David’s DR 420 test flights

By | October 9, 2020

By today’s standards my new ‘DR420’ is a blast from the past – balsa and ply, with spruce spars, full length flaperons, and mechanical mixing. She’s also the first model aircraft I’ve designed in over 40 years!


Maiden – test flights 1 & 2 – Black Hill

Her ‘maiden’ consisted of two flat field test glides and two 40 min test fights in a gusty 6-12 mph:

I didn’t touch the flap function for the first 20 mins just used aileron and elevator and struggled to get above 30-40 feet in less than ideal conditions. Applied about 30% down flap and she was 80 feet up quite quickly.

Stalling straight showed no tendency to drop a wing and recoveries were easy and with little loss of height. She did sometimes drop a wing in turns but usually not. I couldn’t establish exactly why but suspect that variations in wind speed caused the inconsistency. Mostly she flew pretty well for a first attempt. I found that:

  1. Flap function made more difference to airspeed than altitude and no constant elevator input was needed. With full up flap she does come down gradually but gains considerable energy. At full down flap she flies close to stall speed but with careful handling is still well behaved. 25-35% down flap was best for climbing.
  2. The aileron response is disappointingly slow even at high airspeed. Increasing aileron throw a bit made little difference. I couldn’t roll her past about 80 degrees no matter what I did and even that took ages and height was lost. I expected to be able to do quick 90 degree banks and turns using up elevator but she was having none of it. Most turns were fairly close to flat.
  3. When I flew her rudder/elevator the flight characteristics were almost identical (even with very little dihedral).
  4. Remarkably, flap position made no difference to aileron authority and no difference to the likelihood of dropping a wing. That’s good.
  5. She looked majestic in flight and sounded great in passing.

Next Day – test flights 3 & 4 – Chinkwell Tor

Beforehand – I increased and aileron throw AND the up-flap throw but decreased the down flap (bearing in mind that these are full length flaperons not separate control surfaces). Conditions were far better, a much steeper slope with a much stronger wind than yesterday gusting 7-17mph at eye level .

In these conditions she flew like a different bird possibly because of the overall higher airspeed but the increased aileron throw was obviously making a huge difference. Mostly I just tested her responses and watched for tip stalls:

1) Aileron response was good at a wide variety of airspeeds and was not affected by up or down flap input. She still won’t do a complete roll but did pretty fast 80 degree banked turns (both into wind and downwind) and half rolls from inverted were no problem. She even did a half-decent Cuban 8.

2) Up flap (now about 30 degrees) response was much stronger and looked identical to down elevator input (it had zero braking effect – quite the opposite!). It worked brilliantly when more penetration was needed but for bringing her down I guess the next thing to try is up flap and up elevator at the same time(???).

3) No sign of tip-stalling in any sort of turn or at any groundspeed (but probably the airspeed was constantly higher than yesterday).

4) Rudder response is great. She can be flown rudder/elevator and does great stall turns.

She really was a great pleasure to fly in these conditions. To anyone reading this who has not had a go at ‘designing your own’ I would say have a go.

With traditional spruce spars, her thin wings are pretty flexible and at speed she literally ‘wriggled’ her way though strong gusts – like a falcon rousing her feathers in flight – quite something to watch. In recoveries from failed manoeuvres I pulled far more G than intended which greatly increased my faith in the light balsa wing structure.

I hope to further improve the aileron response by increasing the flaperon differential (reduce the amount of down aileron). Mind you, if it doesn’t work I’m still a happy chappie. If I’d wanted an out-and-out aerobatic glider I would have designed her differently.

I guess that’ll be my next project.

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