Prop Setting For Climb

[bstratt]

Reading the post about constant speed props reminded me of a question I had but forgot to ask a while ago.

I have a normally aspirated plane with a constant speed prop. I was flying along at 10,000 ft with an rpm of 2,400 - manifold pressure with the throttle firewalled was around 55%, IIRC. I wanted to climb to 11,000ft. Would the climb have been any faster if I had increased the rpm? Decreased rpm? Left the same? or it makes no difference as I had already reached power limits?

 

[mikea]

Reading the post about constant speed props reminded me of a question I had but forgot to ask a while ago.

I have a normally aspirated plane with a constant speed prop. I was flying along at 10,000 ft with an rpm of 2,400 - manifold pressure with the throttle firewalled was around 55%, IIRC. I wanted to climb to 11,000ft. Would the climb have been any faster if I had increased the rpm? Decreased rpm? Left the same? or it makes no difference as I had already reached power limits?

Obviously, the throttle is fully open.

You might make small changes with the prop to see what happens, but I think you'll see in the POH that it doesn't make a lot of difference how you spend the HP you're getting.

What I did was tweak the mixture a bit to see where I could better performance. That's how I got mine to 15,500.

I have a JPI but I didn't see much change there in HP. You do see it in EGTs.

Early on I asked why you choose low RPM vs. higher. My CFI said just to lower the noise. I guess you can also go with "Click and Clack's" theory that an engine has a limited number of "R's" in its life, so the less of those you use per minute the longer the engine will last.

 

[Areeda]

What does your POH say?

As an example I picked up the POH for a C206 NA:

At 10,000' max MP listed is 20" and BHP goes from 63% @2500 to 52% @2100 and it's similar at altitudes where MP maxes out.

While it might not be much I believe all piston engines will produce higher horsepower at higher RPM if MP is fixed.

Joe

 

[Lance F]

I have a JPI but I didn't see much change there in HP. You do see it in EGTs.

Early on I asked why you choose low RPM vs. higher. My CFI said just to lower the noise. I guess you can also go with "Click and Clack's" theory that an engine has a limited number of "R's" in its life, so the less of those you use per minute the longer the engine will last.

I believe that every cerificated engine is tested to achieve full TBO at max rpm (although there may be a higher rpm approved for short time intervals to give more take off HP). I see no reason to limit rpm for longevity issues. Noise, vibrations and fuel consumption are other matters.

Although I am a true believer in engine monitors, the HP funtion is one I do not use. It is not accurate over a very wide range of operations and it is meaningless in LOP operations,

 

[Graueradler]

I would think that throttle wide open, pitch for redline RPM would be max power available.

 

[Dave Siciliano]

In simple terms, higher RPM to go fast; lower rpm to go far.

We did some experimentin with the Baron. Left the MP constant, raised rpm in increments of 100 at a time and found we could raise fuel flow also (LOP) and picked up a few knots each time.

2200 rpm; 35" of MP = mixture of 16.5' 2550 rpm; 35" = mixture of 18.5.

LOP, each gallon burned roughly equates to 13.7 HP on my place. 2 more GPH = 27 more ponies.

Best,

Dave

 

[CJones]

Reading the post about constant speed props reminded me of a question I had but forgot to ask a while ago.

I have a normally aspirated plane with a constant speed prop. I was flying along at 10,000 ft with an rpm of 2,400 - manifold pressure with the throttle firewalled was around 55%, IIRC. I wanted to climb to 11,000ft. Would the climb have been any faster if I had increased the rpm? Decreased rpm? Left the same? or it makes no difference as I had already reached power limits?

I would *think* that you would be able to make the climb faster if you advanced the prop to higher RPM (more HP for given MP).

In real-world, though, for 1000' I would just leave everything set and "trade speed for altitute." (don't you HATE hearing that 1,000 times at airshows!?)

 

[gismo]

Reading the post about constant speed props reminded me of a question I had but forgot to ask a while ago.

I have a normally aspirated plane with a constant speed prop. I was flying along at 10,000 ft with an rpm of 2,400 - manifold pressure with the throttle firewalled was around 55%, IIRC. I wanted to climb to 11,000ft. Would the climb have been any faster if I had increased the rpm? Decreased rpm? Left the same? or it makes no difference as I had already reached power limits?

Unless there is something seriously restricting airflow at higher RPM (not likely in a slow turning aircraft engine, but common in cars), you will always generate more useful horsepower (and thrust) at a higher RPM. Both engine and propeller efficiency go down with an RPM increase, but those losses are overwhelmed by the nearly linear increase in airflow through the engine and power output is very much proportional to that (minus increased frictional losses).

Increasing from 2400 to 2700 RPM should result in very close to a 12.5% boost in power assuming the fuel/air ratio is the same.

 

[bstratt]

In real-world, though, for 1000' I would just leave everything set and "trade speed for altitute." (don't you HATE hearing that 1,000 times at airshows!?)

That's in essence what I did although it was more of a "step" climb. Pull up, watch altitude and speed - when speed fell too much, level off and build speed again - repeat as necessary!

 

[poadeleted20]

...you will always generate more useful horsepower (and thrust) at a higher RPM.

Lance is correct -- if you want to climb out of 10K in a nonturbo'd airplane, you get the best climb rate with the prop redlined and the throttle firewalled.

 

[tonycondon]

theres a reason its prop full forward for takeoff.

 

[cwyckham]

In simple terms, higher RPM to go fast; lower rpm to go far.

We did some experimentin with the Baron. Left the MP constant, raised rpm in increments of 100 at a time and found we could raise fuel flow also (LOP) and picked up a few knots each time.

2200 rpm; 35" of MP = mixture of 16.5' 2550 rpm; 35" = mixture of 18.5.

LOP, each gallon burned roughly equates to 13.7 HP on my place. 2 more GPH = 27 more ponies.

Best,

Dave

The reason you were going faster and burning more fuel is that you were increasing the power output of the engine by increasing the rpm while keeping the mp constant. So it's not really high rpm to go fast, low rpm to go far. It's high power to fast, low power to go far. Of course, you can only achieve max power at max rpm, so if you want to go really fast, you'll also have high rpm.

Chris

 

[tonycondon]

faster RPM isnt always going to make you go faster. there is a limit based on propeller efficiency. the prop, like the wing, has a best Lift/Drag angle of attack. This is the AoA (pitch) that you want the prop set at for the best speed. it is not necessarily the fastest the prop will spin.

 

[cwyckham]

faster RPM isnt always going to make you go faster. there is a limit based on propeller efficiency. the prop, like the wing, has a best Lift/Drag angle of attack. This is the AoA (pitch) that you want the prop set at for the best speed. it is not necessarily the fastest the prop will spin.

The power that is transfered from the engine to the prop is proportional to the rpm times the torque (and the torque, I believe, is proportional to the mp, right?). Therefore, the amount of power transferred to the prop will always increase with an increase in rpm at fixed mp. However, as you say, the power transferred to the air is also dependent on the propeller efficiency.

I guess I don't really know, but I'd be surprised if in practice the efficiency dropped off that much as long as you stayed below the prop redline. Certainly, in the attached 182 table, at, say, an mp of 18 an increase in rpm gives you an increase in %bhp and therefore TAS.

So how do you figure out what the prop's best angle of attack is and how to achieve it? Is that sort of information given in the performance section under best range? I don't have the rest of the 182 POH. The Diamond DA20A1 POH talks about endurance but not range, which is just silly.

Chris

 

[jesse]

The Diamond DA20A1 POH talks about endurance but not range, which is just silly.

Range is something the POH really can't account for. I'd much rather be seeing endurance numbers than range. Range varies with winds aloft.

 

[Bill]

I wanted to climb to 11,000ft. Would the climb have been any faster if I had increased the rpm? Decreased rpm? Left the same? or it makes no difference as I had already reached power limits?

I always climb full prop, and yes, I've put it forward just to climb 1000ft, and then pulled it back again once level. That is why we have that nice blue knob, so we have a climb prop in climb and a cruise prop in cruise.

 

[cwyckham]

Range is something the POH really can't account for. I'd much rather be seeing endurance numbers than range. Range varies with winds aloft.

I don't know of any really easy way to come up with best range without the POH telling me. Luckily, I know how to acount for winds aloft, so all I need is the still air range which is absolutely something the POH can account for.

What the heck am I going to do with an endurance number? Best endurance will be achieved at a much lower airspeed than best range and is probably useful for traffic reporters and people trapped above a cloud layer hoping that the weather will improve before they run out of gas.

Best range, however, tells me how far I can go between airports which seems to be much more useful. I certainly would like to know what speed and/or power setting will give me best range in case I'm suddenly in a position where I'm far from airports and starting to worry about my fuel.

Chris

 

[jesse]

I don't know of any really easy way to come up with best range without the POH telling me. Luckily, I know how to acount for winds aloft, so all I need is the still air range which is absolutely something the POH can account for.

What the heck am I going to do with an endurance number? Best endurance will be achieved at a much lower airspeed than best range and is probably useful for traffic reporters and people trapped above a cloud layer hoping that the weather will improve before they run out of gas.

Best range, however, tells me how far I can go between airports which seems to be much more useful. I certainly would like to know what speed and/or power setting will give me best range in case I'm suddenly in a position where I'm far from airports and starting to worry about my fuel.

Chris

I may have misinterpreted what you said. Hell, I don't think I was even thinking.. I'll look in the Diamond POH after I get some food in me.

 

[tonycondon]

yea chris, i think that the basic idea is to keep the tip speeds below Mach .7. any higher than that you run into shock waves and serious drag issues. looks like, at least on the 182RG, the optimal AoA for the prop coincides with an RPM that is higher than redline. It is quite possible to get some props out there to go faster than Mach .7, so the optimal RPM would be below redline.

 

[cwyckham]

yea chris, i think that the basic idea is to keep the tip speeds below Mach .7. any higher than that you run into shock waves and serious drag issues. looks like, at least on the 182RG, the optimal AoA for the prop coincides with an RPM that is higher than redline. It is quite possible to get some props out there to go faster than Mach .7, so the optimal RPM would be below redline.

Gotcha. On a fast aircraft, you could be getting tip speed issues below redline because you're adding the aircraft velocity to the blade tip velocity (vector addition, of course) so that effectively the redline would change with TAS. Cool. Probably more of a problem on a Piagio than a 182

Chris

 

[tonycondon]

yea, or on planes with long props, like cessna 180s or stearmans, and lots of aerobatic airplanes

 

[CowboyPilot]

In the RV8 and its n/a IO360, I'll get better climb with the increased RPMs. In the turbo Cardinal, same thing but better because of the increased efficiency of the engine.

I was always taught that if I want to climb quickly with a CS prop, use all the RPMs I've got available.

Regards.

-JD

 

[Skip Miller]

It is quite possible to get some props out there to go faster than Mach .7, so the optimal RPM would be below redline.

Your tax dollars at work: here's what happens when you speed the prop way over Mach 1. the highlights are mine... -Skip

(found by googling "Thunderscreech")

A project which hoped to produce the fastest propeller-driven plane resulted in the Republic XF-84H, aka the Thunderscreech. The intent behind this project was to take a jet-propelled airframe known to be capable of supersonic flight (that is, a late-model turbojet-powered F-84) and modify it for use with a turboprop engine. This would give a high-speed plane with propeller acceleration and endurance. Note that there was never actually any intent for it to reach supersonic speeds, though that claim appears in many references.


The test flights proved that it was a fast (though it never flew over 720 kph, due to handling problems) and quick-accelerating aircraft, among the fastest prop jobs of its day. It also, however, held another record, one hinted at by its nickname. It was arguably the loudest fighter plane ever built. The howl of the 6000 shaft horsepower (and that's without the afterburner) twin-turbine engine combined with the roar of the prop made standard ear protectors irrelevant. Worse, during, run-ups the blade tips went faster than sound, reaching Mach 1.18 (it was a constant-speed prop, with thrust adjusted by changing the blade pitch). This produced multiple sonic booms which could pummel ground crew members insensible in short order. Due to prop torque the plane was a major handful to fly at high speed. And, as with any experimental plane, it had development problems. One pilot reported that 10 of his 11 flights were cut short by emergencies of varying degrees. Another made one flight and refused to get back in the cockpit. The plane - two were built, one of which survives - deserves an article all to itself, or perhaps extensive mention in an "it seemed like a good idea at the time" article. Among other claims to fame, it was the only turboprop aircraft to have an afterburner. (This was never lit in flight, probably to the benefit of the pilots.)

 

[tonycondon]

thats just plain cool skip.

BTW the new "scimitar" blade is shaped that way with transonic speeds in mind, an attempt to get higher tip speeds without loss in efficiency.

 

[Keith Lane]

Your tax dollars at work: here's what happens when you speed the prop way over Mach 1: -Skip

(found by googling "Thunderscreech")

The plane - two were built, one of which survives - deserves an article all to itself, or perhaps extensive mention in an "it seemed like a good idea at the time" article. Among other claims to fame, it was the only turboprop aircraft to have an afterburner. (This was never lit in flight, probably to the benefit of the pilots.)

I'm pretty sure one of these was a static display piece at the old Bakersfield airport (KBKF), on a pole at the passenger dropoff area, till the new terminal was constructed. I think I might even have a picture of it somewhere.

 

[flyingcheesehead]

That is why we have that nice blue knob, so we have a climb prop in climb and a cruise prop in cruise.

Keep in mind, though, that you have that whether or not you even move the blue knob. For instance, IIRC your 182 redlines at 2400 and that is also max continuous. You can leave it at 2400 for the entire flight. While you're in a climb, it will function as a climb prop. When you level off for cruise and your airspeed increases, it will increase pitch as you accelerate and when you hit cruise speed, you have a cruise prop.

Whoever invented CS props did a good thing for us pilots. Can you imagine if the blue knob controlled prop pitch rather than RPM? You'd have to adjust the prop to the right angle every time you changed airspeed or power.

 

[flyingcheesehead]

The howl of the 6000 shaft horsepower (and that's without the afterburner) twin-turbine engine combined with the roar of the prop made standard ear protectors irrelevant.

Wow! And I thought the Garretts were loud.

Just think how loud it would have been with afterburners on! (A turboprop with afterburners? Weird...)

 

[Dave Siciliano]

The reason you were going faster and burning more fuel is that you were increasing the power output of the engine by increasing the rpm while keeping the mp constant. So it's not really high rpm to go fast, low rpm to go far. It's high power to fast, low power to go far. Of course, you can only achieve max power at max rpm, so if you want to go really fast, you'll also have high rpm.

Chris

Chis:

Something is always lost when trying to explain things in simple terms. Yes, I kept MP constant and FF changed when I changed RPM. I was trying to convey things in simple terms: higher RPM gives more power; climb better, go faster.

Lower RPM gives less power, go slower, longer. Most folks here aren't trying to to max. range at 1.0 to 1.15 above max wing efficiency. So, excusa me! I was trying to be concise

Best,

Dave

 

[cwyckham]

Chis:

Something is always lost when trying to explain things in simple terms. Yes, I kept MP constant and FF changed when I changed RPM. I was trying to convey things in simple terms: higher RPM gives more power; climb better, go faster.

Lower RPM gives less power, go slower, longer. Most folks here aren't trying to to max. range at 1.0 to 1.15 above max wing efficiency. So, excusa me! I was trying to be concise

Best,

Dave

I am seldom accused of being concise. I'm sure you know more about the use of constant speed props than I do. I was just trying to be very clear that the statements you are making are only true at a fixed manifold pressure. High rpm for fast and low rpm for far is not true, in general.

For example, in the attached performance chart, you can go faster at 2100 rpm and an mp of 21 than you can at 2400 rpm and an mp of 18. Power is a combination of the mp and rpm, and power determines the tradeoff between fuel burn and speed.

Chris

 

[Dave Siciliano]

Oh, I don't know that I know it all. But changing RPM with MP pressure constant is something I do each and every flight. As a matter of fact, I can't fly full RPM and MP in my plane during the climb or something called CHTs will also climb to new regions henceforth undiscovered on the scale of my graphic engine monitor

In the case given in the original example, he was WOT (wide open throttle) and wanted to climb; so, I assumed his MP would stay the same. If he changed RPM, his FF would increase, but he might not even notice. More power for the climb--yes.

Actually, I fly LOP and keep a constant MP and RPM except for departure and arrival. I pretty much manage power by changing fuel flow.

Guess I'll have to be more careful now that I know you and Tony are gunnin for me


Best,

Dave

 

[AirBaker]

Whoever invented CS props did a good thing for us pilots. Can you imagine if the blue knob controlled prop pitch rather than RPM? You'd have to adjust the prop to the right angle every time you changed airspeed or power.

Variable pitch props are out there...

 

[flyingcheesehead]

Variable pitch props are out there...

Yes, they are, but thankfully they're a lot more rare than the constant-speed variety.

 

[Bill]

Whoever invented CS props did a good thing for us pilots. Can you imagine if the blue knob controlled prop pitch rather than RPM? You'd have to adjust the prop to the right angle every time you changed airspeed or power.

And a real godsend for those who do aerobatics. In some ways, Chip's Extra is easier power management wise, just shove all knobs forward, and the govenor gives max power while protecting the engine from overspeed (like the downhill side of a loop).

In a Citabria, you have to be on the throttle and mindful of not overspeeding the engine.