinner higher up and it takes less work to push it out of the way. This is why fuel burn in general improves with altitude.

Fuel burn doesn't necessarily "improve" with altitude. For maximum no wind range (i.e. max efficiency) the IAS is constant (as is the AoA and the efficiency/range) at any altitude the airplane can fly but the TAS increases with altitude. If you think about it carefully you should see that this implies that the fuel consumption varies exactly with TAS in that case (10% higher TAS at high altitude requires exactly 10% more fuel flow if the range is unchanged).

Likely what makes you associate higher altitude with "improved" fuel burn rate is that we typically operate at lower IAS with a higher cruise altitude and it's actually the fact that we're coming closer to the IAS for best range that "improves" the fuel burn. And if you were to fly up real high at an IAS that's slower than best range speed you'd actually see a decrease in efficiency compared to flying the same TAS at a lower altitude where the IAS was closer to best range speed.

The above is not my opinion, it's all fact unless I've forgotten something or wrote it incorrectly. For a more detailed explanation take a gander at Dr. Byington's white paper(s) on the subject:

http://www.db.erau.edu/research/cruise/

That page requires you to take a short quiz which you can skip by going to:

www.db.erau.edu/research/cruise/piston.frame.html

 

[peter-h]

Many thanks for that detail, Lance

One of the things I was trying to get my head round was that a turbo-normalised airplane, TIO-540, 250HP, will do a max of say 165kt IAS (and say 170kt TAS) at low level, but will do perhaps 195kt TAS at FL180, with the engine still supposedly producing 250HP (same MP etc as at SL).

If HP increased simply with TAS, this should not be happening.

 

[Dave Siciliano]

Peter: I'm seeing that in my 58P on all my trips in the flight levels. IAS may be 175 to 180 at 4 or 5,000 feet with TAS a bit higher. At FL180, I'm indicating 155 to maybe 160 and TAS is up around 210 depending on all the variable discussed; standards day, etc. Same power settings and fuel flow. My best glide is 122 knots. Always something to learn about this stuff no matter how long one does it <g>

Best,

Dave

 

[gismo]

Many thanks for that detail, Lance

One of the things I was trying to get my head round was that a turbo-normalised airplane, TIO-540, 250HP, will do a max of say 165kt IAS (and say 170kt TAS) at low level, but will do perhaps 195kt TAS at FL180, with the engine still supposedly producing 250HP (same MP etc as at SL).

If HP increased simply with TAS, this should not be happening.

Yep. HP will be proportional to TAS only if the IAS is constant and I'm sure you'll find that the IAS with 250 HP will be less at FL180 than at low level even though the TAS is higher at FL180. I long ago came to the conclusion that whoever decided to mark the dynamic pressure gauge (aka ASI) in our airplanes with MPH or Kt did us all a disservice in that it tends to create mistakes in the way we look at the effects of speed vs dynamic pressure. If that gauge was simply marked with PSI, Bar, or Pascals pilots would be a lot less likely to get confused about all this IAS vs TAS nonsense.

 

[peter-h]

I still don't understand how a constant-power (turbonormalised) engine produces a higher TAS at altitude.

 

[skidoo]

I still don't understand how a constant-power (turbonormalised) engine produces a higher TAS at altitude.

Because it can produce the same power level in thinner air that has less drag. Less drag = more speed for the same power.

A NA engine will have less power available at altitude.

 

[peter-h]

That what I thought, but that also means that power is not required simply in proportion to TAS.

 

[PilotAlan]

It seems to me there's a flaw in the way the question is framed.
The same amount of thrust will result in the same IAS regardless of altitude.
BUT the prop becomes less efficient with altitude, so more power is needed to maintain the same thrust.

The same power will result in a decrease in IAS as altitude increases.
But TAS is increasing at a faster rate due to drag on the airframe decreasing faster than the decrease prop efficiency.

 

[gismo]

That what I thought, but that also means that power is not required simply in proportion to TAS.

Once again, power required in proportion to TAS if and only if CAS is held constant. Because power required is pretty much proportional to the cube of CAS, a small decrease in CAS results is a much larger decrease in power required. When one cruises at higher altitudes using constant power the CAS decreases but TAS increases. The extra power required for the higher TAS comes from fact that less HP is needed to overcome drag at the lower CAS. Make sense yet?