Dumas Bearcat Conversion

(lots of pictures so be patient)

The picture above pretty well sums up this project. I’m very satisfied with the overall fit, finish, and flight characteristics of the converted Dumas free flight kit. As I mentioned, the Bearcat is originally a 30-inch wingspan (~170 sq. in. of wing area) free flight model kit produced my Dumas Products (www.dumasproducts.com). My inspiration for this project came from a similar conversion of this particular plane by Tom Hunt. Tom’s conversion was performed a few years ago, and I think it was completed before the arrival of the now well-known GWS (Grand Wing Servo) flight systems. Tom’s conversion included a 280-sized electric motor and larger control components, i.e. full size receiver and mini servos, and ended up with a model weighing in at a little more than 11 oz. My goal was to utilize the latest micro servos from either Hitec or GWS (5~6 grams each), micro GWS 4 channel receiver (~4.5 gram), micro GWS 2-amp speed control (~1 gram), GWS IPS-A (Gear ratio: 5.86) gearbox/motor combination, and a light weight battery pack (< 2.25 oz). With these components and building the structure reasonably light, my objective was to end up with a plane that weighed less than 9.5 oz. That might not seem like a very big difference when considering Tom’s model, but in the small radio controlled parkflyer sized electric models it’s HUGE!

Notice that in this picture there are no ailerons, we’ll get to that in a bit.

This picture is sort of out of order, but it gives you a good idea of how the plane is framed up. Quite a few sticks, but that is what makes it both light and strong.

This picture shows the Hitec HS-50 servos mounted on some ¼” X ¼” hard balsa sticks and the receiver and speed control Velcro to a 3/32” balsa plank.

The picture above is a picture of the method I chose to fasten the wing to the fuselage. I decided to make the wing removable on this model to gain access to all the control components and to change out the battery pack. The front of the wing is held down with a dowel that inserts into a mating hole in the fuselage and the back of the wing is held down with a small nylon screw. I chose a nylon screw to reduce weight. The nylon screw threads into a threaded brass insert that can be purchased at most hardware stores…


Here is a picture of the nylon screw and brass insert I used to hold the wing on…

The picture above shows a close up the GWS motor/gearbox combo mounted to the front of the fuselage with small wood screws. It’s important to make sure that the prop shaft has a few degrees of down and right thrust (when standing behind the model). With a 10x4.7 GWS prop and a 7.2 V battery pack the GWS IPS-A draws about 1.67-amp while generating about 129 grams of static thrust.  


The picture above is a snap shot of the 1st Bearcat flight. It sure was spooky.


This is where I get into what happened after the first flight without ailerons….


1st attempt:

First of all my flying spot has a nice concrete parking lot surrounded by grass.

I eased into the throttle and the Bearcat quickly picked up some speed, I then applied some elevator and it rose quite nicely off the ground. I gave it some left rudder to begin to turn and I immediately noticed that it liked to stay in the left turn. I gave it some right, then full right, and it started to come out, but I got scared and cut the throttle. About then it touched down just in time on it's wheels and rolled out. Whew! Almost really bad…

2nd attempt:

I thought I would increase the rudder throw to max to see if I just needed more throw. This time I decided to make a right turn after I got enough altitude and made the turn much wider. Great a successful turn…! So on the next turn I performed another right turn (slightly more aggressive) and immediately noticed that it liked to stay in the right turn as well. I applied some left rudder to counteract the right turn. This time I pretty much had the same response as the first flight and so I cut the throttle and it sort of hit wing tip first and then rolled out on its wheels. Whew again, only this time I sort of scratched up my pretty covering job.

3rd and last attempt of the night:

Frustrated and puzzled I thought I'd try one more, you know, the feeling where you just can't leave the damn thing alone. I took off, and it rose off the ground just perfect, then I applied some left rudder and sure enough same thing only this time I wasn't so lucky. I hit pretty hard and cracked only a few sticks so I had to quit for the night. I'll fix it up and try it again. At this point I'm pretty disappointed and confused, but by no means ready to give up. I'm like my dad; I'll tear it up before I make a wall hanger out of it!


I'm starting to think I need more rudder area. I put the stock amount of dihedral in it, so I don't think it's and issue of not enough, maybe too much?

The pictures above show the quasi-major surgery I had to go through to add ailerons. The next paragraph describes how I came to realize that ailerons were a necessity.


OK, so after some serious thought and talking to some of the guys on www.ezonemag.com, we decided that since the rudder on the Bearcat was located quite high in relation to the center of gravity of the plane, applying rudder in the opposite direction of a turn was aggravating the problem. One unfriendly aspect of tall vertical stabs is that you may experience an adverse yaw-roll couple. Think of it this way; rudder deflection is rotating the aircraft about the yaw axis (imaginary vertical axis that passes through the CG of the aircraft). However, this deflected rudder surface is mostly above the roll axis (imaginary axis that passes through the aircraft CG in a fore-aft orientation). The fact that the rudder surface is above that axis means that a roll torque also results when the rudder is deflected. That roll torque is actually opposite of the intended direction of turn (right rudder deflection imparts a right yaw but a left roll - adverse coupling).

Here are some pictures of the Bearcat in flight. Now that I have ailerons, she is a real fun plane to fly.

I posted some video of my converted Dumas Bearcat on www.silentflightonline.com
You can go to the site and click on imagebase, then Parkflyer-Inflight Photo/Video, you can sort through the files until you find my Bearcat.