Props

The differance in a donut and a circle?

Don't be silly.

Thrust is the mass of air accelerarted.

That can be a big mass accelerated a little (helicopter) or a little mass accereated a lot (27k on a 6x3) ..

If you assume a 'perfect' propellor te static thrust is propartinal to the blade area times the pitch speed, and the pitch speed is proportional to the pitch,

So static thrust should be roughly proportional to diameter squared times pitch times RPM

Yeah right.

But it ain't, since (A) so much of the prop blade is stalled in the static condition unless the prop's pitch is very low, (B) because of the large disc vortex when there's no forward movement, (C) the drag of the cowling and fuselage that affects a large part of that "diameter squared" (should be pi*r squared) area, and (D) the lack of any useful thrust from the prop's central area. Most fixed-pitch props have abysmal acceleration performance in the early part of the takeoff roll and the thrust will increase as speed increases until the AOA drops off and the thrust then decreases again. In doing some acceleration/mass/speed/horsepower calculations a few years ago, we found that of the 150 rated HP in a Cessna 172, the force required to accelerate the airplane from standstill to takeoff speed worked out to about 28 HP. Some of the loss went into the total drag of the airplane, some (probably a lot) into the prop's drag, some into wheel friction, and some was never even generated because the fixed-pitch prop on that airplane won't let the engine come up to 2700 RPM during takeoff and climb, the rated revs where the 150 HP is produced. When we figure that the internal-combustion engine is about

Dan

All agreed...the only thing was to point out that thrust *is* related to pitch. As well as diameter and RPM.

I don't think there are any usable props where the airflow is actually stalled for more than an instant on startup and acceleration. A higher pitched prop will cause the inflow velocity to increase, which decreases the angle of attack. However there are some which with reduced diameter and reduced engine speed have very low static thrust, till the plane picks up some speed and the engine picks up RPM. Pylon racers do stall the blade immediately after throttle up, that is why they take off much quicker when held down till the engine gets to full RPM.

Pardon me, but I'm kinda lost here in Propeller 101. I don't see how static measurements apply with much accurately to any kind of real life engine/aircraft performance, since aircraft rarely find themselves in an envoronment similar to the static thrust environment (except maybe in a hover). Especially when most flights are made with variable throttle levels, in unpredictable propeller attitudes(p factor), and forward speeds. My question is: What does static thrust tell us about how to select and prop an engine for a particular application?

Phil AMA609

Yes but it seems almost half of us now have an aircraft we hover. So it is extremely important to them.

Here's a website that discusses propeller pitch and blade stall:

Dan

It only tells you how the prop performs when the plane isn't moving. The only real use if to determine if the combination will have enough thrust to pull the plane out of a hover, or if the plane will just sink.

Simple physics says that if you have 5 pounds of thrust and a 4 pound airplane, it will come out of a hover. If you have tons of thrust it will go into orbit, at least if you can move rocket fuel instead of air.

Indeed. prop pitch does influence absorbed power. static it absorbes more with more pitch, because the propwash is faster, and thus more air mass is moved; Dynamic, because the plane will move faster at the same angle of attack (=thrust). Power still is force times distance/second. I have a calculator available on

Blades stalled mostly is hogwash, except for very high pitches. A good foil will not stall up to 15 degrees effective AoA, which must consider inflow speed. It will operate under increased dynamic drag though, but still at max lift capability.

Pitches near the root of a blade can be 35 degrees or more, and at the root the inflow is very disturbed and the outflow is experiencing some bounce off the cowling, reducing airflow further. Good airfoils can fly at AOAs up to 17 and 18 degrees.

Dan

The 35 degree blade angle has nothing to do with effective AoA. The root blade section moves slowly, whilst the inwash moves pretty fast. It is not so much the inflow that is disturbed, but the outflow, so the root does not operate at such high AoA as might be expected. For the inflow, the center section has the highest speed, which tapers off towards the tips, as in any vortex flow. Also, the center 1/2D part of the prop contributes not much to the prop thrust due to low blade speed. (section lift relates to v^2)