Mig-21 flops but mig-21 profile soars! - RC Groups
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Jun 13, 2003, 10:12 PM
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Mig-21 flops but mig-21 profile soars!

Ok well yes I'm still trying to get the Mig-21 flying. Last report it only managed to maintain launch altitude, and gradually descend. I tried lots of things to no avail so I decided to do a profile Mig-21. Other than the fuse everything else is the same and it flew pretty good. It was on a Mf-480 with an AP-29L so not stellar but a little dive would make a good loop.
So I think that means I have a ducting problem. So back to the drawing board I guess.
I did get a slight improvement by changing the nose cone. I had a diamond (but rounded) shaped cone in the front of the duct. I think it brought the FSA down to around 90% or less. So I made another that was basically a 3D profile or like a T. So in the air it looks like a cone but restricts a lot less airflow. The profile was a bit lighter too I'd say around 9-10oz but I don't have a scale. It feels lighter anyway. I think if I stick a brushless in the 21 it will go like stink though. No cheater hole as they bother me lol. It flew a little better with the new nose cone too. Very shallow climbs, and had to watch the turns as it wants to bleed speed. I think I'm going to open the ducting up some more stick with the 3D profile nose cone, and go with tailerons only to cut down on servo weight. I'm sure with a brushless it would fly great just don't have the spare change for a new brushless motor and controller right now.
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Jun 13, 2003, 11:11 PM
EDF rules... :)
AirX's Avatar
Pictures please...

Eric B.
Jun 14, 2003, 10:51 AM
EDF Head
Haldor's Avatar
What cellcount?
Jun 15, 2003, 12:57 AM
Registered User
Just popped on for a minute but I'll get pictures as soon as I get ahold of my camera. It's on loan to a friend, and well he hasn't been too keen on giving it back yet. I don't know why...maybe I don't want to know lol. Anyway it's on 10x1300's with a griffon 55 ESC and AP29L MF480 combo. It's using 4 mini servo's at the moment for simplicity of hookup. I think I'll take two out of the ailerons.
I've been real busy and the net is out at work so I get little posting time. It's supposed to be fixed soon though.
Jun 15, 2003, 09:07 AM
EDF rules... :)
AirX's Avatar
Cool... always have had a fond place in my heart for this plane.

Eric B.
Jun 15, 2003, 11:03 AM
Registered Useless
scififlyer's Avatar
That's a long hole to suck air through a MiG-21 fuse. Your DF probably thinks its trying to drink a milkshake through a straw in a blizzard! Also working against you is the loss of flow when you are turning (just like the real thing), the AoA changes the direction of the airflow away from straight on. Try to avoid hard turns, smooth over equipment in the ductwork, and use cheater holes for r&d purposes before investing in a new brushless (unless you already have one).
Jun 15, 2003, 01:43 PM
Registered User
Ok I think I've found the problem! I could kick myself really as it's so fundamental. Well it's only my second try at this scratch building EDF's so I guess I need some slack. First try with actual ducting last one was a V-1 buzz bomb.

Anyway I've got plenty of intake my problem is on the exhaust, and with my design . Scale wing, and 90% FSA on the exhaust and I know what the problem is. I don't think it's generating a high enough efflux to make it get up to speed. Because on handlaunch it just stays at the height you throw it with some very shallow climbs (can hardly tell it's going up).
I think due to the wing design it needs more speed to really get up on the wing, and go. Right now handlaunching is pretty easy and I wanted it to be. I'm going to try to make a thrust cone and reduce it slowly about 5% at a time, and see what I get. I've got plenty of ESC so I shouldn't burn it up. Besides I should hit the sweet spot long before I start really putting a strain on the motor.
So it'll be awhile before I figure it all out. Then hopefully I'll have my camera back too. I'll cut a new fuse with the correct extake diameter, and try tailerons only. If it goes good I'll go ahead and start doing things like making a canopy lol. Right now it sort of makes me thing of a flying cucumber lol.