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I'm modifying a RC plane into a bi-plane with floats and it keeps stalling. I don't have a bigger battery or better motor to add so how do I fix this? This is the biggest challenge I've ever faced.

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    $\begingroup$ Are the floats just hunks of foam? How do you know it's stalling and not just way too much drag? $\endgroup$ – Ron Beyer May 29 '19 at 1:23
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    $\begingroup$ From the photos it looks like you've taken the wing of some other model, lashed it to the fuselage with elastic bands, and slapped on some bits of foam to act as floats. Many things could be wrong with this, and there may not be a simple fix. Find a local aeromodellers' club and take it along. They'd be much better placed to make suggestions at first hand than we will. $\endgroup$ – CatchAsCatchCan May 29 '19 at 2:26
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    $\begingroup$ Do you not have an elevator? Can't you just give the plane some nose-down trim with elevator? I'm suspecting that you do not have an elevator; if so that is important to share w/ us. In fact I'm suspecting that this plane has NO moving flight controls and is controlled ONLY by differential engine thrust from the 2 wing-mounted engines-- is this true? Maybe you could add an elevator even if it just something you set on the ground at a fixed position for trimming. Not quite enough info in the question as of now to justify a full-fledged answer but-- $\endgroup$ – quiet flyer May 29 '19 at 3:51
  • $\begingroup$ Has the airplane ever been flown in the current configuration except for no floats? I suspect you added the lower wing and the floats all at the same time? $\endgroup$ – quiet flyer May 29 '19 at 4:25
  • $\begingroup$ Is the center propeller functional, or fake? What is the pin with the white ball head next to the propeller? $\endgroup$ – bogl May 29 '19 at 7:00
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Your model is based on the FX-803 Piper Cub model shown in this video. It's made from expanded polystyrene wings and fuselage, with a two channel radio that controls thrust from two motors mounted below the wing, with overall thrust controlling pitch and differential thrust controlling yaw. There are no moving control surfaces, and the propeller on the nose is a cosmetic windmilling attachment.

Your extra wing, lashed on with elastic bands, may or may not produce lift, but adds additional weight and drag below the thrust line, inducing a significant nose down moment. Your carved foam floats add additional drag even further from the thrust line exacerbating the nose-down problem, and further increasing the weight.

The original model was built and trimmed to give flying characteristics that could be controlled by simple thrust adjustments. There is no moving surface with which to counter the added drag, and even if there were, it is unlikely it would have sufficient authority to overcome the problem.

The modifications may also have moved the centre of gravity significantly, possibly beyond the point where it is fixable.

The model shown in the video has sprightly performance on limited power. The additional weight and drag you have added could easily overwhelm the available power.

You could try adding a fixed elevator to provide significant down force at the tail, but this will further compound the drag issue, and could also affect centre of gravity.

Realistically, I doubt you will be able to get this to fly. If you do, please come back and post a video!

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This plane appears to have no moving control surfaces at all-- it appears to be controlled purely by differential thrust from the two wing-mounted motors. You could consider modifying the horizontal stabilizer to give the plane an elevator, even if just works as a ground-adjustable trimming device. You need to fix the elevator in a slightly lowered position to counteract the stalling tendency you are experiencing. Don't overdo it! It could be as simple as piece of stiff cardboard taped to the back of the horizontal stabilizer, that you can bend into a lowered position. In fact it looks like you may have already done something like this with the vertical fin, to give some right rudder trim.

Or you could just try moving the CG forward by adding weight to the nose. This is probably the best solution as will enhance pitch stability as well as help address the trim issue. Again, don't overdo it! Small changes will be best--

Or try a combination of both approaches.

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  • $\begingroup$ It is hard to give authoritative advice without even knowing the relative incidence of the two wings. Maybe one stalls already when the other is just starting to produce lift? How is it flying without the floats? Since their drag adds a nose-down moment, an elevator should definitely be added. $\endgroup$ – Peter Kämpf May 29 '19 at 4:18
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OK! Is there a doctor in the house!

Try moving the lower wing back until it stops pitching up. "Staggering" the wings may also reduce interference between the two, which may be part of the problem once it starts to pitch up. It should fly better overall with the lower wing further back.

Try this modification and hand launch to test. If it works, then reset your floats as needed.

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