Mechanically controlled variable pitch props vs the constant-speed variety

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rdj

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There are a few posts close to this topic, but none that really answer this question: why are there so few manual adjustable pitch prop hubs for smaller engines (Rotax, Jab, VW, etc.)?

All of the bigger engines seem to use constant-speed units, with hydraulic governors and their associated complexity. The smaller engines, however, are almost invariably run with fixed-pitch props, resulting in the inevitable selection of either a cruise prop with poor climb performance or a climb prop with poor cruise performance. This problem was solved almost a century ago with solutions like the Beech-Roby manually-adjustable hub. The design doesn't seem that complicated.

A variation of it can be seen in the manual for the Duc propeller: Duc Hélices (Stupid frame-based site btw--follow the link to 2/3 blade variable pitch, then 'more info'..)
The blades have a finger at their root which engages a slotted disk that can be rotated forward and back by a simple screw mechanism. As the disk moves forward and back, the position of the slot rotates the blade via the finger. The design seems fairly straightforward.

Seems to me, if the Duc mechanism was adjusted such that moving it to one end gave a climb pitch, and the other end a cruise pitch, you wouldn't need much more than a lever in the cockpit labeled 'climb' and 'cruise' to get the best of both worlds.

However, that's not what's out there. There are very few controllable-pitch hubs of any sort for the small engines, while the large engines invariably have the complex constant-speed systems that provide a fixed RPM. The latter seem simpler to operate at first glance, but every time I study them closer I found warnings about don't exceed this manifold pressure or the engine blows up, square-this-n-that, etc. Furthermore, the few products available for the smaller engines are usually the more complicated controllable-pitch solutions as well, such as the Airmaster. Which leaves me wondering, what is it about adjustable pitch that 1) requires constant-speed operation and 2) makes simple manual adjustment undesirable on smaller engines?

Bob
 

autoreply

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The nut that connects the yoke to the pedal.

Even a shallow dive is enough to overrev your engine. The RF5 has a manual pitch prop (low pitch, high pitch, 90 degree pitch). Switching at the wrong moment either meant you were going down without power or you'd ruin the 20K US$ engine. Starting in the 90 degree pitch was impossible.

The electric props that stabilize on a given (desired) rpm seems like a great solution to me.
 

Himat

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The nut that connects the yoke to the pedal.

Even a shallow dive is enough to overrev your engine. The RF5 has a manual pitch prop (low pitch, high pitch, 90 degree pitch). Switching at the wrong moment either meant you were going down without power or you'd ruin the 20K US$ engine. Starting in the 90 degree pitch was impossible.

The electric props that stabilize on a given (desired) rpm seems like a great solution to me.
An electronic govenor on the engine would prevent the user to over rev the engine.
But it dont stop the pilot from going down without power.

(Electronic gizmos like this often have drawbacks. A RNoAF pilot crash landed a Bell/Augusta 412 because of the torque limiter prevented him from using 100% power on both engines. The helicopter have two engines, but the gearbox can not handle 100% torque from both. To preserve the gearbox the engine management system limit the torque. Ok if you need twin engines for hot and high operations or the safety of two engines. Not OK if you need all the power to arrest the descent.)
On the other hand, an electronic controlled electric prop is probably a better solution anyway.
 

Dana

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They're not commonly seen on smaller engines because the planes fly relatively slowly. Adjustable (constant speed or otherwise) props really make sense when the aircraft has a wide speed range; a prop that's good for climb at 60 won't do very well cruising at 200. But when you're climbing at 60 and cruising at 100 there isn't that much loss. Aircraft owners often can choose between a "climb prop" or a "cruise prop", but the actual difference is pretty small (2" pitch difference for the A-65 in my Taylorcraft IIRC).

GSC does make an in flight adjustable prop for smaller engines.

-Dana

Place a half full glass of water before a pessimist, optimist and an engineer:
The pessimist says the glass is half empty.
The optimist says the glass if half full.
The engineer says the glass is larger than it needs to be.
 

autoreply

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But it dont stop the pilot from going down without power.
Yes it does. I was referring to where you put the prop in high pitch at low speed, essentially stalling your engine.
Electronic gizmos like this often have drawbacks.
Yeah, some people think so. Virtually nothing is simpler than keeping a prop at a given RPM by varying the pitch.

I've flown both (mechanically, electric and hydraulic) and would be willing to spend a considerable amount of money on an electric of hydraulic prop if the alternative was a mechanical one and so will probably most pilots who've flown both.

Just think of it. You take off with the prop at fine. Then you climb and at 2000 ft you level of. If you forget to change the pitch just a single time when leveling off, well, good-bye to your engine...
 

Himat

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Ah, I wasn't particucalry clear in my statement.

An electronic govenor would prevent the pilot from over reving the engine if he selected fine pitch at altitude and speed.
To save him from the braking of the fine pitch a high speed there would have to be an selector inhibit.
Same, there would need to be an selector inhibit to stop the pilot from selecting coarse pitch at slow speed, stalling the engine.
And with all this in place, let just make it fully automatic, as you say an electric or hydraulic constant speed prop is better.

Just remember as in the helicopter case I mentioned and others, with a lot of automatic systems, youd better know their working envelope and underlying constraints. If not you could be caught out.

@Dana
I realy like that one with the engineer, but after years of working (as an engineer) I would rather state that the glass could be considered to be larger than it needs to bee.
 

BBerson

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My G109 has a mechanical three position (climb, cruise, feather) Hoffmann prop. Mostly installed on motorgliders for the feather, but also the heavy motorglider would not climb very well with a fixed pitch unless it was very low pitch.
The Hoffmann weighs 23lbs and is not cheap (something like 10 or $12,000, last I heard)
BB
 

Dan Thomas

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Any adjustable prop has to be really strong to take all the forces that a propeller takes. A fixed-pitch prop's unitized construction makes this easy; the adjustable prop has to do it with minimal weight and volume increases. It makes it much more expensive, whether it's a two-position prop or a constant-speed prop. And then there are the manufacturing hassles of so many small engines that need different bolt patterns, pilot bores, diameters and pitches and blade profiles. Very low production numbers increase the costs.

Dan
 

rdj

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Thanks guys, I think I'm getting a feel for how the controllable props work. Most of what I've read about them has the pilot going full forward on takeoff, and then at some cruise altitude pulling back to another RPM setting and leaving it there for the rest of the flight. Seemed like two speeds to me. But, I understand now, if it wasn't RPM limiting in the full forward position it would over-rev the engine and blow it up once the pilot entered cruise. In other words, the high-RPM (fine pitch) position is not limited to an equivalent fixed-pitch climb prop pitch, but can go even finer for more performance. Correct?

If you take another case, the one mentioned by BBerson where the prop is 3-position, I presume the G109 controller limits the pitch excursion between climb and cruise positions to those equivalent to a fixed-pitch climb and cruise prop. As Dana notes, the pitch difference would be small, but the engine wouldn't over-rev once you entered cruise and the pilot was playing tetris on the GPS instead of paying attention to his RPMs. Then again, maybe my presumption is wrong and the engine could over-rev on the G109 if the pilot isn't on top of it?

Perhaps an explanation of the 'why' behind my question is in order; hopefully it won't throw the thread off on some wild tangent :) The issue BBerson describes is the same as mine. I'm working on a motorglider design where I would like to be able to feather the prop. There's another thread on here lamenting the very few options available that don't cost as much as the engine. Furthermore, most of the options available seem to be designed as constant-speed units first and support feathering second, if at all. The only mechanical options are invariably old, heavy, expensive and custom-built for a particular airframe (such as the Grob). I was curious as to why that's the case, and what advantage constant-speed has over a mechanical range limited to fixed-pitch climb and cruise settings. It appears the answer is because constant-speed props invariably have a much wider pitch range than that determined by the equivalent best fixed-pitch climb and cruise props, and a governer is the best way to manage that range without damaging other expensive components. Right?

Going even more off-tangent, this all began in my head when I started pondering whether or not I could design my own manually-controlled prop hub, using a redrive on a VW engine to give me access to the axle of the hub. If the prop hub rotated on bearings around a cylinder which had a sliding fore-aft groove, and the propeller blades had fiingers which rotated around in this groove, I could (theoretically) change the pitch by sliding the cylinder back and forth, much like the Duc hub does. Ignoring all the numerous mechanical and vibration issues, practical operational questions included: what range of pitch is desirable, what range of pitch is safe (for the engine), what are the drawbacks of simple mechanical operation (push-pull cable), and is there anything to be gained from the added complexity of adding a governer and making it fully constant-speed (probably using a servomotor and electronic governor). These are the questions I'm trying to sort out initially. I'm leaving the whole mechanical and vibrational mess for another day.
 

BBerson

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The Hoffmann prop exploded view is shown in the manual here:http://www.hoffmann-prop.com/documents/E0107.72.pdf
Very large undertaking to make this prop, I would say.

Actually, I tested the sink rate of the G109 with the prop feathered and unfeathered:
sink rate with prop feathered= 250fpm
sink rate with prop not feathered = 250fpm (no detectable difference at 62kt, I did not test at high speed where there may be some difference).
BB
 

autoreply

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If you're looking for a motorglider, going with fixed pitch and a folding prop might be the way to go, for both a tractor or pusher prop, your wallet and your brain (might explode from the design effort of a GOOD 90 degree variable pitch prop). Something along these lines:
Future of gliding - Front Electric Sustainer

Would reduce drag by a large amount, even if it's not so streamlined, but simply consists of flat propeller blades that roughly lie against the fuselage.
 

Dana

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Many aircraft with fixed pitch climb props will overspeed the engine if run at full throttle in level flight... but probably not as dramatically as an adjustable prop in fine pitch.

-Dana

Campaigns to bearproof all garbage containers in some national parks have been difficult, because as one biologist put it, "There is a considerable overlap between the intelligence levels of the smartest bears and the stupidest tourists."
 

Dan Thomas

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Going even more off-tangent, this all began in my head when I started pondering whether or not I could design my own manually-controlled prop hub, using a redrive on a VW engine to give me access to the axle of the hub. If the prop hub rotated on bearings around a cylinder which had a sliding fore-aft groove, and the propeller blades had fiingers which rotated around in this groove, I could (theoretically) change the pitch by sliding the cylinder back and forth, much like the Duc hub does. Ignoring all the numerous mechanical and vibration issues, practical operational questions included: what range of pitch is desirable, what range of pitch is safe (for the engine), what are the drawbacks of simple mechanical operation (push-pull cable), and is there anything to be gained from the added complexity of adding a governer and making it fully constant-speed (probably using a servomotor and electronic governor). These are the questions I'm trying to sort out initially. I'm leaving the whole mechanical and vibrational mess for another day.
The propeller blade pitch-change mechanism has to be fairly rigid and robust. Depending on the geometry of the prop, the blades will want to either go to a higher pitch (caused by center of pressure ahead of blade pitch axis) or to a lower pitch (caused by centrifugal twisting forces; the masses of the blade behind and in front of the pivot axis want to come into the same plane of rotation). Both forces can be fairly large and in some constant-speed props one or the other are used as an adjunct force for pitch change.

And the pitch mechanism must be reasonably reliable, too. A failed pitch link could make things ugly indeed.

The propeller blades exert enormous outward (centrifugal) forces. Each blade of a typical fixed-pitch metal prop, such as found on the good ol' 172, can put 25 tons of force on the hub at the blade root. Much higher forces, of course, with larger, heavier, or faster props. The blade bearings must be able to take this force as well as the blade thrust and drag forces and the pulsations of the engine, and still allow the blade to rotate freely enough. Remember that the prop is usually the engine's flywheel and that the engine kicks those blades ahead every time a cylinder fires, then those blades pull the hub forward in the next compression stroke. All of this stuff causes bearing wear and "blade shake." We look for blade slop in the hub at inspection time.

Dan
 

rdj

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Dan, totally agree. I've already come to the conclusion that designing the variable-pitch hub would be as large a project as the design of the plane itself. However, as an electrical engineer who knows just enough about mechanical things to be truly dangerous, that wouldn't stop me (although it would naturally slow me down quite a bit :) ). Given BBerson's glide data though, maybe it really is an unnecessary complication. I've heard similar anecdotal tales from other sources--as long as the prop is not windmilling, the drag effect is fairly small.

However, BBerson also notes that he needs the variable-pitch feature of the prop in order to have an effective climb rate. So I might still need to go the variable-pitch route on my own motorglider, although there are more off-the-shelf options out there if feathering is not a requirement.

And thanks for the Hoffman manual link BBerson, I was looking for something like that. Gives me an idea of what it takes to design/build such a hub.
 
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Back in the thirties, some of the Thompson trophy racers had "shiftable" two-position props that used an inflatable bladder in the prop hub -- one position for takeoff, another position for racing (cruise).
 
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