I was wondering if anone ever tried, or has made one. I figured it might not be too hard to make one with some counter weights and springs. I dont have the equip to make one but wondered if anyone has ever tried. Not made to be adjustable in the air, but maybe have an allen screw that can adjust a speeder spring assembly on the ground to dial in the RPM you want.
I dont know, thought it might be a good idea.

sean

Kavan used to make an inflight adjustable prop. Was expensive and blade replacements were expensive too. I think Hobby Lobby used to sell it for about $125.

Just a spring and counter weight wouldn't make a CS prop, it would just reduce the RPM change somewhat over a fixed pitch... you need a governor that senses rpm and changes pitch to control rpm... They must be separate systems. I have put a lot of thought into this one myself and there are two possibilites... some sort of electric pitch adjustment using a motor and an electronic sensor to measure RPM... or use a propeller planform and airfoil that provide a strong negative pitching moment and oppose that with a spring... as airspeed goes up reducing propeller angle of attack and lowering thrust the spring would force the blades to higher angle of attack... I'm still thinking about building something like the second one I mentioned to use on my full size pitts.. the crankshaft is solid so there is no way to put a CS prop on it.

Ty

I have a homade one on a Q 50. Had running on the stand but not in a plane yet. As you say, springs and weights.

well i know how a real aircraft cs prop works, i am an A&P, and one of the first ones made used a spring with counterweights, like i first said. Also it was not pilot adjustable. The problem is that it constantly needed tweaking to work on full size acft. I thought that it may work though for our models. Flypaper, you say you have one, have you used a tach to confirm that it actually stays at a realtivly same RPM once it comes on speed? Acropilot, when you say a "strong negative pitching moment," are you talking about creating a prop that trys to decrease its pitch as it creates thrust, and opposing that with a spring? I dont really understand what you are trying to explain. I do see how you say it will just decrease the rpm range instead of making it cs. I think the electronic idea, with maybe some sort of variable position solenoid in the hub using slip rings and brushes for power from a cs circuit may be the best idea. Or maybe even some sort of swash plate, but that may be too impractical and complicated. You say you are going to put it on a full size pitts? Will the FAA approve that? or is it certificated as experimental? Also, what do they put in the nose of a pitts anyways? if its a lycoming or continental you may be able to get another CS that will allow the oil to get to and from the hub. Does your engine block have a mount and drive for a governor? Someone must make an STC for that. I am sure it would cost a small fortune to get and put in though. lemme know what you think!!

sean

sean

Bob Kress used to have/sell a variable-pitch unit. I'm not sure if it was constant-speed or if the pitch was pilot-selectable, but it could vary pitch in flight.

Sean:
Tachs at 7000 wide open throttle. At idle, pitch is about three in. pitch. When you watch the hub closely, you can see it change pitch as you open the throttle. the weights are on the back of the prop hub on a 45degree angle facing forward. As the speed increases centrifugal force pulls he blades into coarser pitch, The springs control how much. Watched a guy do an airshow routine here. When he was done I watched him put heavier weights on to go to higher pitch on cross country to Ottawa. The one I built is pretty crude as it more of a test prop. The prop is its own governor.

There's a guy around here with the usename "mec" who's built a governed CS prop asembly for a turbo-prop model engine. Pitch is linked inot the turbine ECU to maintain constant RPM. Search on his username for a long thread with pictures.

Out of curiosity...does "constant speed" refer to engine rpm or airspeed?

I think Hanno Prettner used one of those Kavan units for a while to try to keep airspeed constant through climbing and diving maneuvers. I could never figure out how he had enough hands to work it, though

Engine RPM... as load increases and RPM decreases, the pitch decreases to maintain a constant RPM.
Obviously, throttle does come into play at a certain point to limit the top speed (or to limit RPM as max pitch is reached). This type of system is ideal for turboprops as turbines make more power as they increase in speed... keeping the RPM up is even more important than in a recip engine.

Look at it like a transmission in a car.

Also at take off, pitch set to get max thrust at a certain rpm. At speed, the engine is still at max thrust RPM at higher pitch. Probably work good on a Reno racer type plane if you could keep it together at the higher rpm required. The fellow in Germany, with the turboprop, had it on an aerobatic plane where it would work well over the wide speed range. One of the hard parts was figuring out a way to keep the blades synchronized at the same pitch without getting complicated. There is an arm going across the centre of the prop on a pivot. Linked to each blade with ball links on an arm.

Some of the rubber-powered contest airplanes, both indoor and outdoor, use variable pitch props. The pitch of the prop is controlled by the tension of the rubber motor. That's about all I know about it.

Jim

ok, thanks for the replys everyone! like i said, nothing i have the tools to build, but thought it would be neat.

sean

Ifix,

The prop idea I mentioned using negative pitching moment relies on the change in thrust accompanied with changing airspeed... When you accellerate the prop AOA goes down and so does the thrust. If this decrease in thrust can be mechanically linked to an increase in prop pitch then you can limit the RPM change with speed... My idea to acomplish this is to use a swept back planform on the prop combined with an undercambered airfoil that will provide a very strong negative pitching moment that is directly related to the thrust.... in other words the more thrust, the more negative pitching moment, the lower the pitch angle. I would use a spring to drive the blades to High pitch... It would take some experimenting to get the right size of spring, but once you had a good ballance you could adjust static RPM using some kind of locknut and washer arangement on the spring. The only thing that I don't like about this is that the prop goes to high pitch at idle and I'm afraid of the engine bogging down.

On my Pitts I have a Lycoming IO-360B4A no drive for a governor, no hole in the crank... it has a sensenich fixed pitch and the RPM's get way up there when I have to get the smash up for some of the unlimited figures (3400 RPM). I'd really like to keep that from happening... I actually think the prop is braking by this time and all of the horse power is going into making shockwaves... I get lots of complements on having the loudest airplane in the IAC :-) Just hope it doesn't blow up. It is experimental..

Ty

Ty:
That method may work also. I had to put the pivot point of the blades at the thickest point of the blades, the neutral point if you will. If one used the centre point of the blade to pivot on , it would try to go into course pitch as soon as the engine started. Easier to think of it as a wing. If you were to move the pivot point ahead of the neutral point, it would try to weathervane and a spring would pull it into courser pitch. The only problem is getting past that course pitch when the prop is static. Have to think about that one. will try to get some pictures up but not having much luck. Got one to work in the test forum, using the cut and paste method but wouldn't work after that.

Gord, I'd like to see how it works... I was putting a lot of thought into the setup with weights and springs on the prop, but I discounted it because I realized that the only way to increase pitch was to increase RPM since the weights are attached to the prop... But my perspective was only full power accelerating from a stop to full speed... the RPM would still increase with airspeed, but to a lesser extent. The real benefit to this design may be the fact that prop pitch will change with power changes allowing for more static thrust at full power and more braking at idle... especially on a model where top speed is very low anyways. Can you email me the pictures

[email protected]

Thanks,

Ty

Ty:
I will try again to get the pictures to work. Some of the books are old where I learned this stuff from. Don't know if a library would still carry them. Air-age Series (science of preflight aeronautics). Copyright 1942. Aviation mechanics Simplified Copyright 1943 ByAustin Technical publishers. Aviation Technitian Intigrated training program. ( powerplant Section Textbook) Copyright 1983 By IAP inc. Hope this helps.

Ty:
Sent an email to the above address with a picture and it came back. Is there a gap between acropilot and ty. I'll try again.

it is an underscore between... hard to get an email address without one these days

here is your pic

here is the pic of my constant speed prop !