Scratch-built Twin Boom 36 Combat Airplane (sort of)

Once in a while I get this motivation to throw something together just to see what I'll do. I had a new (but modified) Thunder Tiger Pro 36 laying around so I thought I'd build an airplane around it. I began with a 63 3/4" Extra 300 wing and chopped 25" out of the middle. This left me with a 39 1/4" tapered cord wing having 351 square inch wing area. I connected the wing halves with a plywood joiner. The next thing to do was to build an engine pod and frontal section of the fuselage for a fuel tank. I built it around a 6 ounce fuel tank. The entire pod is made of  liteply in an octagonal shape for strength. The firewall is 1/4" plywood. The engine was machined as a radial mount so there wasn't any space wasted and the weight kept to a minimum.

The photo illustrates the octagonal shape of the firewall to strengthen the engine support. This is critical, and must be assembled correctly. The back plate of the engine pod is directly mounted to the plywood joiner. I included 5 degrees of right thrust and 1 degree of down thrust in the firewall.  The next item was to layout the tail section and determine the length, attaching points, and tail fin orientation.

The crossbar between the booms is there only to hold the booms in place while I do the math. I ended up with a 1:1 coupling ratio. This is rather short, and the airplane with be twitchy on the elevator even with the correct CG.. Before I coupled the booms I built the horizontal stabilizer and installed it on the booms. That way, I can zero out the wing incidence while attaching the booms.

I did end up lengthening the booms another 3 /12" inches to provide good balance management. After the booms were set I pre-sanded the booms to flow nicely into the wing on the top & bottom. The tail feathers were zeroed out in X,Y, and Z axis relative to the wing. The  2" straight ailerons were cut to taper towards the wing tips. This will soften the aileron effect at high-speed.

Here you are viewing the completed access hatch for the fuel tank. It has a tab on the front and two screws at the rear. This photo also shows the cowling completed. It's a very tight fit around the engine!

This is a relative comparison of the Twin Boom to an electric Edge 540. As you can see, the Twin Boom is shorter, wider, and thicker cord. You can also see the extreme short coupling. The coupling was lengthed after this photo was shot. I also added landing gear supports so I can taxi the airplane.

Covering this airplane was sort of difficult with all the components in place but it worked out OK. The topside is bright yellow and the bottom side is plum. I will mix some plum accents on the top.

This photo illustrates the accents and final assembly of the TB 36. Note: this photo was taken after the maiden flight! The first flight was rather exciting! I had way too much travel on the controls and WOW, was it tough to handle. After some re-tuning on the controls and adding exponential it flew on RAILS! The CG is very critical only tollerating about +- 1/4" variation!

This little bird is very fast and super agile. I wouldn't recommend it to anyone that doesn't have a fair amount of pylon and or combat stick-time under their belt. The roll rate is about 5 per second and it loops so fast you can't hardly follow it.

I added small winglets to help the low-speed handling. It glides in nicely but it doesn't slow down to much on the final approach. It takes a lot of runway to get it on the ground.

SPECS:
WINGSPAN: 39 1/4" tapered cord, swept-back leading edge
FUSE LENGTH: 30"
WING AREA: 351 sq.in
WING LOADING: 21 ounce 
WEIGHT: 3 pounds 5 ounces Ready to Fly
ENGINE: Thunder Tiger Pro 36

CONCLUSION:
The TB 36 got its 11th flight yesterday and a complete inspection. The radial mount was working loose from the engine and needed immediate attention. The screws were too short to properly secure the engine. I had to re-drill the crankcase making through holes and re-tapping the threads. I secured the screws with LocTite RED. This really ought to do the job. Other than that, the aircraft is in perfect health.

To give you some idea how agile the aircraft is, the HIGH RATE on the elevator is 3/16", and the airlerons are 5/16". The LOW RATES are 1/8" on the elevator, and 1/4" on the ailerons. Both High & Low rates have -35% exponential to keep it manageable. I have never owned an aircraft with such short control surface travel and respond as hot this one. The tipstall for all practical purposes is gone, thanks to the over-sized (flat plate style) winglets. The aircraft has an exceptional dead-stick glide rate and can easily be returned to the runway with adequate altitude.

The aircraft was flown on an assortment of propellers so I could determine which one gave best overall performance (with the mild-mannered 36 engine). None of the props approached supersonic tipspeed, and all the props provided acceptable performance. However, the best overall prop was the APC 10.5x4.5 "3D" style, followed by the APC 9.5x6 "Schimitar". The two Pylon props I tried were OK but disappointing for short take-offs.

Over all, the aircraft fles excellent. It has excellent speed but certainly not the fastest kid on the block, however an OS 46AX would correct that... The ground handling is superb and the take-off is short and aggressive. Its inverted flight is perfect, not needing any elevator correction. The landing is smooth and stable but it does require a fairly long approach to bleed off speed. The little Twin Boom 36 is certainly a keeper.

      
mark.fuess@comcast.net