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Power rule of thumb for float planes

Old 03-26-2019, 04:07 AM
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kargo
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Default Power rule of thumb for float planes

Hi Gang,

I have been searching your very awesome thread for a gouge on how much power I need for my float plane (a Bud Nosen Citabria 40, an auction find). I've seen in a couple of places that how well they jump off the water is more about how the floats are setup. Do the standard rules apply (75 watts per lb for scale, 100 for aerobatic, etc), or is it something else? I plan on using foam core floats from Seaplane Supply, as well as their float setup gouge (to the best of my ability anyway).

I saw specs somewhere that said it should weigh to ready to fly 4.5 to 5 lbs. If it weights 6lbs all setup on floats, is about 600watts good enough?

Thanks in advance for your answers!

kargo



http://www.seaplanesupply.com/veebottoms.htm
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Old 03-26-2019, 06:43 AM
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Wildflyer
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I put some floats on a big Piper Cub I bought for $10.00 I researched Cunningham floats, found details online.
Made the floats out of rigid pink foam with a 1/4" plywood keel, covered in shipping tape.
Plane flew great and I doubt I had 100 watts per pound.
watch your angles and step location.
Floats should be 75% of the length from rudder hinge line to prop.
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Old 03-26-2019, 11:36 AM
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solentlife
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The trick is the pivot point of the floats and relation to CoG. Its why many insist on CoG at or just behind Step.

If you are ahead of the Step ... you need more power to rotate the aircraft to lift off.

If you have a reasonably well powered model on wheels ... floats will be fine as long as the step relationship is good.
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Old 03-26-2019, 04:10 PM
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Wildflyer
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Many, many years ago, there was a design for a twin engine seaplane called the "Liki Tiki" or very close to that.
The designer did not understand step location. The article told how great a flyer it was off a hard surface on wheels, but that he had a terrible time getting it off the water. It told of all of his efforts to get it to fly from water.

One look at the plans told you why, he had the step 3" behind the CG. it created so much drag that he could not rotate to take off.

This plane had 2 .35 size nitro motors on a 72" wing, once airborn it flew like a rocket.

https://outerzone.co.uk/plan_details.asp?ID=9036
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Old 03-26-2019, 05:23 PM
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kargo
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Thanks Gang! I think I have a good idea of what I need now. Be careful to place the step in front of the CG. Seaplane Supply has an excellent article on this, but the summary from all of you sure helped bring it together!

I'll also stop trying to buy my power system before I have a final weight

kargo
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Old 03-26-2019, 08:01 PM
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solentlife
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I usually aim for CoG to be on step ..... that is a good compromise that stops you lifting off too soon with not enough speed but allows model to rotate ...
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Old 03-28-2019, 02:30 AM
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Wildflyer
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I would make the step at or slightly behind the CG. If your float design allows it you could adjust it a bit after you fly it.
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Old 03-28-2019, 04:26 AM
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kargo
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Thanks again Gang, I've been getting a lot of help from many sources. I can't wait to get to work on the Citabria...
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