Lift due to Thrust

[hitech]

Now I understand your thinking charge,  but the effect in this topic would not really be any consideration in prop design.

So now a few basic prop principles.

On all, possibly almost all, WWII fighter had constant speeds prop. This works simply by increasing or decreasing blade angle to maintain the same RPM at almost all flying speeds until the prop is mechanically unable to increase or decrease pitch. In speeds from stalls to  well above max level speed the prop will maintain the same RPM.

2nd it is always preferred to make a longer prop then a shorter one for efficiencies sake. But mechanical limits come into play very quickly. Tip speed is normally just compensated for in the gear box of the prop. Most WWII gear ratio's were 1.5 - 2.0.  A 109G10 is 1.6 a P51D is slightly over 2.0.

In almost all cases prop length is simply determined simply by ground clearance.  After which other methods must be used to increase thrust.

At slower speeds the pitch will continue to increase until  the RPM setting is reached. This has the effect of turning the prop past stall AOA of the prop, so it is wasting lots of power in drag to keep the rpm down. As you increase speed the planes speed with lower the  AOA on the prop, and it will gain efficiency as it's AOA  starts coming out of the stall regime.

Now once you have made the prop as long as possible with out striking the ground, you need to do something to absorb the power. The choices are add more blades, or add more area to the blades. Making the blades wider changes there aspect ratio and increases induced drag just like it does on a wing.

Adding Blades also decreases efficiency because of disruptive air flow from the previous blades. These effects decrease efficiency at high speeds, but increase the efficiency in the climb speed ranges.

But none of these choices and trade offs have an effect on how much thrust is produced in the lift direction, that is simple trigonometry. Also when doing lift calcs you must also take climb angle into account for required lift. If a plane is traveling straight up or straight down zero lift is required. So the total lift required (including thrust from the prop in the lift dirction) for non turning flight is the cos(ClimbAngle) * Weight.

HiTech

[Ardy123]

On all, possibly almost all, WWII fighter had constant speeds prop. This works simply by increasing or decreasing blade angle to maintain the same RPM at almost all flying speeds until the prop is mechanically unable to increase or decrease pitch. In speeds from stalls to  well above max level speed the prop will maintain the same RPM.

HiTech,
I'm guessing this means at lower velocities, the prop must have its prop pitch at less of an angle to keep the RPMs up, thus its 'bite' is less. This would imply that at slower speeds the prop isn't pushing as much air as at higher speeds, thus there is an optimal speed at which you will get the most thrust out of your engine. Is this assumption correct?

Thanks

[Krusty]

I'm not sure. Shouldn't it be the other way around?

Lower RPM = sharper "bite" to slow down the prop with more work/friction?

Higher RPM = shallower "bite" to make less work and thus the prop spins faster with less resistance?

Again, I'm not sure.

[Ardy123]

I'm not sure. Shouldn't it be the other way around?

Lower RPM = sharper "bite" to slow down the prop with more work/friction?

Higher RPM = shallower "bite" to make less work and thus the prop spins faster with less resistance?

Again, I'm not sure.

When I meant speed, I was talking about the velocity of the airplane, because as the aircraft flys faster, the sharper the bite the prop can be while maintaining the same RPM.

[hitech]

I'm not sure. Shouldn't it be the other way around?

Lower RPM = sharper "bite" to slow down the prop with more work/friction?

Higher RPM = shallower "bite" to make less work and thus the prop spins faster with less resistance?

Again, I'm not sure.

Yes and no crusty, depends on your definition of bite.

As the plane moves faster the angle relative to the prop hub  increases, but relative to the air flow (AOA) it decreases.

Ardy see above, I am assuming byte means AOA, hence slower is more .

HiTech

[Ardy123]

Ardy see above, I am assuming byte means AOA, hence slower is more .
HiTech

'Slower' = Slower RPM I'm assuming, so when wep is hit and the prop is at max RPM, the prop is also at max AoA, hence why you can't change the RPMs with wep on?
Also, because as you increase the AoA, the prop spins slower, more 'bite' doesn't necessarily mean more thrust correct? I'm guessing that the relationship between prop speed and prop AoA in regards to thrust is non-linear. Is that correct?

[hitech]

No ardy slower = the planes speed. I was not referring to the effect of RPM settings.  Ardy you never directly control the pitch of the prop. You only set the speed of the governor.

For any given RPM setting as the plane speeds up or slows down the prop rpm stays the same.

As the plane flys faster the prop angle increases, but the prop AOA decreases.

HiTech

[Baumer]

I think this paragraph explains it clearly in relation to climbing and diving;

http://www.pilotoutlook.com/airplane_flying/constant_speed_propeller

"When an airplane is nosed up into a climb from level flight, the engine will tend to slow down. Since the governor is sensitive to small changes in engine r.p.m., it will decrease the blade angle just enough to keep the engine speed from falling off. If the airplane is nosed down into a dive, the governor will increase the blade angle enough to prevent the engine from overspeeding. This allows the engine to maintain a constant r.p.m., and thus maintain the power output. Changes in airspeed and power can be obtained by changing r.p.m. at a constant manifold pressure; by changing the manifold pressure at a constant r.p.m.; or by changing both r.p.m. and manifold pressure. Thus the constant-speed propeller makes it possible to obtain an infinite number of power settings."

[Ardy123]

I think this paragraph explains it clearly in relation to climbing and diving;

http://www.pilotoutlook.com/airplane_flying/constant_speed_propeller

"When an airplane is nosed up into a climb from level flight, the engine will tend to slow down. Since the governor is sensitive to small changes in engine r.p.m., it will decrease the blade angle just enough to keep the engine speed from falling off. If the airplane is nosed down into a dive, the governor will increase the blade angle enough to prevent the engine from overspeeding. This allows the engine to maintain a constant r.p.m., and thus maintain the power output. Changes in airspeed and power can be obtained by changing r.p.m. at a constant manifold pressure; by changing the manifold pressure at a constant r.p.m.; or by changing both r.p.m. and manifold pressure. Thus the constant-speed propeller makes it possible to obtain an infinite number of power settings."

So the Governor also controls the throttle if the selected RPM cannot be reached by merely adjusting the prop pitch?

[Die Hard]

Nope. It's a very simple device that only controls prop pitch via oil pressure or centrifugal weights. One possible exception is the Fw 190 which had some sort of mechanical computer that controlled engine management.

[Ardy123]

Nope. It's a very simple device that only controls prop pitch.

I'm assuming the governor does not have a full 90 degrees rotation on the prop blade. If thats the case you could easily bust your engine by setting the RPM to low, or diving to fast with a high RPM setting and overspeed your engine.

[Die Hard]

I would think the pitch limits varies from design to design. Most modern props can feather the blades and so could many WWII designs on multi-engine planes.

[Baumer]

The governor only controls engine RPM. So once you get to the limit of propeller pitch, then the RPM will change due to the flight condition.
 

[Die Hard]

Yup. You see that in AH on some fighters if you dive them too fast the engine RPM increases.

[Ardy123]

Yup. You see that in AH on some fighters if you dive them too fast the engine RPM increases.

but no damage happens...nor if you set the RPM too low. Is that then just a FM/modeling issue?