Thrust and Thrust Available

[chris]

Hey guys,

I don't understand why thrust available decreases with increasing airspeeds. I know it has something to do with the fact that the props lose efficiency as you go faster... but why is this?

I have read that the relative wind changes (and hence the AOA changes) as you go faster... the faster you go, the more the AOA decreases, and thus the amount of thrust produced decreases. But why is the RW changing?

On a similar note... Fine pitch props are not good for cruising and lose efficiency at high speeds. Is this because the pitch (and hence AOA) is relatively low on this setting, and thus, not much thrust is produced?

Thanks.

 

[mtsu_av8er]

[ QUOTE ]
Hey guys,

I don't understand why thrust available decreases with increasing airspeeds. I know it has something to do with the fact that the props lose efficiency as you go faster... but why is this?

I have read that the relative wind changes (and hence the AOA changes) as you go faster... the faster you go, the more the AOA decreases, and thus the amount of thrust produced decreases. But why is the RW changing?

On a similar note... Fine pitch props are not good for cruising and lose efficiency at high speeds. Is this because the pitch (and hence AOA) is relatively low on this setting, and thus, not much thrust is produced?

Thanks.

[/ QUOTE ]

All of these answers, and more, can be found in your trusty Aerodynamics for Naval Aviators !!! In all honesty, that's one of the best books on aerodynamics that I've ever read.

Better have some coffee and a calculator handy, with lots of paper, pencils and erasers!!! It's pretty cool, though!

 

[ananoman]

[ QUOTE ]
I don't understand why thrust available decreases with increasing airspeeds. I know it has something to do with the fact that the props lose efficiency as you go faster... but why is this?

I have read that the relative wind changes (and hence the AOA changes) as you go faster... the faster you go, the more the AOA decreases, and thus the amount of thrust produced decreases. But why is the RW changing?

On a similar note... Fine pitch props are not good for cruising and lose efficiency at high speeds. Is this because the pitch (and hence AOA) is relatively low on this setting, and thus, not much thrust is produced?

[/ QUOTE ]

Thrust available from a piston engine is usually greatest when the aircraft is not moving and the engine is making full power. In this situation the AoA of the propeller is at its maximum and is purely a function of blade angle. As you accelerate the angle of the blades remains fixed (assuming a fixed pitch prop), but the air that the prop is trying to 'grab' is moving. This lessens the props angle of attack. You operate much the same way. You can generate maximum 'thrust' when standing still, pushing against a wall. If I made you run on a tread mill that was moving very fast, the thrust you could generate against the wall would be much less.

The only good way to illustrate this is with a vector diagram, which is not possible here. So, I would second the advice to obtain a copy of Aerodynamics for Naval Aviators.

The fine pitch prop is not good for cruising because it already starts with a low angle of attack. This is necessary so the engine can make full rated power at low airspeed, generating maximum power and thrust on take off. As the aircraft speeds up, the angle of attack is further reduced and you must retard the throttle to keep the engine from overspeeding. Since you cannot use as much power or generate as much thrust as a higher pitched 'cruise' prop at higher speeds, your aircraft will be slower. The constant speed prop fixes this problem by allowing a low angle of attack for take off and a higher angle of attack for cruise.

 

[B767Driver]

[ QUOTE ]

If I made you run on a tread mill that was moving very fast, the thrust you could generate against the wall would be much less.[ QUOTE ]


Great analogy.

I had a difficult time understanding the difference between power and thrust relating to recips. The thrust produced is relative to some initial velocity. The horsepower produced is in regards to the internal mechanics and doesn't care about intial velocity (this is not true for jets or turboprops as some power output is achieved by imparting exhaust to the slipstream).

As stated above with the treamill analogy, one could exert the same amount of power on the treadmill at a high or low speed, but the thrust would be different.


To address the original post, propulsive power available is affected by propeller efficiency and horsepower. As the velocity of the aircraft increases...prop efficiency is worsened. As altitude increases...horsepower is decreased.

So an increase in speed or altitude will reduced the power available to the recip powerplant.