Manual pitch proppeler barely influence flight

[JG13Wulf]

Hello,

testing on the 109 and the 190 A8 that both have complete manual setting of their proppeler angles, I noted that in level flight from 2200 to 2800, the speed of the plane barrely get modified.

From 2500 RPM to 2200 RPM setting (going high pitch), I don't notice real speed modification.
From 2500 RPM to 2800 RPM setting (going to fine pitch) it's the same, the speed isn't modified.

As I was in level flight with steady speed,

1) going to high pitch should have reduce a bit the traction force and then make my speed reduce slowly.
2) going to fine pitch should have make the propeller more "dragging than pushing", making the plane speed reduce quite obviously.

 

I take 2500 RPM as a reference value. I considered it was the best RPM. Getting lower RPM would make 1) and getting higher RPM would make 2).
But it's simple to understand that even if 2500 isn't the good reference value, we should see at least a speed difference beteween High and Fine pitch.

The only thing I noted during few more test was that the blade pitch setting seems to have some effrect on the acceleration and deceleration speed.
Mean that if I take a slow plane and try to reach high speed with high of fine pitch, I will reach the high speed but time to reach it is different.

To increase / decrease speed, then the prop pitch seems useless. Only throttle control seems to have influence.

I didn't tried on Spitfire and P51 as they use automatic pitch control based on manual RPM setting.

I didn't tried yet with the P47.

RPM A8.trk RPM 109 2.trk

[Yo-Yo]

Manual propeller pitch is not a magic wand - using prop rpm setting you let the prop governor to do absolutely same job you did manually, so any plane with automatic rpm control is absolutely equivalent to the plane with manual pitch control.
The key factor is not pitch by itself but the rpm where engine power is in equilibrium with the power prop absorb at any certain pitch.
For the most of engines maximal power for the certain MP is obtained at max rpm, so reducing rpm (does not matter using automatic rpm governor or directly increasing pitch) you just reduce engine power.  Approximately, the power is proportional to the air mass flow, thus - rpm, so reducing rpm from 2700 to  2500 (as I checked carefully)  you will have only 2.5% change in maximal speed, as it is proportional to (power)^(1/3).

I can not imagine what amount of speed changes did you expect, though... 

[grafspee]

OP said that there was no speed change at all, it should be At 500kph 2.5% gives 12.5kph difference it should be noticeable then

[Yo-Yo]

41 minutes ago, grafspee said:

OP said that there was no speed change at all, it should be At 500kph 2.5% gives 12.5kph difference it should be noticeable then

It requires careful measuring.

 

[sam777777]

Manual RPM does influence climb rate and acceleration though. Particularly in the 109K4 without MW50, running manual @ 2600RPM vs Auto gives a not insignificant advantage at low speeds and climbing. I can beat a Spit mk IX in low speed flat scissors with manual pitch, bursting up to 2700/2800RPM momentarily is fine.

Instantaneous accel can be higher too, leaving the pitch on auto means cutting the throttle and then throttling up has significant lag time (109 goes very coarse when low throttle). With manual you can cut throttle (giving that braking effect of lower throttle, high-ish RPM) then throttle back up very quickly. Though you risk breaking things.

Edited June 23, 2021 by sam777777
RPMs

[NineLine]

5 hours ago, Yo-Yo said:

It requires careful measuring.

 

Yeah I had trouble myself, I'll chat with him tomorrow and see if he can give me some tips on measuring it, thanks all.

[grafspee]

In K-4 rpm handle is basically linked with throttle, if you retard throttle to minimum you get minimum rpm in P-47 you can link throttle with rpm lever so behavior of throttle movement will be exact the same as in K-4.

So w/o mw50 you cant use full throttle, but once you get above 30k you can open up to 2800rpm it give significant power increase at high alt even w/o mw50

Edited June 24, 2021 by grafspee

[Yo-Yo]

6 hours ago, sam777777 said:

Manual RPM does influence climb rate and acceleration though. Particularly in the 109K4 without MW50, running manual @ 2600RPM vs Auto gives a not insignificant advantage at low speeds and climbing. I can beat a Spit mk IX in low speed flat scissors with manual pitch, bursting up to 2700/2800RPM momentarily is fine.

Instantaneous accel can be higher too, leaving the pitch on auto means cutting the throttle and then throttling up has significant lag time (109 goes very coarse when low throttle). With manual you can cut throttle (giving that braking effect of lower throttle, high-ish RPM) then throttle back up very quickly. Though you risk breaking things.

 

I do not think it is an objective fact. The engine at 2600 rpm and the same MP gives you, roughly, 26/28 of its power or about 7% less. Prop efficiency changes are negligable, so can not overcome power loss. 
All directions for all warbirds stated, that MAX POWER, so max acceleration, max climb are achieved at maximal permitted rpm and boost.

 

[grafspee]

I use different method in K-4. I use auto prop but when i about to cut throttle i flip in to manual mode, then i flip to auto mode when throttling up. This way I avoid that lag. It is cheating, this way is impossible in RL plane  unless you have third hand

Edited June 24, 2021 by grafspee

[sam777777]

4 hours ago, Yo-Yo said:

I do not think it is an objective fact. The engine at 2600 rpm and the same MP gives you, roughly, 26/28 of its power or about 7% less. Prop efficiency changes are negligable, so can not overcome power loss. 
All directions for all warbirds stated, that MAX POWER, so max acceleration, max climb are achieved at maximal permitted rpm and boost.

 

 

Cheers for the reply! I'm talking about without MW50. Under those conditions you can't go full throttle so the RPM governor is never goes above ~2400RPM. Oil temps are always low even at "full" throttle.

 

 

[grafspee]

28 minutes ago, sam777777 said:

 

Cheers for the reply! I'm talking about without MW50. Under those conditions you can't go full throttle so the RPM governor is never goes above ~2400RPM. Oil temps are always low even at "full" throttle.

 

 

2600 is max rpm w/o mw50

2800 is max rpm w mw50 or above crit alt.

Edited June 24, 2021 by grafspee

[Yo-Yo]

21 minutes ago, sam777777 said:

 

Cheers for the reply! I'm talking about without MW50. Under those conditions you can't go full throttle so the RPM governor is never goes above ~2400RPM. Oil temps are always low even at "full" throttle.

 

 

Sorry, missed this fact... too hot here  +32C. Of course, you can set higher rpm preserving max allowed MP. By the way, there is another case you need to use manual pitch: high altitude cruising, you need to increase rpm to get enough MP.

[JG13Wulf]

I agree max acceleration is achieved with maximum RPM. That's why we have fine pitch to take off and climb. But reaching max speed is not a question of acceleration. With a low acceleration, you can reach a high speed. It would just be non effective and take lot of time. But it's not about this I made this report.

I think that the proppeller actually are not so bad for climb and acceleration in DCS (I won't say perfect because I've no idea about how each prop should react).

But there is actually a difference on the acceleration and climb ability of the plane if you select different RPM.

 

The problem to me is that a fixed pitch proppeller should be best effective for a particular speed.

When under this speed, the plane should be able to reach it. If the pitch is a bit too high (let's not go to extreme), the plane should reach the max speed but it would accelerate to it slower (as said before). => This seems to be quite ok in DCS.

But when above this speed, the proppeller should start to be less effective and then to slow the plane. 

 

 

This is why historical fixed pitch prop race plane were seaplane. They used high pitch proppeller to be able to go as fast as possible. But they needed long runway because they won't accelerate as fast as a low pitch proppeller plane.

 

The thing here I'm looking for is that when at stable high speed with a manual german proppeller (= fixed pitch proppeller) and change the pitch, the plane don't seem to have any modification. When going from coarse to fine pitch manually, RPM increase but the speed of the plane is not modified.

 

I don't see anything that tell that a plane with fine pitch can't reach high speed. The only thing seems to have influence on the plane max speed is throttle.

Edited June 25, 2021 by JG13Wulf

[Yo-Yo]

2 hours ago, JG13Wulf said:

I agree max acceleration is achieved with maximum RPM. That's why we have fine pitch to take off and climb. But reaching max speed is not a question of acceleration. With a low acceleration, you can reach a high speed. It would just be non effective and take lot of time. But it's not about this I made this report.

I think that the proppeller actually are not so bad for climb and acceleration in DCS (I won't say perfect because I've no idea about how each prop should react).

But there is actually a difference on the acceleration and climb ability of the plane if you select different RPM.

 

The problem to me is that a fixed pitch proppeller should be best effective for a particular speed.

When under this speed, the plane should be able to reach it. If the pitch is a bit too high (let's not go to extreme), the plane should reach the max speed but it would accelerate to it slower (as said before). => This seems to be quite ok in DCS.

But when above this speed, the proppeller should start to be less effective and then to slow the plane. 

 

 

This is why historical fixed pitch prop race plane were seaplane. They used high pitch proppeller to be able to go as fast as possible. But they needed long runway because they won't accelerate as fast as a low pitch proppeller plane.

 

The thing here I'm looking for is that when at stable high speed with a manual german proppeller (= fixed pitch proppeller) and change the pitch, the plane don't seem to have any modification. When going from coarse to fine pitch manually, RPM increase but the speed of the plane is not modified.

 

I don't see anything that tell that a plane with fine pitch can't reach high speed. The only thing seems to have influence on the plane max speed is throttle.

 

You mix PITCH and RPM. The curves you posted actually about PITCH. Fixed pitch. As the plane gains speed the propeller pitch INCREASES to keep the absorbing power constant. So, the prop efficiency stays near its maximum level. This is a result of correct prop design. For sure, lowering rpm, thus, pitch at the certain speed in some cases you can add for example, 0.5% to the efficiency (because the efficiency as any function changes very slightly near its extremum), but the amount of engine power losses will be way higher.
Generally, in the only one case (high speed, high altitude, high Mach number) rpm reducing can increase efficiency a bit more, reducing tip losses at transsonic region. But there are a lot of adversing factors like MP reducing, thus, the FTH lowers, drag rises...

 

[grafspee]

Take note that changing engine rpm from 2600 to 2400 for example require very small change in blade angle, probably barely visible from outside. This small change will not change prop efficiency to make huge difference in top speed.

Only reason why constant speed units were developed is to allow engine to run in safe rpm range while maintaining high efficiency and power.

[JG13Wulf]

10 minutes ago, Yo-Yo said:

You mix PITCH and RPM. The curves you posted actually about PITCH. Fixed pitch. As the plane gains speed the propeller pitch INCREASES to keep the absorbing power constant. So, the prop efficiency stays near its maximum level. This is a result of correct prop design. For sure, lowering rpm, thus, pitch at the certain speed in some cases you can add for example, 0.5% to the efficiency (because the efficiency as any function changes very slightly near its extremum), but the amount of engine power losses will be way higher.
Generally, in the only one case (high speed, high altitude, high Mach number) rpm reducing can increase efficiency a bit more, reducing tip losses at transsonic region. But there are a lot of adversing factors like MP reducing, thus, the FTH lowers, drag rises...

 

 

I agree the pitch increase automatically when you gain speed to keep RPM (power) constant.
With constant speed proppeller like allies or german automatic system, the proppeller is set automatically to keep RPM constant.

 

I don't point out a problem near extremum when the curb is high. I 100% with you that slight modification will not make lot of speed modification. I'm discussing about large modification, in particular when you try to go over the extremum.

[grafspee]

1 hour ago, JG13Wulf said:

. I'm discussing about large modification, in particular when you try to go over the extremum.

You cant test it in dcs. Try set full fine pitch in k4 at 500kph.

Edited June 25, 2021 by grafspee

[JG13Wulf]

10 minutes ago, grafspee said:

You cant test it in dcs. Try set full fine pitch in k4 at 500kph.

 

YEs, I mean the max fine pitch you can put without destroying the engine (so max 2800 RPM)

[grafspee]

In order to fully test it, we would need variable gearbox for prop, so our engine can stay at constant power while our prop's rpm can go indefinitely high.

Racing seaplanes take off run was very long when equipped fixed racing prop, not only due to low prop efficiency at low speed, but also because engine could not rev up to max rpm as well pilot could not advance throttle too much because of detonations limiting factor.

I did numerous test in P-47 to determine how much take off performance deteriorate at high pitch settings, i testes pitch for continuous power level flight at about 300mph cant remember exact value. I set this pitch manually before take off in P-47. At begging engine rpm was so low that i could not draw more then 35" and doing something like 2000rpm and 35' i accelerated extremely slow, barely managing to lift off.

Edited June 25, 2021 by grafspee