Hollow props..

Bonjour.

Does anyone have any experience of making hollow composite propellers? I'm flying with Retro Discovery engines and I am rather fed up with the instability of the wooden props. I do re-pitch them in the same way as carbon props and sometimes they stay, sometimes don't. The pitch also seems to change with humidity and temperature changes. Normal CF props, like Bolly's and Eathers are too heavy for Retro.
My prop dimensions are about 13,5x6-4,5, with rather wide and thin blades. I know that hollow propellers are made but for bigger rc models. In a stunt prop the construction methods would have to be a lot more delicate.
I have some random ideas:
-I would prefer making the top and bottom parts separately and join them after curing. That way it would be possible to use higher pressure to press the laminate against the mold surface. Perhaps some elastic piston could be used, like in molding free flight l.e. torsions. I'm a bit worried about the strenght of that joint; it would propably not be nice if the propeller splits in flight.
-Perhaps some lightweight filler material would be a good idea. Rohacell or something similar would be ok but difficult to shape accurately. Perhaps some putty made of Epoxy and phenolic microballoons?

Regards, Lauri M

LAST EDITED ON Aug-31-06 AT 05:27 PM (CST)
 
Sergei Belko of Ukraine, makes very light foam core propellors. I have several of these props that I obtained with the engines that he also manufactures.

The props I have and all that I have seen were Three blades of varying diameter with 5.5 inch pitch.

However it's possible that he makes more or would do so if asked.

His props (and engines), are of excellent quality and work very well on his engines. They are very light and very stiff.

The one major drawback to the props is that they cannot be repitched.
I would think that this would probably be the case with any hollow or foam filled prop.

I do not currently know how to contact Sergei, but I believe Windy Urtnowski of Pro Stunt Products knows how to contact him.

I would mention that some of the Eather props that I've used in the past were fairly light (at least compared to the Bolly's).

The Belko Props, however, are much lighter than either of those.

PS: One of your countrymen and a beloved friend of mine and many others here in Tucson AZ is Rene Berger. He definitely is frequently in contact with Sergei and has a supply of the propellors I discussed.

Randy,

which of the Belko engines do you have - his own design .60 (with straight cylindrical muffler) or the Rustler-Belko Metamorph .60 with a fancy side/rear muffler?
I'm curious about your ownership experiences. I have the Metamorph .60 and a few foam-core 3-bladers, but have only run it on the bench.

Lauri:

I, too am looking at alternatives to the supplied props for the D-R .60.

I'm curious . . . how did you determine that the CF props like the Bolly and Eather are too heavy for the Retro?

I have yet to run anything but the stock props, but am interested in finding something that works with less diameter, maybe three blades.

Hi Lauri,

Brian Eather has been looking at the possibility of hollow (well, foam or balsa core) props specifically for the Retro - I'll ask him if he has given it any more thought.

Are the laminated Beringer props more stable than the Yatsenko props?

Rick, Belko has some 3 blade composite props for his engines - which run very much like a Retro, so might be worth a try. I've run an Eather 3 blade, but didn't find it an improvement.. more experimentation perhaps required.

My experience is the same as Lauri's - Retro's like light props. The difference in gyroscopic effect is very noticible, particularly in the outside squares.

It would be easy to lay up a prop with a light weight kevlar core. I've made fiberglass props with a carbon fiber core so I could color the glass but still keep it stiff. Later I just painted the CF props. They use carbon/kevlar mix to keep high priced tennis rackets light. If you can "adjust" a CF prop it means it's resin rich. When it's layed up right it's lighter and stiffer and you can't adjust it after it cures.

"""Normal CF props, like Bolly's and Eathers are too heavy for Retro."""

This is absolulty NOT so, matter o f fact the Bolly s work much better in wind and heat than do the Stock props suggested for the motor. And there have been many people use the Bolly 13 x 6 and cut down 14 x 6 (13.4x6) on these engines. # flyers just used these 2 at the US NATs

These have been used on the DR for several years now with great results

Hi Lauri,

some use also the Beringer props on their Retro's. Gilbert published an article on how he does them in his newsletter. It is in French but I think you slowly get used to French
Gilbert uses well stored wood with a defined humidity. That keeps his props from changing shape. They aren't cheap but when looking at all the work that's in them (they are all hand made) it's worth the price.

Richard Kornmeier once also tried some carbon props on his Retro's but quickly went back to wood because the props caused the crankshafts to break. Never heard from anybody else about this problem but finally he settled back to wood props.

Good luck

Christoph Holtermann

>This is absolulty NOT so, >Regards
>Randy

Hi Randy.

" I could not fail to disagree with you less"

I also have seen quite a few Retro-planes with Bollys etc, including one Chinese national team member (12x6 3-blade). I cannot say if they work better or not, different people like different things. My guess is, that with heavier prop it is possible to get the motor run well also in planes with "normal" nose construction. "Normal means what 99,9% of stunt designs have, hardwood motor mounts glued strongly to plywood doublers. But then you can forget that instantaneous change from 4-stroke to 2-stroke and back what makes Retro so special. You end up with something more like constant-rpm -setting. But as people say, someone likes the daughter, someone father.
If you look into the engine compartment of a yatsenko-model, you will notice that engine bearers are not in direct contact with fuselage skin sheeting. They only touch the plywood formers and nose ring. This leaves the engine mounting quite flexible and lets the motor to vibrate at a certain rpm during the shift from 4- to 2-stroke. That together with the lightweight prop is extremely important if you want to make the Retro run as it's designed to run.
Propeller weight has also a dramatic effect to the trim of a plane. Not only to how sharp corners it makes but also to the symmetry of outside and inside turning. Yatsenko-planes are allmost all-in-line designs and with a heavy prop they tend to create a tracking problem. Especially when people first compensate the lessened sensitivity by moving balance point back.

>
>Hi Randy.
>
>" I could not fail to disagree with you less"
>
> I also have seen quite a few Retro-planes with Bollys etc,
>including one Chinese national team member (12x6 3-blade). I
>cannot say if they work better or not, different people like
>different things. My guess is, that with heavier prop it is
>possible to get the motor run well also in planes with
>"normal" nose construction. "Normal means what 99,9% of
>stunt designs have, hardwood motor mounts glued strongly to
>plywood doublers. But then you can forget that instantaneous
>change from 4-stroke to 2-stroke and back what makes Retro
>so special. You end up with something more like constant-rpm
>-setting. But as people say, someone likes the daughter,
>someone father.
> If you look into the engine compartment of a
>yatsenko-model, you will notice that engine bearers are not
>in direct contact with fuselage skin sheeting. They only
>touch the plywood formers and nose ring. This leaves the
>engine mounting quite flexible and lets the motor to vibrate
>at a certain rpm during the shift from 4- to 2-stroke. That
>together with the lightweight prop is extremely important if
>you want to make the Retro run as it's designed to run.
> Propeller weight has also a dramatic effect to the trim of
>a plane. Not only to how sharp corners it makes but also to
>the symmetry of outside and inside turning. Yatsenko-planes
>are allmost all-in-line designs and with a heavy prop they
>tend to create a tracking problem. Especially when people
>first compensate the lessened sensitivity by moving balance
>point back.
>
> Lauri M.

Hello Lauri

I have quite a bit of first hand experince with The DR motor and the Yatsenko Planes working with Bolly props flying these motors and on Yatsenko airplanes. Maybe the air is differant here But they seem to work very well.

My post was just about the Bolly and engine combination, as I don't have much experince about the Planes being shall we say less than robust on the engine mounting, so I will take your word on that. I have however found that when I run any engine in a 4-2 cycle back and forth quickly it has always had the best engine run with a very stable robust motor mount.

Will be interesting to see how the lighter props work when you make them

>"Normal means what 99,9% of
>stunt designs have, hardwood motor mounts glued strongly to
>plywood doublers. But then you can forget that instantaneous
>change from 4-stroke to 2-stroke and back what makes Retro
>so special. You end up with something more like constant-rpm
>-setting.

and

> If you look into the engine compartment of a
>yatsenko-model, you will notice that engine bearers are not
>in direct contact with fuselage skin sheeting. They only
>touch the plywood formers and nose ring. This leaves the
>engine mounting quite flexible and lets the motor to vibrate
>at a certain rpm during the shift from 4- to 2-stroke.

This is very interesting, I can't even imagine what the mechanism might be. Could you expand on how this works?

Lauri,
it is my understanding that some of the "hollow" props of the Eastern Europan origin are made simply by squeezing a thin sheet of (styro?)foam between upper and lower halves of the mold, laid up with cloth. The foam provides enough pressure to make good contact with the mold.
I haven't tried this myself, so can't provide any more details.
Unless you already have a mold ready, it might be easier to cure the unstable pitch problem by laminating the wood props with thin fiberglass, after proper pitch has been set. This should not result in too much weight gain.

Hejssan.
Thanks. Beringer props are very good for retro, too. I used them in Spain. Original pitch is maybe slightly too much for my Sharks so I usually make it 1/4..3/8" less, measured at dia.290mm. Perhaps I could compensate this just by leaving the tip section a little thicker, I tend to carve them very thin. Laminated construction makes them more stable and so they also more difficult to re-pitch so that they keep their shape.
From Yatsenko props, I prefer Andrei's ones. The quality is not so constant as in Yuriy's props but buying a lot and choosing the good ones helps a lot. I personally think that they are better, especially in windy conditions. In calm I prefer Beringers props as they give more power in top. On the other hand, in thin air, which happens quite often in Switzerland, I sometimes have to reduce the venturi diameter. Propably, and much rather, I would get rid of this need by increasing the blade area. (Sounds stupid but seems to work.)
I know the Belko props but won't use them. I think that the plane turns nicer with a bigger diameter 2-b.
I will try to make those hollow propellers during the winter. I think that it's the ideal solution.