Hi,

Are you linked to this ?

https://www.rcgroups.com/forums/show....php?t=2292224

This is interesting, but maybe not useful for all applications.

Can you give more details on how the pitch is governed ?

I can think of an application. With a friend, we are in some kind of unofficial challenge. The deal is a speed record. The plane must be flown FPV, the cam must be mounted on at least one servo. The takeoff power must be limited to 500 W.
The race consists in flying 500 m away and back (using GPS) as fast as possible.

My plane shows 500 W on takeoff, with a 5.25x6.25 propeller. During the race, at around 160 km/h, the power drops at 400 W or below.

Takeoff (hand launch) is difficult, because of the high stall speed and the poor static thrust of the 5.25x6.25.

Here, if we imagine the extra weight of a variable pitch system is meaningless, maybe it would be useful to takeoff with a propeller larger in diameter, but with a lower pitch than the 5.25x6.25, so that we can get more thrust for the same 500 W.
Then, in flight, I could switch to a higher pitch to keep the watts near 500 W and the thrust high.

Here variable pitch could be a solution, if not too heavy, and if we consider it's not cheating of course.
The need would only be for a two positions switch (low pitch / high pitch).

Maybe we can find some others applications, but the "good" and the "bad" must be accurately estimated, which is uneasy but possible.

EDIT: if I consider an ideal variable pitch system, no extra weight, I could replace my 5.25x6.25 by a 5.75 propeller with a pitch varying from 5 to 6.2.
This can lead to an increased static thrust (900 g vs 700 g on takeoff) and a very small increase in max speed (167 km/h vs 164 km/h)

variable pitch propeller automatic / adaptive propeller; works only and exclusively for the aerodynamic forces that are generated when the rotation starts.
the blade is always oriented in the air current that passes through the predetermined angle of incidence, the value of this angle determines the power required for operation. The greater the angle, the greater the power absorbed by the propeller, also for its characteristics of alignment with the wind that strikes it, always ensures traction, non-cavity, it does not stall. The propeller current has been calibrated for a maximum power of 1200 watts to about 9500 RPM; the diameter is 13.77 inches. on my site is shown the characteristic diagram propeller considering only values below minimum detected during the test flight, ie, the return can not be less than the estimated value of 71% in its maximum condition.

for your race, it is not applicable

I know, that was just for an example.

Do you have a link to your site ?

I'm not sure I understand correctly how it works, you mean that the blade chord is always oriented parallel to the wind that strikes it ?

So very low pitch in static, larger pitch at higher speeds ? what about "cruising" conditions, part throttle ?

it is a compromise between a fixed propeller and a variable pitch. it guarantees always thrust you in any flight condition. Maximum thrust at full power when starting, when it is in flight power almost halved.
www.alaiamodelli.com

I'm sorry but I don't see any data on this site ?

well now I have done repair.
go to product / propeller / find out.

Originally Posted by yomgui

Bonsoir,

I thought of a way to read the UIUC data, without being too complicated or requiring huge computations.

If one can measure static thrust and the corresponding rpm, and so the corresponding pitch speed.

On the 1st picture below, there is a typical propeller polar, with Ct (thrust coefficient) vs J (advance ratio J = v/nD).

The red arrow shows how the static thrust is related to Ct, and the green arrow shows how the pitch speed is related to J (and P/D ratio). Of course, this supposes there is no "unloading" which is not correct, but it isn't very important.

More important is the shape of the thrust curve.

You can make a graph with thrust / speed. Then the static thrust and the pitch speed are landmarks to help you draw the thrust curve, like on the 2nd picture.

This last one shows an APC 10x5E vs GWS 10x6DD, with very similar results, and the different way of understanding pitch. APC sees the pitch as the geometric pitch (angle of the chord line, from the leading edge to the trailing edge of the blade) while GWS seems to see the pitch as pitch speed = zero thrust speed (I think in fact they just measure the angle from under the blade, which is a bit less than at the chord line, and it simply happens that it nearly leads to pitch speed = zero thrust speed).

congratulations for your technical skills, I saw that we perform the same type of calculation. If interested I can share with you my experiences.

Thanks, I have no particular skills, I'm just used to work with propeller polars.

This thread is more about general subjects on propellers, you can check the main subjects Louis suggested.

Bye

Hey guys is there a simple explanation for what prop pitch provides? IE speed, thrust.

Thanks

where can i buy a graupner cam 11x8 in the states . amazon sells them but not that siize

Originally Posted by bigdbc

Hey guys is there a simple explanation for what prop pitch provides? IE speed, thrust.

Thanks

Both of those are a result of the prop's work, imagine a screw entering wood, the tips of the prop would follow a helical path much like the threads on a screw. And that makes the prop want to "carve it's way forward" against the air the way a screw does into wood.

But a prop is working in a compressible medium so there is some "slippage". When a prop has a pitch of 6" that is the theoretical distance it will advance in one turn. but, in reality, slippage and drag means it will be less, maybe 20% or so less on average.

In theory the higher the pitch is on a prop the further it will travel in a revolution so that makes it potentially faster. Also it is taking bigger "bites" at the air so it will be generating more thrust.

Jack

Originally Posted by jackerbes

Both of those are a result of the prop's work, imagine a screw entering wood, the tips of the prop would follow a helical path much like the threads on a screw. And that makes the prop want to "carve it's way forward" against the air the way a screw does into wood.

But a prop is working in a compressible medium so there is some "slippage". When a prop has a pitch of 6" that is the theoretical distance it will advance in one turn. but, in reality, slippage and drag means it will be less, maybe 20% or so less on average.

In theory the higher the pitch is on a prop the further it will travel in a revolution so that makes it potentially faster. Also it is taking bigger "bites" at the air so it will be generating more thrust.

Jack

Nice explanation.
Thanks for that Jack.

Originally Posted by cmdl

this would be nice. i'd like to learn more about props. recommended for sticky.

Dang
APC Props HAD a lot of performance data on their props, including thrust, horsepower and so on at different RPM's. Their web page indicates that the data will be available again soon. Well, it's been a few months now.

http://www.apcprop.com/v/downloads/P...B/datalist.asp

Hi
I have had an email contact with APC
They plan to offer again data in january 2015 (?)
Wait and see
Louis