Propellor Pitch

Billy Pilgrim

Can anyone explain how propellor pitch is measured? I know that the pitch, 5" or 6" or whatever, refers to the distance that the propellor will move forward in one complete revolution but what does that mean? If the propellor is sitting on a test bench it won't move forward at all, whereas if its on a sports plane moving at full throttle it will move forward a considerable distance.

coolwings

It does not matter if the model is held down, or if it is in flight.
The propellor still moves air from infront of the prop-disk, to behind it as it revolves.

Pitch is the measured of the length of the cylinder of air thus moved by the prop in one complete revolution, assuming 100% efficiency. It therefore gives a theoretical speed for the model, for any rpm. But not all glowfuel props have the same efficiency, some are less than 50% efficient. So for example in these cases theoretical speed = rpm x pitch x 50%

So if you have a 10 x 6 prop on a .40 engine, your model will go faster if you change the prop to a 10 x 7.
Or it will accellerate faster with an 11 x 6, but have the same top speed.

This depends on the engine having spare capacity enabling it to run at the same rpm as before, even though you have increased the load on it.

Billy Pilgrim

Thanks for that. Perhaps you could help me with another question thats been niggling me? How does a plane with a symmetrical wing fly? Surely if a plane with such a wing is flying straight and level then the lift would be equal top and bottom?

coolwings

Sure....
First most "symmetrical" wings are not fully symmetrical, a semi-symmetrical wing may have say only 30% of the curve underneath that it has on top. Therefore when heading directly into the wind, there will still be more lift than downlift. So it flies up.
Incidentally, you can simulate the effect on the model's stability of having a downward pull quite easily...imagine a chestnut "conker" dangling on it's string...it sways back and forth when deflected by the slightest breeze (turbulence) ... that is your trainer with flat bottom wing,....now imagine I attach a second string to the bottom of the chestnut, and I pull down 1/3rd of the pull up, not enough to drag it down, but it becomes rock solid, unaffected by sideways windforces. That is your semi-symmetrical wing.

Now lets look at a fully symmetrical wing, there is no lift when flying straight ahead...wrong. We attach the wing to the model with the leading edge raised higher than the training edge. So when the plane points forward, the wing is pointing ever so slightly uphill, and forward. This is called wing incidence angle, normally it is about 1.5 to 2 degrees raised in front.

Finally there is the "flat plate delta model, where the wing is flat with no airfoil, and it is inline with the axis of the model, ie no incidence. No lift when going forward .... right .... so all we do is lift the model's nose a little to fly it straight. We create incidence by flying slightly uphill to go straight.
Concorde did it on takeoff and landings, lifting the nose to get incidence, and once it was waaay up there it could level out to almost horizontal, and go faster with less drag.

the problem with symmetrical wings is the lift is achieved at a greater speed, not good for beginners trying to land in one piece.....

Billy Pilgrim

Lovely and clear. Thanks