UMX 1/20 XP-77 (420mm)


New member

I've been building some simple UMX planes lately, using electronics taken from a Sport Cub S2. This time I wanted to try something a bit more interesting than square fuselages and retangular wings...

The XP-77 is an experimental light fighter developed by Bell in WW2 as an 'emergency fighter', built from easy to come by materials. The plane was mainly constructed out of wood and was small compared to contemporary fighter aircraft. 2 prototypes were built, but ultimately the project didn't work out.
View attachment 219751

To determine a good scale for the model, I looked at Eachine warbirds. Those have a 400mm wingspan, and I thought a wingspan in that range would be good for my XP-77 as well. 1/20 scale corresponds with ~420mm wingspan, and I went ahead with that. To scale my plans I used GIMP, and after struggling with the software for a while i got it printed. In terms of materials, I'm primarily using 1/16 inch (1.6 mm) modelling foam.

View attachment 219797


Work on the frame is practically done, and I've started on the foam 'covering' and mounting the electronics.

Update #1 23-2

Done with the covering sheets and canopy (first try turned out nice). Test fit here:

Update #2 25-2

Finished the wings today, aileron servos will be mounted later. The spar is made from 2x 1.6mm foam and a popsicle stick in the middle, and the ribs are foamboard pieces. I used masking tape for bending the leading edge section, as seen in one of the pics. Practically all that's left to do is the very front of the nose and the tail sufaces. And some paint, if the thing ends up flying. Which it should, because without the tail and aileron servos, and with the battery it weighs around 37 grams. It should end up at around 42 grams AUW.

Update #3 27-2

95% done. Just gotta install the control horns and finish the last bit of the nose. I've been having a little trouble with the ailerons, so I'll first try a powered glide test without them and mount them on later. I've got some light gray water-based spray paint which I'll apply if the thing flies. I'll be sure to record the maiden.


I finished the nose, fixed some other things, and briefly tested it. Gonna add some of those bent pieces between the wings and fuselage, now that i think about it. (True) maiden flight hopefully this week. If that goes well, i'll paint it. Stay tuned.

And with the bent wing pieces


Update #4 23-3
After testing a little, i noticed the tiny ailerons basically did nothing, and due to torque from the motor, the thing only flew in left-hand circles. Consequently, I (finally) enlarged them:



Update #5 25-3

Painted and finished.


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New member
are you going to post your plans?
I'm sort of finding out how I build along the way (true scratch build ;)), and I should get a CAD program if I'm gonna do that. So no plans for this one, I'm sorry. But thanks for the suggestion/reminder, I'll consider it for further projects.


New member
Update #1 23-2

Most of the fuselage done - see original post.
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New member

Take some advice from me: 2+ g/cm2 wing loading and a symmetrical airfoil don't keep an airplane up well. Should have went with a bigger scale...

SSgt Duramax

Junior Member

Take some advice from me: 2+ g/cm2 wing loading and a symmetrical airfoil don't keep an airplane up well. Should have went with a bigger scale...
Good effort though. When I was going through the pics I was thinking that you might want to scale up the wing a little. It is hard to keep things perfectly scale this size. When I do planes that small, 500mm works out the best for me. Not saying others can't, but that just seems to be what has worked. I haven't tried a warbird that small.