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Prop pitch at high altitude?

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Duncannon

Active Member
If a prop is pitched for a certain airspeed, will it perform or react the same at high altitudes as it would at sea level? For example, if an RV (particularly an RV 9), has a prop that’s pitched so that it gets close to top end at 140 kts ias, that may be considered under pitched at lower altitudes compared to a prop that could fly over 160kts. But if you climb to 15k feet, 140kts ias would likely be at least 180 kts true airspeed (people on here have stated that 13—17k feet is the sweet spot for 9’s), so that wouldn’t seem underpitched in that circumstance. Never having flown an RV (or any EAB for that matter), I don’t know what the typical indicated airspeeds are at higher altitudes.

If you planned on flying at much higher altitude, could you be better off pitching the prop a bit lower than someone who plans to slice through the air at 160-170kts below 5k feet? This is assuming the aircraft is equipped with a fixed-pitch prop.
 
Are you running a 180 HP? Or a 320? My buddy has a -9a with a CS prop O320 160 hp. It seems to run out if power much above 10 to 12k.. it cruises at 158 knots.
 
Duncannon said:
If you planned on flying at much higher altitude, could you be better off pitching the prop a bit lower than someone who plans to slice through the air at 160-170kts below 5k feet? This is assuming the aircraft is equipped with a fixed-pitch prop.
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In simple terms, yes.

The way to get additional power at higher altitudes with a fixed pitch prop. is to have the ability to turn the engine faster than would be possible if the airplane was equipped with a prop pitched to use the majority of the available power down low.

So technically a propeller often referenced as being a climb prop., is also a higher altitude cruise prop.

This is a very generalized statement because that prop. will be different for someone based at an airport with a ground elevation of 150 MSL vs one based at an airport at 5000 MSL.

This has been discussed quite a bit here in the forums over the years and emphasized by experienced RV builders / owners.
The early versions of the build manuals (back in the days when RV's were almost universally using custom made fixed pitch wood propellers) there was a section on choosing propellers that Van had written. It went into quite a bit of detail explaining this particular subject.
 
rvbuilder2002 said:
In simple terms, yes.

The way to get additional power at higher altitudes with a fixed pitch prop. is to have the ability to turn the engine faster than would be possible if the airplane was equipped with a prop pitched to use the majority of

So technically a propeller often referenced as being a climb prop., is also a higher altitude cruise prop.
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This is basically what I was wanting to know. Admittedly I’ve not read much of the threads regarding propellers. I plan to go and look them up.
 
Duncannon said:
This is basically what I was wanting to know. Admittedly I’ve not read much of the threads regarding propellers. I plan to go and look them up.
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There is a lot of good discussion (and misunderstanding ;) )
One of the common themes you will find from people that Know, is that to discuss what is happening with a particular propeller, numerous details of the operating environment need to me known.
Saying "I had such an such of an indicated airspeed at such an such RPM" doesn't tell much, because there is no way to know what amount of power was being produced (that particular RPM may have been attained at wide open throttle or with the throttle pulled back 1")
This is just one of the reasons that a manifold pressure gauge is valuable even on an RV equipped with a fixed pitch propeller.
 
I found, when running an 8 with fixed pitch, that the true airspeed follows RPM very closely but not the indicated. For example, I usually cruised around 2650 rpm and that gave about 165 knots regardless of altitude. Took more power to spin it that speed down low and the indicated airspeed was higher, but true speed was pretty close.
 
oaklandaviator said:
I found, when running an 8 with fixed pitch, that the true airspeed follows RPM very closely but not the indicated. For example, I usually cruised around 2650 rpm and that gave about 165 knots regardless of altitude. Took more power to spin it that speed down low and the indicated airspeed was higher, but true speed was pretty close.
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I agree.. that seems to be the case with my -4.. makes sense really. A fixed pitch prop is like a threaded fastener, but with a little slippage (prop efficiency?)
 
Duncannon said:
I would assume an o320.
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I know Vans only recommends the 320 for the -9, but the OP was talking about 180 KTAS and 13k to 18k.. I don’t think the 320 is going to do either of those.. put me in the 360 fan club for the RV-9.. it’ll get you higher, faster, and won’t run out of steam up there either. You can then throttle back to 320 fuel savings.. I’m convinced that a 360 -9 would burn less fuel on a long cross country flight (2 or more hours) than a 320 because you would get to cruise altitude quicker, and be able to match fuel flow once there.
 
rvbuilder2002 said:
So technically a propeller often referenced as being a climb prop., is also a higher altitude cruise prop.
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I'm skeptical of this. I would think that a propeller's pitch would always be best at certain combinations of RPM and true airspeed. A climb prop has a fine pitch, so it should not be very efficient at high true airspeeds and not-super-high RPMs, regardless of whether those high true airspeeds are at high or low altitudes. At a given high speed (and, let's say, high altitude, but it shouldn't matter), a climb prop will need more RPM (and more GPH, all else being equal) than a cruise prop.

But I could be missing something; My intuition comes from limited experience, and I realize that propeller performance is impacted by all kinds of factors in nonlinear ways. I would be very curious to see that guidance in old kit instructions about choosing propellers.

oaklandaviator said:
I found ... that the true airspeed follows RPM very closely but not the indicated. For example, I usually cruised around 2650 rpm and that gave about 165 knots regardless of altitude. Took more power to spin it that speed down low and the indicated airspeed was higher, but true speed was pretty close.
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Yeah, that's pretty much what I would have guessed.
 
AirShowFan said:
I'm skeptical of this. I would think that a propeller's pitch would always be best at certain combinations of RPM and true airspeed. A climb prop has a fine pitch, so it should not be very efficient at high true airspeeds and not-super-high RPMs, regardless of whether those high true airspeeds are at high or low altitudes. At a given high speed (and, let's say, high altitude, but it shouldn't matter), a climb prop will need more RPM (and more GPH, all else being equal) than a cruise prop.
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Engines lose power with increases in altitude.
With a fixed pitch prop that equates to a loss in RPM, at a higher rate than the less dense air reduces the loading on the prop
So, having a lower pitch allows you to spin it up at a higher RPM at higher altitudes. I’m talking 13 K to 14 K.
Spinning at a higher RPM helps get some of the lost power back just like it does with a constant speed if you choose to turn the knob and increase the RPM.
But don’t take my word for it, go find Van’s old writings on the subject ;)
 
rvbuilder2002 said:
Engines lose power with increases in altitude.
With a fixed pitch prop that equates to a loss in RPM, at a higher rate than the less dense air reduces the loading on the prop
So, having a lower pitch allows you to spin it up at a higher RPM at higher altitudes. I’m talking 13 K to 14 K.
Spinning at a higher RPM helps get some of the lost power back just like it does with a constant speed if you choose to turn the knob and increase the RPM.
But don’t take my word for it, go find Van’s old writings on the subject ;)
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I have been studying this for a while now, with the thought of re pitching my fixed pitch Catto to produce more RPM, hoping for more speed when up high. But all the data I can find, suggests changing the prop pitch increases the RPM proportionally, but only increases the true airspeed by a few knots. I was hoping an inch change in pitch would give me 5 knots more cruise, but the data doesn't seem to support this when above 12K.

For you guys with constant speed props, when you are above 12k, and change the RPM by 100, how much change do you see in True airspeed?
 
PilotjohnS said:
I have been studying this for a while now, with the thought of re pitching my fixed pitch Catto to produce more RPM, hoping for more speed when up high. But all the data I can find, suggests changing the prop pitch increases the RPM proportionally, but only increases the true airspeed by a few knots. I was hoping an inch change in pitch would give me 5 knots more cruise, but the data doesn't seem to support this when above 12K.

For you guys with constant speed props, when you are above 12k, and change the RPM by 100, how much change do you see in True airspeed?
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In case people are getting the wrong idea from my previous comments…
I never meant to imply that if you want to make your RV faster, and you have a fixed pitch propeller, select a lower pitch value.
I think everyone probably understands that a fix pitch prop is a compromise, and for any given pitch value, you will have an ideal density altitude for best cruise performance.

What I was trying to convey, and what Van wrote about in the past as well, is that a fixed pitch prop doesn’t have to be as big of a compromise between climb and cruise as many people think, if your desired cruise altitudes are up higher than a lot of people tend to fly if they live in the flatlands.
 
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