DH84 Dragon

[Manish Chandrayan]

Trevor, I have seen the youtube video of your Rapide multiple times. Did not know about your blog till you earlier post. Beautiful models and nicely flown

[MattyB]

Posted by Chris Reid on 17/02/2019 17:56:35:

Thanks for the compliments guys.

I've done the weighing and power checks now. The latter proved a bit of a puzzle as I'm only pulling 210W on 7x6" props from both motors from 2x2200 3S lipos in parallel; my standard for larger models. The Hobbyking spec for the Turnigy 2863/8 1100Kv motors says they pull 336W each at 18A and 15v, which I should have spotted is rubbish, as others have recorded on forums various. The model weighs 4lb 12oz ready to fly which around 40W/lb, rather less than some of the tables suggest for good performance. However, as fellow clubmates and I have found, brushless motor watts go further than old brushed motors watts, on which the tables are based. The static thrust is 2lb 2oz which ought to be enough. It's a vintage biplane not a pattern ship!

I've done the CG calculations which are not wholly straigtforward for a swept winged biplane. I hope I've got it right. I just need a fine day and a friendly test pilot now. I'm a safe flyer but I'm not up to first flight out of trim surprises.

I have a lightweight foamie parkflyer Spad from HK that was originally designed for 2S, but because of their strange design decisions ended up on 3S. I put it on an diet to be able to fly it on 2S in small spaces, and it has ~45W/lb.

It is perfectly flyable (from a hand launch, ROG is impossible due to it's small size), but was very hairy on the first flight. Given how much time and effort you have put into building this beauty I would encourage you to think of a way to get at least 60W/lb for your first flights, whether that is by going up to 4S or fitting new motors. The additional cost will be forgotton when you see it soaring confidently overhead, but it would be horrible to lose it because of a powertrain miscalculation.

Edited By MattyB on 18/02/2019 17:32:43

[Chris Reid]

I've done some more work on the power front. I cut down a couple of 8"x6" props a bit to give me the largest prop that would fit in the nacelle/fuselage gap. A power check gave me 260W and almost 8oz more thrust which was the improvement I was looking for. Foolishly I then tried a 4S battery pack. The fresh batteries gave a surprising 500W dropping to 460W after 10 secods or so. More than enough, and not necessary, so I decided that 3S batteries would do for the test flying. I reinstalled the 3S batteries only to find that one of the 25A ESCs had given up - too many amps for too long! The ESC should have survived but it has taught me not to overpower cheap Hobbyking ESCs. A 40A one will be substituted. Even that's not going to be straight forward as the original ESC signal plug and socket join is buried in the wing. I will have to cut the signal lead off the busted ESC and join it to the new one. I have some Futaba plug and socket kits to make up a new socket so I wil probably do that.

This has delayed the test flying, but the weather has reverted to normal March so I haven't really missed any flying opportunities. I hope to get to it soon. If Dragon flies well the big decision will be whether I can face drawing up a decent plan for RCM&E. I don't have the energy I once had.

[michael harper 2]

Any news on test flight ?

[Chris Reid]

Hi Michael,

I've been a bit tardy in getting the Dragon airborne, although she is ready to fly. On one recent trip to the field my favourite test pilots were elsewhere. Brilliant weather this week, but family issues and a large garden have taken priority.

It will be soon.

[kc]

It's always worth waiting for the best test pilot for a really nice scale model.

Just noticed your interesting staple method for retaining the wings - my thought was what happens when the balsa wears a bit - will the staple drop out? Have you ever thought of putting a magnet to ensure the staple cannot ever drop out?

I am sure we are all looking forward to seeing the plan in RCME if that's any incentive for you to draw the plan......

[Chris Reid]

I'm not too worried about wear in my wing link staples holes. I toughened them with some cyano before covering, and I don't plan to fly inverted so gravity should keep them in! If the holes get too sloppy, they can be plugged with a balsa dowel and re-drilled. When I used carbon rod and tube wing joiners on other models, it was pretty obvious that friction and the flight loads kept the wings in place, and a rubber band and hooks link was pretty superfluous. The Dragon centre section didn't lend itself to a rubber band link, hence the staples.

I've already committed in principle do a plan for RCM&E, and I've written build notes and taken photographs as the project went along. I've been waiting for successful flight testing before contacting Graham Ashby again. As I've aged, the time to get round to these jobs seems to increase as the incentive to do it declines. If I do it it may be my last big design effort.

CR

[michael harper 2]

Offer still stands if you want it drawing up, even hand drawn on drawing board.

[Chris Reid]

Very belatedly, a test fly attempt has been made. Not without problems however. The tail wheel on the model is linked to the rudder by small rubber bands between two hooks. This ensures that wheel loads are not transmitted to the rudder servo. On the day, one of the bands failed leaving the tail wheel connection rather sloppy. Unwisely, I let my test pilot have a go anyway. The model revealed a strong swing to the left on take off which full right rudder did not control. When the model lifted off, it turned hard to the right and dived in; fortunately with no real damage.

Much food for thought. I am going to try some high speed taxy trials with the tail wheel properly connected to see if the swing is controllable. If not, I'll maybe fit a pusher prop on one motor and run it in reverse to even out the torque effects. The trouble with a twin is there is not much prop wash over the rudder before it has enough airspeed to work. I am also going to add some weight to the nose, as the leap into the air looked a bit like aft CG to me.

Finally, I'll contact my full size Dragon pilot to see if the real thing swings on take off. A lot of twins of the period did. Not too much of a problem on a grass airfield as one could start the take off run out of the wind and swing into it! There are lots of Dragon models flying, so a fix must be in there somewhere.

[Chris Bott - Moderator]

That's great to hear that it's OK, Chris.

It sounds like you're considering just the right things to sort it out.

Good luck for next time and keep keeping us posted.

[Chris Reid]

Some more small progress. Small regluing of the pilot's seat dislodged by the test flight minor prang. Two oz of lead added to the nose to move the CG forward a fraction - the wings are straight but slightly swept aft of a tapered centre section, which made the result of my original graphical calculation a bit uncertain.

Finally, I bought two APC 8x6 electric props, one of which was a pusher. They needed 1/8" trimmed from the tips to clear the fuselage. I switched the leads on the starboard motor to get reversed running and put the pusher prop on that side. It looks good. The motors are now turning inwards, each at 8,700 revs at full power, pulling 21A from the 2x3S lipos in parallel. That's 215W, giving 2lb 6oz of static thrust- 6oz more than the original 7x4 props. That's around 50W/lb, enough for a scale 30s biplane.

With any luck that should fix the swing at take off. De Havilland did the same, ie reverse one prop, with their Hornet for the same reason: the Mosquito was a bit of a beast on take off, it needed a lot of rudder and differential throttle to go straight, and easily got out of hand. When it stops raining and warms up a bit, I'll give the Dragon another go.

Merry Christmas to all my readers.

[John Rickett 102]

Having completed a DH84, I'm naturally interested in the aeroplane and in browsing the web for relevant information, I spotted this thread.

The previous post on the subject was over two years ago, however to reply and hopefully offer some help on how to overcome take-off swing with a twin (or multi) you could try mixing rudder with throttle. I've only tried it with i/c power but I believe there's no reason why it should not work with electric motors provided each motor has its own speed controller.

Taking a twin as an example, the principle is to set up throttle on two separate channels, then make each one a slave to the rudder. By reducing the slave output to say 10% and with it being positive on one channel and negative on the other, the result is for an individual throttle servo to advance a small amount in accord with the direction of rudder movement. So, for example the left motor will advance with right rudder and vice versa, if the rudder is left at the neutral position, then both throttles open together as normal.

If the radio allows, a master switch could be incorporated to inhibit the mixing once airborne, though in practice I've found the servo movement at the top end to have inconsequential effect on motor revs.

The amount of engine differential needed is minimal and only required until sufficient speed has been built up for the fin/rudder to do its job. I've used this method on 3 models now and found its very effective in being able to maintain the desired straight line.... it especially helps with taxing.

[Chris Reid]

Thanks John for your very interesting input. My own installation is pretty simple with the throttle control going to two ESCs via a Y lead. My very simple Orange TX has some mixing but not up to your ideas. I am pretty confident that my reversing one motor as described above will be a fix. That, and a good rudder/tailwheel link, which has always worked well for me before. However, my advancing years means that avoidance of other people to cut the risk of covid19 has prevented further test flying. I've recently had a first vaccination so hopefully I'll be able to go flying again soon.

Chris

[Denis Watkins]

Watching with interest Chris, and see that you have worked twins earlier.

The DH84 gives us the best chance of flying a scale twin, as the motors are as close together as is possible.

The lads at my field would try to fly my sport twin at the field and swing wildly, unable to complete a take off.

Of course, they saw me fly the model routinely and both started to investigate my technique !!

Their findings were simple. My thumb on the rudder already held a small input, and the throttle was teased

progressively slowly to higher revs, taking account of the ground-run into wind.

Just normal progressive throttle to have some dampening of surprises at lift off.

[John Rickett 102]

Pleased to hear that the vaccines are filtering through, though unless someone has their own private flying field, I doubt anyone is or will be for a while, enjoying any flying just now.

If you can't incorporate throttle mixing, the other simple thing you can do is to accelerate slowly, not holding the model at full throttle and then letting go. Getting the airflow over the fin and rudder with the least amount of prop-wash will help in combatting the swing.

If the model always wants to swing in the same direction (probably left) then you could also apply some right rudder trim before starting the run and take it out once in the air.

[Chris Reid]

Hi Guys,

Great that interest in the Dragon has revived after a too long break in test flying. Will it ever be safe to go out again?

You prompted memories of my earlier twins Denis, and I am aghast to find it was 19 and 17 years ago. The Islander was marginal on power with twin Graupner Speed 400s. It would take off slowly on tarmac but had no swerving issues. It had a nosewheel which probably helped. It became much nicer to fly later with brushless inrunners replacing the Speed 400s. Vroompire was much more lively on Speed 400s. Actually a bit fast for my reflexes at the time. Hand launched there were no problems.

Chris

[Denis Watkins]

I remember reading the Islander article Chris, it was, and is still inspiring.

Just look at that aeroplane, it has everything for us. Wide body, loads of wing, and proper vertical stabilizer with good

rudder authority. Horizon Hobby did a very tasty ARTF at one time.

Electric was so heavy when we started out and became feasible over a very short time, as you know.

At an earlier time, one of the competitive lads who flew electric racers, assured me that his speed 400s were only

good for 40 flights before performance fell off ?. Imagine that, as I bet loads of 400s are still flying sports models.

[michael harper 2]

Just ordered plans for the Islander from Sarik Hobbies.