View Full Version : prop pitch

08-01-2008, 11:38 AM
Do any of you guys think that messing with prop pitch makes a difference? I was doing some tests, and I am not seeing any difference when manifold pressure is the same and you vary the prop pitch. I would think that if the nose is pointing down, or level and you reduce prop pitch to zero, that acceleration should go up due to reduced drag of the prop.

I am beginning to think that it is better to leave prop pitch at 100%....

08-01-2008, 12:59 PM
I don't notice much change in speed, I think that the real benefit of prop pitch is the ability to run your engine cooler.

i.e. if I am in a turning dog fight at 110% throttle and WEP enabled and start to over heat, if I find I have a few seconds to spare, I can throttle back, open the rads, prop pitch down to 70% and the engine is back to normal in only a few seconds. I find it is much quicker to cool the engine this way than just flying with the rads open. Keeping in mind you don't want to do that with a bogie right on your six...

08-01-2008, 01:15 PM
Good tip! http://forums.ubi.com/images/smilies/25.gif I guess its not modelled that prop blades produce lots of drag....

08-01-2008, 01:18 PM
On multi-engined AC that support prop feathering you can indeed reduce the drag of a dead engine, but I don't think it's modeled in enough detail for prop pitch to make a difference, just feathering.

08-01-2008, 01:27 PM
Mike, it makes a difference but what type of prop system the plane has, how much power the plane
has and what speed you are running at all make a difference.

With most Allied planes it is Constant Speed Prop like on P-47's for example. With those and
most like German automatics you are adjusting prop rpm.

Try running at 70% power instead of full out and see what rpm lets you cruise fastest, it will
be less than 100%. If I cut power and rpm is high, that is how I slow the plane down from high
speed and control my speed on approach to landing.

In a dive at full power I get more speed if I lower the rpm as I go past level max speed.
IRL the prop tips travel as fast as the plane moves forward plus the radial motion, at some
speed the prop tips may go transonic or even supersonic which may be modeled as extra drag.

There is the thing about less rpm means less engine heat as you noted.

Stronger in 4.01 and 4.02 than now but still some effect is gyroscopic effects of the prop.
When aiming and correcting the nose a bit to the side there was/is (not all planes I think)
some up/down nose movement. Lowering rpm did lessen that. If you fly something with that
problem, try dropping pitch to 70-80% before firing in practice and see if it helps.

Try getting a P-47 up to 500kph then lower pitch to 30% then lower power to 30% and see how
you slow down. Run it back up to 500kph, pitch at 100% and lower power to 30% and see how
you slow down. IRL that would -maybe- damage the prop/engine but in game, check it out.
I find that low power and 100% rpm wants to run at a certain speed depending on settings,
it makes a good speed control in that sense.

So yeah, CSP pitch makes difference. It makes difference on the other types too.

08-01-2008, 05:27 PM
Except for the transonic blade tips, (anyone know if they're modelled??) a particular engine will always make LESS power when at less than full manifold pressure and RPM. Reducing RPM slightly in a dive makes sense, as the prop pitch governor often doesn't quite keep up with the speed.

If you're getting more LEVEL, steady state speed by reducing either power or pitch, I'd like to see a track of it!

Mike_espo, regarding the "drag of the prop" - it's the thing that is pulling the plane forward by pushing air backward, and as such has effectively negative drag (along the direction of the plane's motion). You need your engine to be at it's peak power RPM so it can deliver peak power, so regulate your pitch to keep it so. Normally this will mean 100%.

08-01-2008, 10:17 PM
In a high speed dive the prop does in game and IRL become draggy, it's up to you to make it
less so or come post about the plane not being fast enough in steep dives.

In a dive that takes the plane beyond level max speed... just run at 60% rpm and 800+ kph and
then push it up to 100% rpm. See which will let you augur in faster.

If the power won't support whatever speed you are at then the prop becomes a brake.
Prop thrust decreases with speed while parasitic drag increases at the same time.
At less than 100% power you will go faster at less than 100% rpm too.
When you don't have the power for the speed you're running then less rpm will help, try it.

08-02-2008, 09:31 AM
Interesting. What I don't understand, is that if your engine is at full power, you're getting an extra few thousand HP helping you along - where does it all go?

Is it simply that the prop becomes *that much* less efficient at high speed? Remember that the CSP will have the prop set to a very co**** pitch anyway in a high speed dive. I wouldn't have thought that prop efficiency would drop so far.

From an engineering perspective, acceleration is found from the sum of all forces, so any forward thrust from the engine/prop should only help.

I'll try it in game, as you suggest.

08-02-2008, 01:31 PM
Crumpp and others have posted the formula here, and may again.
For a given power you get most thrust at the least speed and thrust decreases as speed grows.
The ideal graph is a line with high end at zero speed going down to zero at some speed X.
That's without considering drag which when added turns a line graph to a curve.

Parasitic drag increases as the square of speed though as you get transonic it's the cube.
So you get decreasing return of thrust for power with more speed minus increasing by squares drag.
The drag wins. In aero texts they say the prop drag gets to be the same as a solid disk the
diameter of the prop before mach 1, with possible exception of special transonic props made
since the war.

When the engine doesn't make enough power to turn the prop at the desired speed, the blades
flatten and the air drives the prop and engine. Some of that extra HP goes to do that.

08-02-2008, 02:52 PM
Cannot remember exact details but doing a test run in a ME109E (Cimea map), I managed to get ~ 410KpH at sea level with 60% throttle and prop pitch.
This was about 10 KpH faster than 100% throttle and pitch, and I could run like this 'for ever'.

So when the need to run away is acute, this info is always nice to know.

08-03-2008, 01:22 AM
In addtion to cooling benefits, selecting lower RPM will conserve fuel on long missions.

08-03-2008, 01:43 AM
I fly for real. If I ran my airplane at 100% prop pitch all the time I would burn more fuel and my engine would be working too hard when it doesnt have to. Thats the benefit of having a controllable pitch prop.