Variable pitch prop

[dirtybird]

I have to wonder why no one has yet made a variable pitch prop for giant scale electrics. I would think it would be easy to do on an electric and it would improve performance quite a bit

[ImaBiggles]

I have to wonder why no one has yet made a variable pitch prop for giant scale electrics. I would think it would be easy to do on an electric and it would improve performance quite a bit
most likely because the advantage of a vario prop on small scale is more significant and practical. As you scale up in size and weight - the advantage dwindles as what the plane needs to fly also narrows. Also a vario prop that can handle 500+ watts is a beefy thing.

[dirtybird]

I would expect they would be beefy and expensive.
As I recall all WW11 fighters had one. I think they were regarded as necessary despite the expense. I think the improvement in performance would be considerable but we don't know unless we try.

[_Dutchy_]

As I recall all WW11 fighters had one. I think they were regarded as necessary despite the expense. I think the improvement in performance would be considerable but we don't know unless we try.

Well....I guess a RC-plane, big scale or not, does not have to bring a large payload up, doesn't go extremely high, if 1 engine stops somewhere we don't mind the extra drag, we don't have to fly to Berlin...and so a lot of other reasons.

But, I also would like the idea on a rc model. :cool:
Meaning that I would enjoy the technical piece of work behind a pitch prop on a scale rc-model but I think that it's totally overkill to put one on a rc-plane. :D

[hul]

this is the only one I've seen: http://www.rc-network.de/forum/showthread.php?t=13540
It's a 4 blade 72x100cm (28"x39"). He uses reduced pitch to limit current at low speed and more pitch at high speed. It seems to be efficient; only 2kW for 20kg (44lbs) of plane. See the plane here http://www.bastelzimmi.de/ under Projekte -> P61.

Hans

[dirtybird]

Well....I guess a RC-plane, big scale or not, does not have to bring a large payload up, doesn't go extremely high, if 1 engine stops somewhere we don't mind the extra drag, we don't have to fly to Berlin...and so a lot of other reasons.

But, I also would like the idea on a rc model. :cool:
Meaning that I would enjoy the technical piece of work behind a pitch prop on a scale rc-model but I think that it's totally overkill to put one on a rc-plane. :D

I was not working at Boeing during the prop era but the old timers that were tell me they achieved up to 40% increase in performance with a variable pitch prop.
It would seem that that increase in performance would be a great help to our electrics.
With a gas engine all you need to do is put in a bigger engine. But with an electric you need a bigger motor, more batteries and then you need to match it to the right prop. Being able to set the pitch for the flight condition would help.

[blucor basher]

IC engines / Electrics = Apples / Oranges

Keep in mind that we're not changing our load to maintain a specific RPM, we don't have the same torque curve that an IC engine has.

[dirtybird]

You are right we don't have the same torque curve an IC engine has.
However there is an RPM where maximum efficiency occurs considering motor,prop and A/C factors. It would be nice to go to that rpm anytime the you are not doing something special, provided it will fly the airplane; for the sake of flight longivity.
Currently the practice is to get a range of props,test them and chose the best during one the things you want to do. And to h*ll with longevity.

[Ed Lyerly]

There is a guy named Ed Sweeney in Colorado who has been tinkering for some time with this (variable pitch setups on electric planes). Most of his stuff has to do with 2 meter pattern type planes (10S). If you want to see his 2m pattern bird fly and hear him talk about his variable pitch arrangement .... buy the 2007 SEFF DVD from Propwash videos and learn more.
Ed

[GordonTarling]

Tom Hunt carried out some experiments a long time ago now and told me that it merely added complexity to the model for little or no gain. Think about it for a moment - a full-sized aircraft uses variable pitch to match engine rpm, airspeed and height in order to gain the most efficiency from a fixed prop diameter. How on earth could you hope to match all these variables correctly AND still fly the model? It COULD be done with various sensors and a microprocessor, but would there be any real advantage?

[flypaper 2]

Constant speed prop might be a better option. Much simpler mechanically than a variable pitch and you don't have to monitor it. Operates on springs and weights using the centrifugal force of the weights. For instance, pull up into a hover and the prop goes into finer pitch and maintains the same rpm. Let the nose down and the plane keeps accelerating till it meets the hp, drag barrier, or pull back the throttle to save amps with high pitch at cruise speed. Built one for a 50cc gas motor about 15 yrs ago as an experiment that worked alright. Should try one for electric as an experiment too.

Gord

[dirtybird]

Tom Hunt carried out some experiments a long time ago now and told me that it merely added complexity to the model for little or no gain. Think about it for a moment - a full-sized aircraft uses variable pitch to match engine rpm, airspeed and height in order to gain the most efficiency from a fixed prop diameter. How on earth could you hope to match all these variables correctly AND still fly the model? It COULD be done with various sensors and a microprocessor, but would there be any real advantage?

The gain will depend heavily on the workmanship and the skill of the builder.
If you continue to think a bit you will realize we are subject to the same limitations as full scale. It just takes different application.
With control over the pitch you would adjust for maximum performance for the flight condition you are interested. Then you could change it for another condition if you wanted to.
For a gas engine it wouldnt be much help as you have so much power to waste you don't care how inefficient you are when out of the primary performace objective.
With future batteries maybe we wont care,but now, we are interested in efficiency under all conditions.
A constant speed prop would be nice but I don't agree that it woild be easier to make.

[mrdakota]

http://www.warbirdpropdrives.com/

[portablevcb]

Variable pitch props on full size are really to take advantage of a narrow power band and widely varying flight conditions. Getting max takeoff/climb power needs a much different pitch than cruise, especially with large altitude changes.

Many, many full size planes fly with fixed pitch just fine, especially sport planes. They might not get that extra 10-20 mph in top speed, get the extra 100-200 ft/min climb, or have another 30 min duration from a tank of gas, but, they do the job. Just like us they pick a prop that fits most of their flying needs. You can frequently hear a pilot talk about a 'climb' prop when he usually flies in and out of small fields and takes short hops, 'cruise' or 'speed' props for longer cross country flying from longer paved runways.

Models are usually so overpowered that having the extra complexity would only be a 'neat' thing. Spectators would probably not be able to tell the difference.

But, having reverse thrust on something like a C-130 model might be interesting :D

charlie

[flypaper 2]

When I was a young lad, apprentice plane mechanic in the 50s, a fellow had just finished an airshow at our airport with what looked like a Pitts. He was adding these big washers/weights to the prop blades so he could go cross country to Ottawa in higher gear for another airshow. :D