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Mechanical propeller pitch control

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Dan Thomas

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I searched here and goggled it. Why aren't there any mechanical pitch controls for airplane propellers but there are on helicopter rotors?

There are mechanical props but they're found mostly on homebuilts now. They were common enough on certified airplanes of 50 or 60 years ago but were replaced by the constant-speed prop, where the pilots selects an RPM and the governor adjusts the prop pitch to load the engine appropriately to maintain that RPM. It's more efficient, more automatic, and helps prevent overspeeding the engine.

A helicopter needs more immediate pitch control to control the lift. RPM has to remain constant whenever the machine is in flight of any sort. Old helicopters required the pilot to adjust the throttle constantly to keep the RPM nailed while everything else was changing, but most now have governors or correlators to adjust power as loads change. Rotor RPM is critical; too much stresses the blades and hub as centrifugal loads get out of hand, and too little RPM results in the blades flexing upward under the load ("coning") and drag goes up, lift goes down; more pitch is needed, adding to the drag problem, and the result isn't pretty.

Seems to me IVO has an inflight-adjustable prop now. An electric motor in the hub changes the pitch.

Dan
 

Starman

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Thanks, I see there are and were mechanical systems out there, that's good. Unfortunately I'm using either direct drive or an in line psru so I can't use the inside-the-shaft actuation. Electric sounds good, I guess if it locked in place during an electrical system failure, would be safe.

I like the idea of using a lever or a hand crank though, and it will need to use a slip ring actuator, a push-pull slip ring should be easy to make. Is there anything like that out there without the inside actuator? now or has been? there must be.
 

Dan Thomas

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Thanks, I see there are and were mechanical systems out there, that's good. Unfortunately I'm using either direct drive or an in line psru so I can't use the inside-the-shaft actuation. Electric sounds good, I guess if it locked in place during an electrical system failure, would be safe.

I like the idea of using a lever or a hand crank though, and it will need to use a slip ring actuator, a push-pull slip ring should be easy to make. Is there anything like that out there without the inside actuator? now or has been? there must be.

Some used a fork-and-slip ring affair similar to the shifters in a manual transmission. Best would be a sealed ball bearing device similar to what's used as a swash plate on helicopters.

Dan
 

Joe Fisher

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There are Beech Robey that were common in the late 40's and early 50's Univar still sells blades for them. They are ether electric or manual using a crank and speedometer cable. They were used on 65 through 200hp Contentals and some Lycomings. The only one I ever flew was on a PT-19 with a Ranger engine. Years later I had an opportunity to fly another PT-19 with a fixed wood prop and the fixed prop seemed to work better.
 

Starman

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Some used a fork-and-slip ring affair similar to the shifters in a manual transmission. Best would be a sealed ball bearing device similar to what's used as a swash plate on helicopters.
I agree, a sealed bearing unit is the way to go.

Since I want to use a tractor prop now I need to be using a psru so might as well get a big prop. Something like 8 ft or bigger possibly.

Is it possible to buy used 8 -10 ft propellers that are close to timed out or need to be shortened beyond limits for fairly low $$? I guess best for this purpose would be one with a broken pitch control.

I was thinking if those weren't too much of getting one of them and clamping some collars on to the prop shanks and running external pitch rods to a sealed bearing slip ring dealybobberwhateveryacallit and operating that with a lever in the cockpit.

If forces are high for a lever then hydraulic boost can be added.
 

HumanPoweredDesigner

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8 to 10 feet is pretty big. I bet it will be efficient at just one speed and inefficient at other speeds even if you adjust the pitch.
 

Starman

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There doesn't seem to be much out there. I've been thinking that since I'm going to be using a psru that it might just be better to make it a two speed unit rather than having a controllable pitch prop. I'm going to check the difference in costs, I think a two speed psru just might favor simplicity ... but heavier? Could be close on weight.
 

TFF

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Turbo-prop engines only turn one speed; all the thrust change is with the prop pitch.
 

Dan Thomas

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There doesn't seem to be much out there. I've been thinking that since I'm going to be using a psru that it might just be better to make it a two speed unit rather than having a controllable pitch prop. I'm going to check the difference in costs, I think a two speed psru just might favor simplicity ... but heavier? Could be close on weight.
We already have tip speed issues with direct-drive and PSRU's propellers. With a two-speed setup you will run into it even more. Jim Bede tried a transmission in the first BD-5 but it was worth less than the trouble it raised. You need a low-pitch propeller for low forward speed, so your propeller will have to have a low pitch. If it turns slowly it will need to be large to get the necessary thrust for takeoff. Then as you shift it, its tip speed gets so high that it loses too much HP to drag (and noise) and you don't gain anything.

The real problem is with the prop blade's angle of attack. If we have a prop that has, let's say, a 15-degree pitch at the 75% blade span, that part of the prop will have a little less than 15° AoA when the airplane sitting on the ground, not moving. A little less than 15° because there's some movement of air into the prop disc. In other words, it's nearly stalled. Anyway, as we start moving forward, the AoA of the blade starts to drop due to the relative wind vector that the blade experiences, and as the AoA drops to near between 2 and 4 degrees, the airplane won;t accelerate any more. We can raise the AoA again by speeding the propeller up, but those tricky tip speeds get us, see? Or we can increase the pitch of the blade, which is what variable pitch is all about, whether directly-controlled or through a hydraulic governor, as in the constant-speed setup usually found in better airplanes.

As always, what we see flying around is what thousands of experimenters and designers have found works the best. Transmissions also add terrific weight, especially anything that would handle 500 continuous horses. Even a 500-hp truck doesn't have a transmission that will take constant power at that level. Two and three-speed transmissions have also been tried in boats, where the tip speed issue is much less trouble, and even there they didn't help.

You might want to see this:
http://www.recreationalflying.net/tutorials/groundschool/propeller.html#prop_theory


Dan
 
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Starman

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That all makes sense and I agree ( haven't read the link yet)

But

this is how I see it, correct me if I'm wrong.

First keep in mind that when I say 500hp from an engine I expect to cruise at less than 200hp most of the time. My plane should go around 180mph on only 63hp. Due to this a transmission can be very beneficial, with a large diameter prop set for high speed pitch during takeoff it will stall and take a lot of torque to turn it at anything useful so in low gear the torque can be doubled or tripled to crank the big prop around enough to get useful thrust anyway, then while cruising at moderate speeds the engine can be putting out rather little effort and can be shifted to a gear so the engine is loafing at lower cruising speeds and just coming off an idle at 150 but starts winding out above 300. With a wide speed range and a high power to weight ratio a transmission can be good I think.

I think it will work reeeelly well, but whether or not it does it will be at the big show this summer.

Unless: The main reason I shy away from hydraulic props is because I heard they are very expensive, but if they aren't too bad then I guess it's not hard to rig up a remote oil supply to run it, then it could still be controlled with a lever in the cockpit, without the governor.
 
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Lucrum

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180mph on 63hp while toting around the weight a 500hp engine is a little hard to believe. What kind of wing area and equivalent flat plate area are you using to come up with this?
 

autoreply

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180mph on 63hp while toting around the weight a 500hp engine is a little hard to believe. What kind of wing area and equivalent flat plate area are you using to come up with this?
You would require 1.4 or so. That's about the same as the Nemesis NXT or the Lancair 360. Seems indeed hard to achieve for a 500HP airframe...
 

Starman

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180mph on 63hp while toting around the weight a 500hp engine is a little hard to believe. What kind of wing area and equivalent flat plate area are you using to come up with this?
I guess that 63 was thrust horsepower rather than crankshaft horsepower (big green grin smiley face goes here) little details like that can make a difference sometimes (ROFL smiley face goes here)

Please check the 'Tandem Wing Pusher' thread to see the plane it is applying to.

Warning, I recommend starting from the back of that particular thread =)
 
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