Building a Kaman free flight helicopter

bnitram Sat Jan 12, 2013 2:44 pm

Hello all,
i want to build this Kaman free flight helicopter:
https://www.coxengineforum.com/t3341-helicopter-babe-bee-power

But i dont want to build it so big. I want to build it half as large as the original with a Cox PeeWee(.020).
Do you think that it will work with this engine and the size?

bnitram

daddyo Sat Jan 12, 2013 6:02 pm

Maybe someone knows the power of the 0.020 vs that 0.049, then scale the model directly? 0.020 / 0.049 = 40% scale?

Might be neat to go the other way too, put a Tee-Dee 0.09 on it and double it's size.

bnitram Sun Jan 13, 2013 11:20 am

Thanks for answer.
That would be the best way but i don't now anything about the power in detail.
But I think that the engine has enough power to fly with the helicopter. What would you think?
The engine has enough power to bring a rc airplane in the air. Then it should also get free flight hlicopter in the air right?

bnitram

Kim Sun Jan 13, 2013 12:46 pm

Hi bnitram,

And if I haven't said so, Welcome to the Forum! I'm not involved in a lot of rotorywing flying, but here are my thoughts:

Engine sizes/ratios between airplanes and helicopters don't transfer equally because of the different characteristics of the two...especially in THIS type where the engine is essentially dead-lifting the helicopter with it's little prop. An .020 on an airplane just pulls it forward and lets the wings do the lifting. The rotors on this type basically provide stability by spinning while the propeller does all the lifting. With the engine out of fuel, the rotors then do their thing in gently lowering it to the ground.

Cox marketed an .020 type called a "Sky Copter" which flew so well, there was a chance that it'd take off and leave you. Perhaps Mark or some other collector here would measure and weigh theirs to give you some workable numbers to shoot for. Hope this might help.

Good Luck with your project ! Kim

bnitram Sun Jan 13, 2013 1:33 pm

Thanks for answer.
So the size and weight are very important.
Do you think if I take the right size and weight it will work?

bnitram

**RknRusty** Sun Jan 13, 2013 1:37 pm

Does anyone here know the power output of these engines? I would wild guess that a Babe Bee is around 40 watts. There is at least one member at RCG that has done extensive power measurements on engines. I'll try to find one of his threads. I don't even remember his name right now. He built a device with a swing arm he mounts the engine to and measures how much it can pull.

**RknRusty** Sun Jan 13, 2013 1:38 pm

bnitram wrote:Thanks for answer.
So the size and weight are very important.
Do you think if I take the right size and weight it will work?

bnitram

If you know the watts per pound of the original, then yours will scale directly.

bnitram Sun Jan 13, 2013 1:44 pm

Hello! Thank's.
I think I found a table with performance disclosures.
http://www.mh-aerotools.de/airfoils/cox_performance.htm

Can it help?

bnitram

pkrankow Sun Jan 13, 2013 3:17 pm

You could set the engine up on a balance beam and weigh the thrust...

With many models, the density decreases as volume increases. This means a larger model ends up with lighter loading due to proportions.

Phil

daddyo Mon Jan 14, 2013 12:43 am

Re:

bnitram wrote:http://www.mh-aerotools.de/airfoils/cox_performance.htm

Looks like the horizontal axis is RPM so if you got the right prop on tweaked for max horsepower, you likely get 26 watts out of the 0.020, and roughly twice that out of a typical 0.049. So just go for 1/2 the weight I'd guess = 1/2 the size.

I remember as a kid putting an 0.020 on a scratch hovercraft and it just sitting there stuck to the ground... But the cox helo flew nice.

Show us your result! BTW anyone ever see an engine tied to a rag fly like a helo?

ado1974 Mon Jan 14, 2013 5:29 am

daddyo wrote: BTW anyone ever see an engine tied to a rag fly like a helo?

like this
https://www.youtube.com/watch?v=ph2PMs2-7G4

https://www.youtube.com/watch?v=67PUWyKdqjI

cheers
a

bnitram Mon Jan 14, 2013 8:06 am

So you think that it works well with a half size and the Cox PeeWee.
Just as I thought. But not with the power but the size of the engine.

bnitram

ahrma_581 Mon Jan 14, 2013 12:28 pm

Since things don't always scale exactly, you might consider building it with a bit longer main rotor blades. Then you can cut them back if needed to get the desired lift. Easier to cut them down than adding length....

daddyo Tue Jan 15, 2013 12:18 am

ado1974 wrote:
daddyo wrote: BTW anyone ever see an engine tied to a rag fly like a helo?

like this
https://www.youtube.com/watch?v=ph2PMs2-7G4

https://www.youtube.com/watch?v=67PUWyKdqjI

cheers
a

Ya! Haven't seen that in a while! Amazingly cheap, ultimate quick build haha.

I'm sure things don't scale exactly but my thought is that were not going 1/20th scale or the like, just 50%. Looks like horsepower is kinda proportional to displacement, but that can sure vary alot depending on model/brand. Also since the helo is mostly made of material along the length of the body, which might likely be most of the weight aside from the motor, that cutting the length in 1/2 would be 1/2 weight. Might want to see what the weight of the motor is between the two sizes to see if that might affect things (are they 2:1 ratio?). If you want to be certain it'll have enough lift, maybe you would make it even smaller than 50%.

bnitram Tue Jan 15, 2013 3:57 pm

Many thanks for the answers.
I think I will begin in a few weeks and i will build the blades a bit larger than normal to test it out.

bnitram

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