Horsepower required for same airspeed at higher altitude

[birdus]

If I need a 75% power setting for max cruise at sea level (say, a 200-HP IO-360, so 150HP), will the same horsepower be required to maintain max cruise (indicated) at 10,000 feet, for example? So, in that case, I believe I would be using full throttle as that would give me about 150 HP, or 75% of sea-level power.

Thanks,
Jay

 

[iamtheari]

I don’t know about the same airspeed part of the question, but I think the power available to a naturally aspirated engine at 10,000 MSL will be closer to 60% than 75%. I cruise there anyhow because I normally cruise around 65% power and it’s usually cooler and smoother up there.

 

[MooneyDriver78]

My perf charts tell me 70% at 10,000’ if I set RPMs at 2700, which I would not do, 2600 would give me 65% and a quieter ride.
That’s assuming std temps and low humidity….here in Florida, you’ll be lucky to get 60%.

 

[geezer]

If you are using 75% at sea level, full throttle at about 8,000 feet will be the same horsepower, as the air density is about 75% there. Density altitude will make a difference from day to day, but that number is close.

At 10,000, the TAS will be reduced about 20% from 8,000 feet.

I believe that will give you the same True Airspeed at 8,000 feet, but naturally, a lower Indicated Airspeed.

Are you getting ready to repower the Luscombe with that IO 360? This question is obviously a joke!

 

[birdus]

I don’t know about the same airspeed part of the question, but I think the power available to a naturally aspirated engine at 10,000 MSL will be closer to 60% than 75%.

I read that you lose 3% per 1,000 feet, so that gives about 75% at 10,000 feet. That's what I'm going off of. Obviously, I could be missing something.

 

[birdus]

My perf charts tell me 70% at 10,000’ if I set RPMs at 2700, which I would not do, 2600 would give me 65% and a quieter ride.
That’s assuming std temps and low humidity….here in Florida, you’ll be lucky to get 60%.

Okay, so your 70% number is close to what I got using 3% loss per 1,000 feet, but then it sounds like you'll need a higher-than-desirable RPM to achieve that. Regardless, that's not really the information I'm after, but still good to know/learn.

 

[birdus]

If you are using 75% at sea level, full throttle at about 8,000 feet will be the same horsepower, as the air density is about 75% there. Density altitude will make a difference from day to day, but that number is close.

I read that you reduce power by 3% per 1,000 feet, but maybe that's not right. Obviously, I need to do some more research.

At 10,000, the TAS will be reduced about 20% from 8,000 feet.

I thought true airspeed went up at higher altitudes. Again, I need to read more.

I believe that will give you the same True Airspeed at 8,000 feet, but naturally, a lower Indicated Airspeed.

I don't understand this, but will keep learning.

Are you getting ready to repower the Luscombe with that IO 360? This question is obviously a joke!

Ha, ha! Clearly a joke! I had the pleasure of meeting and chatting for a few hours with George Pereira for a few hours in his dining room down in Sacrament, CA a couple years ago. While there, I bought plans from him for his GP-4 design which I plan on building. Realistically, I'll start on it within a few years. This question is about that plane.

 

[midlifeflyer]

Okay, so your 70% number is close to what I got using 3% loss per 1,000 feet, but then it sounds like you're need a higher-than-desirable RPM to achieve that.

Remember that density altitude does two things. Saps HP and makes airfoils less efficient. The performance charts are trying to account for both.

 

[birdus]

Remember that density altitude does two things. Saps HP and makes airfoils less efficient. The performance charts are trying to account for both.

Right, but I would've thought that increased pitch would make up for that, thus obviating the need for max RPM.

 

[asicer]

I read that you lose 3% per 1,000 feet, so that gives about 75% at 10,000 feet. That's what I'm going off of. Obviously, I could be missing something.

3*10=25? Or is your engine making 105% HP at sea level?

 

[midlifeflyer]

I thought true airspeed went up at higher altitudes. Again, I need to read more.

True airspeed is higher at altitude for a given indicated airspeed. But the reduction of available power and thrust means you might not be able to get to that indicated airspeed.

that's because...

I don't understand this, but will keep learning

True airspeed is the actual speed of the aircraft through the air regardless of air density. Indicated airspeed is the impact of the air on the pitot tube. That is affected by air density.

Think in terms of how much faster you must go through the air (TAS) in order to get the same impact (IAS) at lower air density and you'll begin to understand their relationship.

 

[midlifeflyer]

Right, but I would've thought that increased pitch would make up for that, thus obviating the need for max RPM.

You mean increased drag requires less power and thrust? Think about that.

 

[geezer]

If you are planning a fixed pitch, the RPM will not be an issue, even at wide open throttle, up there the RPM will be less than at sea level.

If your propeller and engine gives red line RPM at sea level, it will only reach 70% at 10,000 feet.

Variable pitch will still be limited to the red line, regardless of altitude.

 

[iamtheari]

My perf charts tell me 70% at 10,000’ if I set RPMs at 2700, which I would not do, 2600 would give me 65% and a quieter ride.
That’s assuming std temps and low humidity….here in Florida, you’ll be lucky to get 60%.

Yowza, people actually cruise at 2600 RPM? I pull back to 2500 by 500 AGL and 2300 when I level off, then keep it there until downwind. But your performance charts are right, those settings are possible and safe. Just not my cup of tea.

 

[RyanB]

Remember that density altitude does two things. Saps HP and makes airfoils less efficient. The performance charts are trying to account for both.

And that rated (200hp IO-360) power output is only going to be true when at sea-level on a standard day.

 

[Tantalum]

Turbo is the answer

 

[kyleb]

Your airplane will fly its fastest at the highest altitude the engine can maintain 75%. Generally, that is in the 8,000' range. 75% is 75%, regardless of the altitude it is achieved at. But air density and drag decrease with altitude, so you get more speed at the same power setting the higher you go.

 

[birdus]

3*10=25? Or is your engine making 105% HP at sea level?

Depends how you do the math, and I don't know how an engineer would do it:

200 0
194 1000
188.18 2000
182.5346 3000
177.0586 4000
171.7468 5000
166.5944 6000
161.5966 7000
156.7487 8000
152.0462 9000
147.4848 10000

Regardless, it sounds like you're completely missing the point of the question.

 

[asicer]

Regardless, it sounds like you're completely missing the point of the question.

No I am not. I'm pointing out your math error. Reasonably accurate math is germane to the question.

 

[birdus]

You mean increased drag requires less power and thrust? Think about that.

Nobody would say that increased drag requires less power and thrust. My point was that at higher altitudes, one increases the pitch of the prop to make up for the thinner air.

 

[Tantalum]

one increases the pitch of the prop to make up for the thinner air

No.

 

[birdus]

Your airplane will fly its fastest at the highest altitude the engine can maintain 75%. Generally, that is in the 8,000' range. 75% is 75%, regardless of the altitude it is achieved at. But air density and drag decrease with altitude, so you get more speed at the same power setting the higher you go.

It can be extremely difficult to get an answer to a question, but I feel like we're getting somewhere here. Still not quite there yet, though.

If I can cruise at, say, 200 knots at a 75% power setting at sea level with an IO-360, that means I'm using 150HP to cruise at that 200 knots. Some people are getting hung up on this, but, for the sake of argument, let's say that that engine will produce 150HP at 10,000 feet at full throttle. Are you telling me that I would leave the throttle alone (75% pushed in) when shooting for max cruise at 10,000 feet? I would've assumed I would would want to push the throttle all the way in by the time I'm at 10,000 feet as that would give me about the same power I was getting as sea level, around 150HP, or 75% of the engine's max sea-level power. My question, then, was at 10,000 feet would that same 150HP give me the same 200 knots of cruise, or would I actually need less power to achieve that same 200 knot cruise speed since the air is thinner at 10,000 feet?

 

[MauleSkinner]

I found a picture of a 172 performance chart…
http://flyingprofessors.net/cruise-performance-charts/
At 2000 feet, standard temperature, 60% power, TAS is 113, which would be a CAS of 109.
At 6000 feet, standard temperature, 60% power, TAS is 118, which would be a CAS of 108.

 

[Checkout_my_Six]

So turning higher rpms wears out engines?

 

[kyleb]

It can be extremely difficult to get an answer to a question, but I feel like we're getting somewhere here. Still not quite there yet, though.

If I can cruise at, say, 200 knots at a 75% power setting at sea level with an IO-360, that means I'm using 150HP to cruise at that 200 knots. Some people are getting hung up on this, but, for the sake of argument, let's say that that engine will produce 150HP at 10,000 feet at full throttle. Are you telling me that I would leave the throttle alone (75% pushed in) when shooting for max cruise at 10,000 feet? I would've assumed I would would want to push the throttle all the way in by the time I'm at 10,000 feet as that would give me about the same power I was getting as sea level, around 150HP, or 75% of the engine's max sea-level power. My question, then, was at 10,000 feet would that same 150HP give me the same 200 knots of cruise, or would I actually need less power to achieve that same 200 knot cruise speed since the air is thinner at 10,000 feet?

You will need less hp to maintain 200 knots at 10,000' than at sea level. "75% pushed in" has no relevance. If you set power to 75% (measured off of the performance charts, using manifold pressure, RPM, etc) you will go faster "at altitude" with that 75% power than you would down low. Unless you have a supercharged, turbocharged, or turbonormalized engine, you'll need to spin the engine faster up high to generate that power, because the lower atmospheric pressure will limit your engine's manifold pressure.

 

[Tantalum]

at 10,000 feet would that same 150HP give me the same 200 knots of cruise, or would I actually need less power to achieve that same 200 knot cruise speed since the air is thinner at 10,000 feet?

You would need less. The air is thinner. I think you might be getting hung up on this idea of percentages and how far you have to push the knob in. 150 hp at 10,000 ft is faster than 150 hp at sea level, and to take it a step further, 150 hp at 25,000 ft is much faster still than either 10K or sea level

It also helps to think of it in terms of manifold pressure. The throttle knob simply moves a small plate in the engine. At 10K feet even if you have it pushed all the way in the air is thinner so you're getting probably somewhere around 20-ish inches of manifold pressure, or about 2/3 to 3/4 of the total power available.. hence the 75% people are mentioning above.. even if it is "pushed all the way in"

This is why people turbo and turbo normalize planes, so they can keep their engines producing power (29+ inches of MP) but take advantage of thinner air up high to go faster KTAS (thinner air, less drag = faster plane)

 

[iamtheari]

So turning higher rpms wears out engines?

 

[MauleSkinner]

A V35B chart I found for 120 hp is
132 KTAS at sea level standard temp (132 KCAS)
141 KTAS at 8000 feet standard temp (125 KCAS)

http://www.fsd-international.com/Hangar/V35B/Pilot Operating Handbook.pdf

 

[birdus]

I found a picture of a 172 performance chart…
http://flyingprofessors.net/cruise-performance-charts/
At 2000 feet, standard temperature, 60% power, TAS is 113, which would be a CAS of 109.
At 6000 feet, standard temperature, 60% power, TAS is 118, which would be a CAS of 108.

Is that 96 HP (.6 * 160) in both cases?

 

[MauleSkinner]

Is that 96 HP (.6 * 160) in both cases?

The same HP either way, but the rated hp of the airplane that chart belongs to, I don’t know.

 

[dtuuri]

If I need a 75% power setting for max cruise at sea level (say, a 200-HP IO-360, so 150HP), will the same horsepower be required to maintain max cruise (indicated) at 10,000 feet, for example?

I think your question could use a slight refinement: "If I need a 75% power setting for max cruise angle of attack at sea level (say, a 200-HP IO-360, so 150HP), will the same horsepower be required to maintain max cruise (indicated) at 10,000 feet, for example?"

Your airspeed indicator is a great AoA indicator. For the same AoA at altitude you will need more power. Yes, you will fly faster, but it will take more power. If you insist on keeping the power the same (as you stipulate), you will be at a higher AoA (slower IAS) albeit at a higher TAS. See the very last chapter in Stick and Rudder.

 

[Adam Weiss]

Yowza, people actually cruise at 2600 RPM? I pull back to 2500 by 500 AGL and 2300 when I level off, then keep it there until downwind. But your performance charts are right, those settings are possible and safe. Just not my cup of tea.

Yes. For long XC, every day at 2600 rpm. Wide open throttle.
CHTs around 385 and oil temp around 190.
Is there any reason why one wouldn’t want to do this (other than economy fuel burn)?

 

[dtuuri]

Yes. For long XC, every day at 2600 rpm. Wide open throttle.
CHTs around 385 and oil temp around 190.
Is there any reason why one wouldn’t want to do this (other than economy fuel burn)?

Believe me, I'm not an expert on this, but maybe one will tell me why I shouldn't be concerned. Between the crankshaft journal and connecting rod bearing is—space. It's where the oil runs through. So the more revolutions the crank makes the more the space increases as the metal wears down. As the space gets bigger it seems to me the power strokes would cause the crank journal to start slapping at the bearing material due to the loose fit and punching through the oil film causing faster wear on both the crank and bearing surfaces. I would think that would shorten crankshaft life and cause bearing wear. No?

 

[birdus]

The same HP either way, but the rated hp of the airplane that chart belongs to, I don’t know.

Yeah, I don't care about the exact engine or power, but the principle. So, 65% of sea-level power will give the same indicated airspeed at any altitude? I'm just inferring since you gave higher true airspeed at higher altitude for that same engine output. Is that correct?

 

[birdus]

For the same AoA at altitude you will need more power.

This is interesting. Same angle of attack = same induced drag. Why would more power be required?

Couldn't find my copy of Stick and Rudder with a brief search. Will look again tomorrow. If the answer is there, I'll read that and you don't need to bother answering. I've read it, but obviously don't recall what it said on this topic.

 

[dtuuri]

This is interesting. Same angle of attack = same induced drag. Why would more power be required?

Couldn't find my copy of Stick and Rudder with a brief search. Will look again tomorrow. If the answer is there, I'll read that and you don't need to bother answering. I've read it, but obviously don't recall what it said on this topic.

I included a link to the book and chapter. The reason is force vs. power. At a higher TAS you need to pull the plane with the same force at a higher rate, ergo, more power required.

 

[birdus]

A V35B chart I found for 120 hp is
132 KTAS at sea level standard temp (132 KCAS)
141 KTAS at 8000 feet standard temp (125 KCAS)

http://www.fsd-international.com/Hangar/V35B/Pilot Operating Handbook.pdf

So I see that the POH specifies that it's a constant 120 HP or full throttle for cruise power, so that's 65% power (120 / 185), so that answers my question partly. The percentage is relative to sea-level power, so in other words, it's a constant engine output or horsepower. Are those speeds a constant indicated airspeed (if you know)?

 

[Llewtrah381]

Turbo is the answer

…and the Jeopardy question is “What do I do if I have too much money?”

 

[dtuuri]

So I see that the POH specifies that it's a constant 120 HP or full throttle for cruise power, so that's 65% power (120 / 185), so that answers my question partly. The percentage is relative to sea-level power, so in other words, it's a constant engine output or horsepower. Are those speeds a constant indicated airspeed (if you know)?

He gives the speed (CAS) and it confirms my post.

 

[Tantalum]

…and the Jeopardy question is “What do I do if I have too much money?”

Interestingly enough I spent some time talking with the owner of a beautiful Comanche 400 and that was part of the rationale behind that airplane.. just give it gobs of naturally aspirated power so even up at altitude it still has a lot of juice