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Old Jul 20, 2012, 02:04 PM
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pb2y (twin) float plane

want to make a float plane with multi engines. i have plenty sets of 4 motor and escs. 4 gram motors, 10 gram motors, 20 gram motors. Im not sure what size plane is realistic, so I made a small mock up and found that prop size limits number of engines on wing. wanted four, but settled for two even then had to run small props still.

about 25 watts (7.4 volts x 3.5 amps) between the two motors.

1404n twin motor test on pb2y float plane (0 min 39 sec)


this is a little one only 22 inch wing span.
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Old Jul 21, 2012, 02:28 PM
An itch?. Scratch build.
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Don't stop at two,....three,...four,.....more ?,........think big......


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Old Jul 25, 2012, 08:41 PM
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yeah. 12 motors. im thinking now. twelve 5 gram motors would be 180 watts. so maybe a 1.5 lb plane total. 46 inch wing span. each prop 5 inch. 30 inches of props across wing would leave 8 inches free at each wing end. 30 inches props and another 30 free would be 60 inch wing. might be hard to keep at 1.5 lbs.

Ive got a 66 inch wing now making. 6 ten gram motors for a larger pb2y coronado
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Old Jul 25, 2012, 08:43 PM
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I could do the one in your photo with twelve 10 gram motors for 360 watts. 2 to 3 lbs total weight with 70 inch wings.
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Old Sep 10, 2012, 07:38 PM
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nose dives and submerges underwater. water flows up and over then into the cockpit. esc gets wet and motor gets skitish.

the whole plane only weighs 150 grams. i might angle both motors up - so that would be adding lots of up thrust.

any ideas? Thanks!
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Old Oct 11, 2012, 07:23 AM
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From force vectors, I think downthrust would work better to keep the nose from burying. It's normal to taxi, and even start takeoff run, with full aft stick. This applies downforce on the tail to keep floats (and hull) from burying. Downthrust would also apply downforce, although with a shorter lever arm than the elevators.

btw, my dad was a radioman/gunner on the PBM Mariner during WW2, the plane you picture in Report#5.

Hope this helps.
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Old Oct 11, 2012, 12:49 PM
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I was thinking a little upthrust would have least torque as in the picture and down thrust the most.

I believe what is happening is that since my plane is small the displaced volume in the hull is 1/8 that of another plane twice the size.

1x1x1 = 1 cubic volume
2x2x2 = 8 cubic volume

yet the torque for the two motors would be much less dissimilar.

see pic 2. Up force from displaced volume, down force from motor

little plane yields 1 F down at nose -> submerges in water
bigger plane yields 4 F up at nose -> flys off water
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Old Oct 11, 2012, 12:52 PM
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im going to double the size of the elevator and hope prop wash will push the tail down the equivalent of 1 F so the hull gets on a plane.
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Old Oct 11, 2012, 01:02 PM
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pb2y pictures

just a few pictures
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Old Oct 11, 2012, 03:37 PM
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There are several forces working on a plane that operates in both air and water: 1) center of floatation (cof)=center of volumn of the underwater part of the hull: underwater part of hull rotates noseup/nosedown about cof 2) center of gravity (cg): entire plane rotates about cg in the air, but rotates about the cof when on the water.

All seaplanes have the center of thrust above the center of floatation. Thus there is always a prop-generated torque trying to force down the nose of the floats or hull. Rotation of the thrust (via rotating the motors) will affect this torque, but only minimally, since it would take an extreme rotation to significantly reduce the vector parallel to the water, which is the one causing the torque.

If the thrust is located ahead of the cof, then upthrust from motors would help, a sort of helicopter effect. Conversely, if the thrust is located aft of the cof, then upthrust would hurt, ie. increase the sinking of the bow.

So, where is the cof relative to the cg? If the plane floats level, then they are stacked one above the other. If the plane floats tail low, then the cof is fore of the cg. This is the usual situation, to insure that the wing has a positive angle of attack upon the start of the takeoff run. If the plane floats level, then likely the designer has specified a positive angle of incidence, again to get wing lift at the start of the takeoff run.

Tractor motors are located ahead of the cg, so thrust is fore of the cg. If the cg is aft of the cof, the usual situation, and the propshaft is short, then the thrust is likely located aft of the cof, and thus, upthrust would hurt. Looking at your plane, I can't tell, the thrust may be very near cof.
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Before you rotate your motormounts, either up or down, an possibly easy fix would be to increase the area of your elevator. Tape some clear stiff plastic to the trailing edge of the elevator, and see if the increased tail downforce (with stick back) keeps your bow from burying. <<I see you already thought of this>>

Once the plane is moving, the bottom surface of the hull will generate a noseup force (just like a flat-bottomed boat planeing while moving fast). At this point, you reduce back stick, and let the hull levelout and accelerate. This planeing effect needs hull speed, so perhaps slowly advancing the throttles would allow the plane to generate the noseup force before the bow buries. Jamming the trottles full on would be the worse case scenario for a seaplane with a tendency to bury the bow - you get max torque w/o any conpensating planeing force.

If the hull is squirrely on the takeoff run, you can use some back stick to drag the aft part of the floats/hull. This will act like a fin aft of the cof, helping you keep the plane traveling on a straight line. You have to remember to remove this extra back stick once the plane lifts off, or you risk stalling.

Hopefully this will give you some ideas on how to fix your plane.
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I noticed a downward extention on your wingtip floats. You don't want them dragging in the water on takeoff. If they drag, then they will tend to swerve the plane: waves or crosswind will sink one float deeper than the other, causing differential drag. With their long lever arm, the deeper float will overpower the rudder.
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Old Oct 12, 2012, 08:31 AM
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thanks! lots of new ideas in your notes. I'll try

0) larger elevator (done)

1) add fin or keep tail in water for straight takeoffs
2) slow down takeoffs (no jamming throttle)
3) see if I still need such long (tall) wing tip floats now after raising motors

then maybe

4) add a few more inches to the nose of the plane.
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Old Feb 05, 2013, 05:35 PM
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lengthened the nose as in the picture.

this time the wing tip kept in the water making it want to do circles. my water rudder was way too small. with 2 cell, it would water plane, but could not get air borne, so i stuck a three cell in and it still dragged a wing tip in the ater, but once free scooted along fine - full throttle. then once airborne it immediatley rotated back into the pond. lost a motor, fried an esc.

im going to try a single momtor on top, get rid of the low hanging wing tip bouy and try again.
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Old Feb 05, 2013, 06:48 PM
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1. Did you measure the cof and cg locations, and compare the results to what I said would happen in the previous posts? It's possible to design a plane by feel, but that takes experience. This seems to be your first multi-engine, seaplane design, so you don't likely have the experience. This leaves you with needing to make multiple experiments, which hopefully will work after you run a bunch of them. This extended process is fine if that is your thing. Or, you can shortcut the process by using the experience of others (not necessarily me, but other, more knowledgeable, sources).

2. Wingtip floats are not supposed to touch the water unless the wing dips. They are there only as a safety device to keep the wing from getting waterlogged due to prolonged immersion in the water. If your floats are touching before your takeoff run, then your model will not (never?) get off the water in a condition to continue flight. You need to shorten the float struts or remove float material to make sure your plane will rest entirely on it's hull only. If you keep dipping a wing and getting a float to drag, it means that there is wind pushing the wing down, or that the plane is not balanced laterally.

3. No water rudder will do you any good if the wingtip float is in the water. The drag of the float has too much leverage to be countered by the water rudder. Most seaplane models, especially small ones, dispense with the water rudder entirely. It adds drag on takeoff which can keep the plane from attaining the air. Also, unless it is very stiff and entirely centered, it will cause the plane to swerve as speed picks-up on the takeoff run.

4. All seaplane models should automatically weathervane, nose into the wind. If they don't, then you need to add more fin area. Could be your plane is dipping a wing because it does not weathervane, thus allowing crosswind to push a wing up (forcing the other wing down).

5. How does your plane fly when you hand-launch it? Perhaps you can speed the experimentation process by hand-launching, and then landing in the water. For instance, if your prop is generating too much nose down torque to fly, then you need to fix that before worrying about the nose down torque keeping you from taking off water. Not sure what you meant when you said it took off, then immediately crashed.
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Last edited by Brooks; Feb 05, 2013 at 06:54 PM.
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Old Feb 06, 2013, 07:35 PM
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yeah. the wing tips drop in the water and sticks as soon as i put the plane in the water. id say the plane is severe top heavy and just tips over any chance it gets. laterally its quite well balanced, but cg must be way too high.

tried again and the wing / wing tip just plunge deep in the water. no chance of getting it out. so im trying as in the picture.

the plane crashed so fast i have no idea what actually happened - over control, excessive motor torque from full power.

plane flies alright when tossed in the air. now with the single motor on top im not sure. i want to get the water taxing set first and have a t least a chance of taking off from water. makes me enjoy the standard two float set up i have in on the garami biplane as in the picture - piece of cake take offs. Still I want to fly a larger hull float plane.

tomorrow ill try again.
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Old Feb 06, 2013, 10:42 PM
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I like the sponsons you've added, they remind me of the China Clippers.

I can see that the single pylon would raise cg quite a bit, perhaps more than the dual engine situation. Since the plane sits w/o tilting on the bench, it's fairly well balanced laterally. You could try balancing it on a pencil run fore&aft to see if there is a un-recognized lateral balance problem that only shows up in the water. Perhaps the foam density of the wings is not uniform. If cg is too high, add some lead weight to the hull, low down.

Once the plane tips and dips a float in the water, it does not re-balance itself. This could be because the tip float is held down by surface tension. You could try coating the tip floats with a hydrophobic substance (cooking oil (Pam), or car/floor wax) to see if that would let them release more easily, allowing the plane to straighten up. Test the hydrophobic substance on a junk piece of foam first, in case it has foam-eating hydrocarbons (particularly floor wax). Pam cooking spray should be safe, I think. Are your floats taped on the bottom? Tape would protect the floats from foam eating chemicals, besides making them slippery-er.

I think you have too much side-area in front of the wing; this excess area forward fights the fin area, making the weathervaneing tendency weaker. Normally, a plane will weathervane, which then uses the breeze-induced lift off the wings to help balance the plane. This is particularly the case if there is dihedral; perhaps you could add some to your wing, which looks pretty straight in the photo. The original plane had polyhedral, I remember from the plastic Revelle model my dad made.

Next time you might, as an experiment, try putting some more area on the fin (tape on a piece of clear plastic cut from a blister pack). Then put the plane in the water, dip a wing, and see if the plane rotates around the dipped float, points into the wind, and then the wing lifts. All this done w/o prop turning.

Hope this helps, you've likely thought of these fixes already, sorry for the redundecency.
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Last edited by Brooks; Feb 06, 2013 at 10:48 PM.
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