my aerobatic plane (WSp 2.31m, weight 10kg) has an (in flight) full adjustable three blade prop.
the prop is 20inch diameter, turns constant 4500rpm. the estimated average speed of the model can be assumed with 100mph.
at the moment i use shortened full symmetrical wooden heliblades.
this configuration gives 12kg thrust on ground.

i believe that this system could be improved in using prop blades with better efficiency. but which one?

is there anybody out there who can help?

what shape?
wide?
Airfoil of the blades (do not know the right term), undercambered or flat bottomed?
washout?
built in reflex?

thanx

mec

I have homade constant speed prop on a Quadra 52 which turns at 7500 rpms, only on a test stand so far. I use a zinger 20/8 cut in two to fit aluminum blade hubs. Not a recomendation, just what I use !!

thanx for input, gord

mec

Helicopter blades have very high aspect ratios. They are forced to use symmetrical airfoils or reflexed airfoils to avoid the aeroelastic effects of the pitching moment of cambered airfoils and extremely high twisting loads on the torsional support of the blades. For aerobatic (non helicopter) aircraft, the blades are much lower aspect ratio, the aeroelastic effects are much less and high lift, cambered airfoils are used because they produce more thrust for a given power. When comparing airfoil camber, it is the mean line camber that counts, not the presence or absence of so called undercamber. For example, a thin undercambered airfoil can have less mean line camber and a less negative pitching moment coefficient than a thicker semisymmetrical airfoil.

An inflight adjustable pitch hub has some of the engineering problems that a helicopter hub has. The hub's ability to sustain twisting loads depends on its design and materials. Who designed the hub? What twisting loads is it designed to carry? The answer to these questions will determine what kinds of blades it should use for best results. If you can't get the answers from the designer then you should stay strictly out of the prop plane of rotation with the engine running and not run the engine in the presence of other people.

Ollie :
I overbuilt the hub assy for test purposes. The blade shaft is 1/2 in solid steel and goes right through the blade hub as one solid piece the aluminum blade hubs attach to this. Not concerned about weight, only strength. the prop blade hubs also have a carbon fibre wrap around the ends of them. As you say, safety is the main concern. Not looking to see a blade stuck in my bellybutton.

You may have over built the hub assemply but, how do you know? Have you analyzed the structure to determine the weakest part and calculated the RPM that will produce failure to see how much higher that RPM is than the maximum RPM that it can see in the intended application? Have you tested it to an RPM much higher than the intended application to test the assumptions in your analysis. Where safety is involved, even the most knowledgable professionals do destructive testing to check their analysis. Until you have done these things, is your conclusion any better than a guess? These questions may be painful but I'm sure not as painful as a prop in the belly button, as you put it.

where can i get an adjustable pitch prop? i am looking for one with the hub assembly

Mec, have you ever considered molding your own blades? There's been a few articles in magazines on this but a quick google didn't turn up anything but I only tried a couple of word combos.

Certainly the helicopter blades are not an ideal solution as they are constant angle rather than constant pitch along their length. If you could mold your own you could use the outer shape and airfoil of something proven from the big makers but morph on your own root to engage the vari pitch hub.

There would still be only one true pitch setting where the blade is perfectly set but it should work over the range better than some heli blades I would think.