Chinook, RSAF's CH-47SD? Converting it to a plane or an autogyro

8 Aug 2015

A piece of foam was carefully drawn and cut. The motor mount was made with about 3 degrees right thrust, the board was mounted with additional foam pieces and motor glued.

The assembly was then glued to the top of the wing and I found that I glued it out of centre. There's too much right thrust.

I used pull-springs because the tail moment was great.
I used 4x1cm tabs of pvc plastic sheet for the springs. I bent the tabs midway onto themselves and superglue them to the surfaces; the surfaces are deflected to the desired directions.
Over-length cotton threads were inserted to the surface horns. The horns were glued in placed and small loops were made on the free thread ends, near the servo horns but about 4 to 5 cm away. Then the free ends were threaded through the servo horns, back through the loops and tied after the board was powered.

The Chinook was flown on 8 August 2015. First few flights would only turn right; to turn left I had to cut the throttle and apply left rudder and sometimes with a bit of down elevator. After the motor was knocked loose, I reglued it straight and that seems to work, I was able to fly left and right. The CG had to be infront of the centre of the front disc. I velcro on additional battery just for the nose weight.

One battery lead broke from the board and the Chinook was retired. It wasn't really flight trimmed but I think I had enough from the few flights.

Observation of Pull-Springs

The torque from the 1cm wide tabs will move the servos if board is not powered. With power on, one of the servos emitted sound, the torque should be reduced. The tabs could be made narrower. I had mounted the elevator spring tab so that the elevator would be up. On hindsight, this may not be good because the up elevator is limited to the torque and if the tab is weak, that might not be enough to bring the Chinook out of a dive. If the tab was mounted so that the elevator would be down, then the servo could pull the Chinook out of a dive, but if the thread broke, it would dart straight to ground.
The surfaces seems spongy, I think the cotton thread I used has a bit too much stretch.
The main advantage of pull-springs is its weight. I didn't weight it, but I think it is a fraction of a gram, very good for long moment arms as it is much lighter then pushrods of wire tipped sleeved carbon fibre rod.

1. Use this method only if the moment is long.
2. Mount the tab such that the elevator will be down. This will mean re-orienteering the WLToys board.
3. Use tabs narrower then 1 cm width for the WLToys board. The tabs could also be made shorter to save some weight. Additional tabs could be glued on to increase the torque if necessary.
4. Use kite string or any string or thread which is less spongy.

4 Aug 2015

I marked out the centres of the two discs on a sheet of 3mm laminated foam.
I marked out where the dihedral lines are to be and cut out the two discs with a circle cutter.
I cut through the surface of the discs, tracing the dihedral lines, thus forming a hinge.
I cut two templates that has the outboard at 45 degrees, pinned it to the discs and used superglue to hold the outboard panels at the correct angle.
An elevator was hinged and cut out from the rear disc.

The printout of the fuselage was glued (glue stick) to the foam. I cut out the fuselage, making both discs at zero degrees decalage and with the photo-print slightly inclined downwards. That way, the Chinook would look as if it is travelling at speed and it ought to compensate for the slight angle of attack of the two discs.

A rudder was cut out from the fuselage.

I slid and glued the front disc into the fuselage, then cut out and glued two bracing pieces before the rear disc is glued on. The braced discs was quite rigid. The rudder seems small but flight test would dictate its suitability.

It looks like this on the starboard side, upside down. The hingelines of the rudder and elevator are clear in this photo.

Flight Tests

I tossed it and it was evident that the CG is too far back.
I taped a 5gm weight and a 1s battery to the front disc, right at the front but the CG was still too rearward for a proper glide.

The next step would be to install the radio gear (5gm plus 8gm plus 5gm?) and pushrods (5gm?) for further glide and flight tests. If the rudder is too small (relative to the lateral area combined from the fuselage and dihedral), I could glue on rudder extension or make a new fin and rudder atop the rear wing.

 

30 Jul 2015

Or how about motor and board atop front disc with elevator cut out at rear disc? Perhaps make the fuselage with a slot to receive the front disc (the connecting foam would be obscured from view by the discs) or make a single gourd shaped planform to represent the 2 discs?

The flight gear would at least be in a good position for CG balancing. The downthrust would be lesser than if the prop was hung below, although the elevator pushrod would be as long as the rudder's. The view from bottom would still be good except for the bit just below the front disc (which could be hollowed up to improve appearance) and the starboard is clear from all workings.

Hey, is that a tandem Nutball?

Or how about making the Chinook into an autogyro? With the CG known in a plane configuration, replace the discs with reversed rotors, mount the motor and board to starboard side, operate elevator with the front or rear disc which is made pivotable?

29 Jul 2015

The photo is of the starboard side of a real RSAF Chinook. I prefer port side. I could only find this photo of someone's plastic model, probably a 1/72 scale.

This is what I shall do to make a RC plane that looks like a profile Chinook with disc wings.

1. Print in colour onto sheet of A4 paper, landscape.
2. Glue onto 3mm KT laminated foam sheet, cut to shape, make rudder hingeline.
3. Cut two discs from 3mm foam sheet to represent the whirling rotor discs, glue the rear disc parallel to the fuselage.
4. Make the front disc pivoting or elevator.
5. WLToys geared motor set on a CF rod with downthrust and side thrust to the nose.
6. WLToys all in one board with the servo horns arranged to give rudder and elevator action.
7. Test glide without propeller, shift battery to adjust or add lead weight to either end of the fuselage.
8. Test fly, see if dihedral is required. If required, try two segments to top of front disc, and if that fails, try to make it into a polyhedral.