|Oct 11, 2010, 02:55 PM|
Having been meaning to post this for some time. Sadly the model is no longer flying.
The MiG21 is an old favorite of mine. I remember building a plastic 1/72 Airfix kit long ago as a young boy. I was toying up with either ordering a new Alfa MiG15 to replace my old worn-out example (almost 100 flights at the time) or the newly released MiG21. The MiG21 won this time around.
Onto the build.
[Photos 6 & 7] First step was to fit a bungee hook. I don't hand launch and the MiG21 won't ROG like the Alfa MiG15 due to the scale sub-fin. The hook is a 1.5mm piece of chromed wire glued to a plate made out of 3 laminations of the 3mm ply Alfa wrapped the box in. The wire is bent to a "U" shape, with a longer end forming the hook. The plate is drilled through for both ends of the wire, grooved (for the wire to rest in), shaped to the fuselage/duct curve, and fitted with 6mm depron front and rear streamlining pieces to make a teardrop shape (in plan view at least).
I chose a spot on the fuse between the nose and the CG - about 1/3 from the nose. It happens to be in the centre of a service panel moulding.
[Photos 8 & 9] Wing spar and servo framework (wing box) was then built. The die cutting is more die squashing, so the fit of parts is a bit ordinary particularly compared to the latest laser cutting. I tacked the bit in place with CA then built up some fillets with a bit of PU for strength. The wings have very slight andihedral, so the spar must be installed the correct way up.
[Photo 10] By all accounts this a very fast small model. I decided to go for a high visibility paint scheme, based on the Russian MiG21 aerobatic team. To save some weight the factory light grey paint was removed with methylated spirits. Lots of rubbing, and rubbing, and rubbing....
[Photos 11 - 13] Next step is to fit the wing box. The fuselage comes with slots cut out for locating the spar. The outline of the wing box fairing is then marked and cut out. The wing box is then slipped into place and the aileron lead run according to the instructions. Make sure it is the right way up so the wings have andihedral, not dihedral. Glued in place with PU, carefully removing excess foaming during curing.
[Photo 14] Wings and tailplane glued onto the front fuselage section using 30 min epoxy. They do not glue onto the aft fuselage. The wings self align, as does the tailplane on the flat plate spar. Remove all paint from the gluing area.
[Photo 15 & 16] A bit of detailing flair. The MiG21 doesn't look right without the tailplane counter balance weights (bamboo skewers), nor the prominent nose probe (carbon tube with a balsa fairing). Not the world's best attempts, but better then nothing (perhaps). Both glued on with 30 min epoxy and microballoons.
Power system is a Don's Wicked 4800kV outrunner, a drop-in fitment to the Alfa fan unit. ESC is a 40A Lightning/Pentium 40A with switchmode BEC. Alfa five blade fan is also fitted. Test power figures were obtained using HC Flightmax packs. Better quality packs should deliver better performance and less voltage sag.
1800 3S 40C pack
1800 4S 40C pack
44.1A - will need some throttle management
Finished weight was 578g with a 1800 3S 40C pack, 629g with a 1800 4S 40C pack. It's no lightweight, but is somewhat better then the 400W/kg rule of thumb for solid EDF flight. Most of the weight is in the painting as I had some awful coverage issues with the red paint.
Maiden flight was an eventful affair. The bungee launch was brilliant, it went off like a rocket. But in the air, on the recommended throws, it was almost uncontrollable. And fast, even on 3S. Any touch of the ailerons had it in a knife edge and rolls were a mere twinkle in the sky. It would also do the typical delta trick of dropping the nose and losing a lot of height through "bank and yank" turns. Which were just about the only turn I could manage
Only needed a bit of up trim, but a lot of exponential is required (50%?).
There is a lot of empty blue sky in the video as it was hard to follow - even for the pilot! Hence the video is mainly just of the takeoff and landing.
Unfortunately on the second flight it came off the bungee a bit shallower. Not really sure why and this is the only bad bungee launch I have had in over two years of use.
I really should have just let it glide in as it had plenty of airspeed, but instead fed in up elevator which killed the airspeed. Then instead of feeding the throttle in I slammed it to full. Predictably (in hindsight) the MiG immediately pitched up, just as Alfa warned it would, dropping the last little bit of airspeed there was. So it did around 5 seconds of high alpha, low altitude hovering before rolling onto its back and crashing in from about 6m altitude.
I don't know if you have seen the video of the demise of Corsair Nut's tricked up GWS A4, but it was almost exactly like that.
So be warned, if the airspeed is slow, feed in the throttle only slowly otherwise it will pitch the nose up to about 60 degrees. Don't expect even 330W to save it
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