Translating Horsepower to Airspeed

[BPM]

Good morning POA.

I’m trying to better understand how the higher horsepower engines in turboprops translate to higher airspeed.

On the one hand it seems obvious that (for purposes of discussion) doubling horsepower would yield higher airspeed. However, propeller speeds are limited (normally ~2,700 rpm’s).

Which leads to my question, if a 300 hp reciprocating engine is able to spin the propeller at max RPM, how does a higher horsepower engine achieve higher airspeed given the max propeller RPM limitation?

My guess is that the pitch of prop on the turboprop could adjust to a higher angle of attack. But I get the sense I’m likely missing something.

I welcome everyone’s thoughts.

 

[Skip Miller]

You are correct that the prop shifts to a higher angle of attack. That is what a properly functioning constant speed prop does when more power is applied.

-Skip

 

[Dan Thomas]

It's not just turboprops. Many piston-engined light airplanes have constant-speed props to optimize RPM, horsepower and airspeed. Se this:

https://www.boldmethod.com/learn-to-fly/aircraft-systems/how-a-constant-speed-prop-works/

 

[BPM]

Thanks for the informative replies. I understand how constant speed props work.

I just didn’t realize that the higher AOA enabled by the more powerful engine was the sole reason for the much faster airspeeds in turboprops.

 

[Velocity173]

It’s not just higher AoA, it’s also prop area. Just like a helicopter’s blade size, a larger prop will produce more thrust. You have to tailor the HP to compensation for the increase in drag of the larger prop. Usually heavier as well, thus more HP needed.

 

[Tscottme]

Also, turboprops operate at higher altitudes in lower ambient temps so they achieve higher true airspeed. Also, turboprops produce more horsepower with lower weight than a comparable recip.

 

[Checkout_my_Six]

adding more blades....will produce more thrust.

 

[PilotRPI]

In very simple terms, drag force = a drag coefficient x velocity squared. Power = force x velocity so Power = a drag coeff x velocity cubed. So it can start to take a lot more power to go just a little bit faster on the same airframe.

 

[kgruber]

Many turboprops are "flat rated"

 

[BPM]

adding more blades....will produce more thrust.

THAT makes sense to me. I wasn’t taking into consideration that turboprop planes have many more blades than their piston powered counterparts. Thank you.

 

[mondtster]

Turboprop or piston doesn’t really matter in this case. What does matter is selecting an appropriate propeller for the application. The correct propeller will be able to absorb/use the power generated by the engine and do it at the engine speed it is rated for.

If a person wants to simplify the concept a bit, use a fixed pitch propeller as an example. A low pitch (climb) propeller may not have enough pitch in it to keep engine speed below redline in level flight with the throttle fully open. Conversely, a cruise prop with too much pitch in it may not allow the engine to run fast enough to make the desired power.

The best thing to see how horsepower relates to speed would be to take examples of the same airframe and run different engines with different power ratings on it. There should be numerous examples of this with which you could create a reference.

 

[Dan Thomas]

THAT makes sense to me. I wasn’t taking into consideration that turboprop planes have many more blades than their piston powered counterparts. Thank you.

More blades are used to absorb more power. Prop diameter is limited by ground clearance or tip speeds, so the only way to use all the power an engine can produce is to use more prop blades.

They're also smoother than two-blade props.

 

[Martin Pauly]

Also, turboprops operate at higher altitudes in lower ambient temps so they achieve higher true airspeed.

This is key. You'll find that the indicated airspeed of a TBM or PC12 at cruise isn't magnitudes higher than a piston airplane. The good true airspeed comes from the fact that turboprops can deliver good power at higher altitudes than piston engines. And it really takes a pressurized airplane to take full advantage of it.

- Martin

 

[sarangan]

Good morning POA.

I’m trying to better understand how the higher horsepower engines in turboprops translate to higher airspeed.

On the one hand it seems obvious that (for purposes of discussion) doubling horsepower would yield higher airspeed. However, propeller speeds are limited (normally ~2,700 rpm’s).

Which leads to my question, if a 300 hp reciprocating engine is able to spin the propeller at max RPM, how does a higher horsepower engine achieve higher airspeed given the max propeller RPM limitation?

My guess is that the pitch of prop on the turboprop could adjust to a higher angle of attack. But I get the sense I’m likely missing something.

I welcome everyone’s thoughts.

Doubling the power will not give you double the speed. If everything were perfect (ie perfectly matched propeller), doubling the power will get you 26% more speed at the same altitude.

 

[Rockymountain]

Doubling the power will not give you double the speed. If everything were perfect (ie perfectly matched propeller), doubling the power will get you 26% more speed at the same altitude.

pretty interesting math. A similar airframe (PA46 variant) the Mirage/M350 and the Meridian/M500 is a pretty good experiment there.

M350 at normal cruise 75% power (263 HP) at 25,000 feet will give you about 210 knots true. The M500 at high speed cruise power 550 ESHP will give about 265 knots true. That is pretty darn close to 26%. The M500 is heavier, but interesting, how close that is.

 

[RyanB]

This is key. You'll find that the indicated airspeed of a TBM or PC12 at cruise isn't magnitudes higher than a piston airplane. The good true airspeed comes from the fact that turboprops can deliver good power at higher altitudes than piston engines. And it really takes a pressurized airplane to take full advantage of it.

- Martin

That, and those airframes are considerably more slick than most of the piston counterparts. Put the two together and you’ve got a faster cruising airplane.

 

[mandm]

This brings the question about true airspeed calculations. At lower altitudes your IAS = TAS but when you go up, do you add 2% per thousand feet?

Sea level 100kts IAS & true
10,000’ 90kts X 1.20 = 108kts true?
1.2 coming from 10,000/10 = 10 x 2% = 20% higher?
So even though your airspeed indicator shows a slower speed, probably 15-20% savings (or more) on fuel flow, your true airspeed is actually higher?

Performance is lower at higher altitudes for NA, so you have lower manifold pressure, thus lower IAS, but IAS is calculated for sea level so increase it for altitude? I always notice that IAS is pretty low over 10,000’ so you either get the advantage of tailwind or not which are typically much more drastic at altitude, but sometimes ground speed would be awfully slow esp with a headwind.

Another question, at sea level or close to it, let’s say 2000’, does it make a difference if you are running max RPM or not? Seems if I’m at 24”/2400, I might get 125-130kts but if I increase to 26”/2400/2500 (there are listed MP/RPM combos in the Lycoming engine manual that do not follow the “rule” of MP must be lower than RPM), the speed increases to 135kts, but the fuel consumption also goes from about 9.5gph to 12gph according to the fuel flow gauge.

And another question, what are thoughts on leaning mixtures at lower altitudes keeping in mind engine temps?

 

[Pinecone]

Performance is lower at higher altitudes for NA, so you have lower manifold pressure, thus lower IAS, but IAS is calculated for sea level so increase it for altitude? I always notice that IAS is pretty low over 10,000’ so you either get the advantage of tailwind or not which are typically much more drastic at altitude, but sometimes ground speed would be awfully slow esp with a headwind.

Another question, at sea level or close to it, let’s say 2000’, does it make a difference if you are running max RPM or not? Seems if I’m at 24”/2400, I might get 125-130kts but if I increase to 26”/2400/2500 (there are listed MP/RPM combos in the Lycoming engine manual that do not follow the “rule” of MP must be lower than RPM), the speed increases to 135kts, but the fuel consumption also goes from about 9.5gph to 12gph according to the fuel flow gauge.

And another question, what are thoughts on leaning mixtures at lower altitudes keeping in mind engine temps?

But remember, you do not cruise at 100% power. So even though max MP goes down, it will still be above your normal cruise MP. Up to about 8000 feet.

If you run the same RPM, but a higher MP, you are running at a higher power. To compare apples to apples you need to use two power settings at the same % power.

Once you set cruise power, you should lean at all altitudes. Full rich is wasting fuel for nothing. If you want max cruise speed, you should lean to Max Power, bur realizing that this is not good for your engine over 65 - 75% power. Below 65% you can't do damage from leaning.

 

[Dan Thomas]

This brings the question about true airspeed calculations. At lower altitudes your IAS = TAS but when you go up, do you add 2% per thousand feet?

Sea level 100kts IAS & true
10,000’ 90kts X 1.20 = 108kts true?
1.2 coming from 10,000/10 = 10 x 2% = 20% higher?

Temperature figures into it, too. Cold air is more dense and hits that pitot head harder, driving the indication up.

 

[dmspilot]

This brings the question about true airspeed calculations. At lower altitudes your IAS = TAS but when you go up, do you add 2% per thousand feet?

Sea level 100kts IAS & true
10,000’ 90kts X 1.20 = 108kts true?
1.2 coming from 10,000/10 = 10 x 2% = 20% higher?
So even though your airspeed indicator shows a slower speed, probably 15-20% savings (or more) on fuel flow, your true airspeed is actually higher?

CAS=TAS at sea level under standard conditions. I would suggest using a TAS calculator instead of the 2% guesstimate method. For example https://e6bx.com/tas/ calculates 90 CAS as 105 TAS at 10,000' and standard temperature.

 

[Checkout_my_Six]

but that thick dense air stays low.....as we climb, regardless of temp....it gets thinner or less dense.

Temperature figures into it, too. Cold air is more dense and hits that pitot head harder, driving the indication up.

 

[Dan Thomas]

but that thick dense air stays low.....as we climb, regardless of temp....it gets thinner or less dense.

The air at 10,000 feet can be cold or warm. It doesn't stay at the same temperature all the time, and so we need temperature as well as altitude to calculate true airspeed.

Look at any whiz wheel. You need pressure altitude and temperature to calculate TAS. Or to get true altitude, for that matter.

 

[mandm]

I don’t understand “lean for max power” and I do recall some POH saying not to lean above 75/85% power, why is this?

Another question is at higher density altitudes, you are taught to lean for max performance while on the ground, which I suspect means lean for takeoff too? But then again I did take some flight lessons at higher altitude airports (7000’-9000’) and I don’t remember leaning for takeoff being a thing. (But you do need the mixture set properly so the engine isn’t coughing prior to takeoff).

Typically in anything with 180+ HP, after starting the engine, bring back the mixture while on the ground, is what I’ve been taught. Full rich for take-off.

 

[mandm]

But remember, you do not cruise at 100% power. So even though max MP goes down, it will still be above your normal cruise MP. Up to about 8000 feet.

If you run the same RPM, but a higher MP, you are running at a higher power. To compare apples to apples you need to use two power settings at the same % power.

Once you set cruise power, you should lean at all altitudes. Full rich is wasting fuel for nothing. If you want max cruise speed, you should lean to Max Power, bur realizing that this is not good for your engine over 65 - 75% power. Below 65% you can't do damage from leaning.

I’ve been taught not to lean (aka keep full rich) when under 5000’ (minus the C172 lean 4 quarter turns after starting which is kind of nothing). In reality (on my Arrow), I lean the mixture slightly keeping in mind fuel flow, oil pressure, temps and EGTs. I know this isn’t an engine monitor but after watching them for awhile you have an idea where it should be at.

The fuel flow on the RPM gauge also equates to a % power. Basically 75% equals 10-12gph.

 

[Dana]

But then again I did take some flight lessons at higher altitude airports (7000’-9000’) and I don’t remember leaning for takeoff being a thing. (But you do need the mixture set properly so the engine isn’t coughing prior to takeoff).

That IS leaning.

 

[Checkout_my_Six]

Oh....I was thinking they were leaning forward while taking off into the climb....

That IS leaning.

 

[Dan Thomas]

I don’t understand “lean for max power” and I do recall some POH saying not to lean above 75/85% power, why is this?

The risk of detonation goes up at higher power settings. Leaning increases that risk, so the POH will tell you not to lean above 75% or whatever.

If you look in your POH, at the cruise charts, you will see that at higher elevations you won't get anywhere near 75% power even at full throttle, in a normally-aspirated engine (no turbo). For instance, the 172N POH cruise chart:



Look at the 6000' level, and at the 20°C above standard temp columns, and see that 2600 RPM is all you're going to get, and that's only 71% power. At a high airport at 6000' ASL, the temperature could easily be higher than 20°C above the standard temp for that altitude (which would be 3°C), and the power output would be even less.

 

[sarangan]

pretty interesting math. A similar airframe (PA46 variant) the Mirage/M350 and the Meridian/M500 is a pretty good experiment there.

M350 at normal cruise 75% power (263 HP) at 25,000 feet will give you about 210 knots true. The M500 at high speed cruise power 550 ESHP will give about 265 knots true. That is pretty darn close to 26%. The M500 is heavier, but interesting, how close that is.

Drag increases as the square of the speed. Power required increases as drag * speed, which makes it the third power of speed. Therefore, speed => (power)^(1/3), assuming otherwise identical conditions.

 

[Dan Thomas]

I’ve been taught not to lean (aka keep full rich) when under 5000’ (minus the C172 lean 4 quarter turns after starting which is kind of nothing).

That leaning idea is from rote learning. Under 5000' and what temperature? The density altitude is what matters, and at 5000' on a warm day the density altitude could be over 7000 feet, and you're running full rich there. That fouls plugs and costs a lot of power.

Yeah, four quarter turns won't do much, likely nothing at all, at idle. It all depends on the control rigging and the particular sensitivity of the mixture control valve in the carb. After start, while idling, you could screw that knob nearly all the way out before it starts to stumble.

 

[Clip4]

Good morning POA.

I’m trying to better understand how the higher horsepower engines in turboprops translate to higher airspeed.

On the one hand it seems obvious that (for purposes of discussion) doubling horsepower would yield higher airspeed. However, propeller speeds are limited (normally ~2,700 rpm’s).

Which leads to my question, if a 300 hp reciprocating engine is able to spin the propeller at max RPM, how does a higher horsepower engine achieve higher airspeed given the max propeller RPM limitation?

My guess is that the pitch of prop on the turboprop could adjust to a higher angle of attack. But I get the sense I’m likely missing something.

I welcome everyone’s thoughts.

Common dude, everyone knows the square root of the increased $$$$ = coefficient of speed.

 

[Pinecone]

Those are old school ways of leaning. And were fine in the days of $1 per gallon AVGAR and $10,000 overhauls.

There is a danger area called the Red Box or Fed Fin where for that power setting and fuel flow you have dangerous cylinder pressures and temperatures. The Red Box/Fin goes away at below 65% tp 75% power. At lower power settings you cannot do any damage by leaning. At higher power settings, you want to be rich or lean enough to be out of the Red Box/Fin. Operating in the Red Box/Fin area can cause damage from high cylinder head temps or high cylinder pressures.

There is a lot written about Rich of Peak (ROP) and Lean of Peak (LOP) operation. I suggest looking at back Pelican's Perch Article on AVWEB for some very good information. Also, articles on Saavy Aviation by Mike Busch.

 

[mandm]

That leaning idea is from rote learning. Under 5000' and what temperature? The density altitude is what matters, and at 5000' on a warm day the density altitude could be over 7000 feet, and you're running full rich there. That fouls plugs and costs a lot of power.

Yeah, four quarter turns won't do much, likely nothing at all, at idle. It all depends on the control rigging and the particular sensitivity of the mixture control valve in the carb. After start, while idling, you could screw that knob nearly all the way out before it starts to stumble.

Airport altitudes in the Chicago area are usually 600-900’ MSL, with density altitude on a hot day it might hit 1700-1800’. If cruising around or practicing maneuvers at MSL 2500-3000’, density altitude would be between 3000-4000’. Typically after takeoff and reaching a safe altitude I bring back the power and mixture slightly, seems there is a pretty big range on the throttle and mixture that appear to do nothing. So I bring it back until you start to notice a difference in the instruments if that makes sense. You can run full throttle and rich mixture burning easily 12gph, pulling it back slightly airspeed is similar +/- 5kts, but fuel flow can be drastically difference, from 12gph dropping to 9.5gph and at altitude 6.5-7gph. I guess if fuel flow is 12gph that is 75% and leaning is safe to do according to the POH.

Oil temps usually increase when not running full rich, from approx 180F to 200F or so. Well below the red line which I think is mid 200s. EGTs increase 150-250F. Have watched many of the leaning videos online and just haven’t been comfortable to keep bringing back the mixture, once it seems any reduction in performance I richen the mixture a bit and leave it.

Another question, would you know/hear if detonation occurred?

What MP/RPM settings would you use for maneuvers? I know to reduce speed to Va, but usually I would increase RPM as that gives the plane more performance, how about the mixture?

In smooth air, do you need to slow down to Va to make gentle 10-30degrees bank turns?

 

[Dan Thomas]

...but fuel flow can be drastically difference, from 12gph dropping to 9.5gph and at altitude 6.5-7gph. I guess if fuel flow is 12gph that is 75% and leaning is safe to do according to the POH.

Another question, would you know/hear if detonation occurred?

In smooth air, do you need to slow down to Va to make gentle 10-30degrees bank turns?

Don't go by fuel flow for percent power. Use the POH's numbers. Leaning it when you're above 80%, say, could get the fuel flows down to what looks like 75% or 65%, but that didn't start from a 75% or 65% standpoint, and you could damage the engine.

You won't hear detonation until the engine blows up. In cars (old cars) you could hear the engine pinging or knocking if you lugged it (too much throttle in too high a gear at low speed). The propeller noise drowns that out. Using the POH numbers will keep you safe from detonation. Detonation is more likely in higher-compression engines with constant speed props, or in turboed engines.

Va (or below) is the speed where the wing will stall before you can overstress it. It's used in serious turbulence, for example. Gentle turns don't come anywhere near the airplane's load limit. You know what breaks airplanes? Flying VFR into cloud and losing control and ending up spiralling out of the cloud at high speed and a steep bank, and the sight of the ground coming at you causes you to pull back. Way over Va, and imposing far too much load on the airframe. Sometimes it's the horizontal stabilizer that fails first, and the airplane then suddeny goes over onto its back and the wings fail downward.

 

[Pinecone]

There is the thing called a POH. In it there are power tables. They tell you what RPM or RPM and MP to set for certain % power.

For airwork, 65% or less is fine. And you can lean to your heart's content.

You will eventually hear detonation, but by then you have already destroyed a cylinder. Just had this happen to a Cessna 182 that I fly at times. Took off, about 5 minutes out, the engine got really rough. Nothing helped. Came back, landed and one cylinder was gone.