Mig 21/J-7 70mm edf scratch build
- Thread starter Funky
- Start date
L Edge
Master member
Make sure you expose a surface of the ESC to the ducting airflow for cooling.
telnar1236
Elite member
To chime in, even "fast jets" will fly fine with a TWR of less than 1. The rule is that to fly level, your TWR must be equal to your D/L and that any excess power can be used to climb. So, like quorneng said "light planes" have a better chance of flying with a low TWR because they tend to have more aerodynamically efficient designs. In contrast, on delta wings (like on the MiG 21) the L/D ratio gets pretty abysmal at higher angles of attack, so you do need plenty of thrust. The compound delta you're using is probably more efficient though. A good rule is that you don't want to go below a 0.8 TWR and that a higher TWR is always desirable.
Another thing to consider is that the EDF duct can result in significant losses in thrust. The measurements we get on product pages for EDFs are in ideal conditions, so I wouldn't be surprised if your EDF only outputs 800-900 grams of thrust or less once installed. The shock cone on the MiG 21 also kills inlet efficiency so it might not be a bad idea to design in some cheater inlets. On your model it looks like the shock cone has a flat back, so you might also get more efficiency by making a second cone to make the back taper back to a point.
This looks like a great project and the pictures of the cockpit especially are pretty neat.
Another thing to consider is that the EDF duct can result in significant losses in thrust. The measurements we get on product pages for EDFs are in ideal conditions, so I wouldn't be surprised if your EDF only outputs 800-900 grams of thrust or less once installed. The shock cone on the MiG 21 also kills inlet efficiency so it might not be a bad idea to design in some cheater inlets. On your model it looks like the shock cone has a flat back, so you might also get more efficiency by making a second cone to make the back taper back to a point.
This looks like a great project and the pictures of the cockpit especially are pretty neat.
telnar1236
Elite member
The CG should be at about 15% to 25% of the wing mean aerodynamic chord. You can do it by hand, but there are also a ton of calculators online that make the process way easier.
Aircraft Center of Gravity Calculator
Calculates Plane's Center of Gravity (CG), the Aerodynamic Center (AC), Mean Aerodynamic Chord (MAC), Neutral Point (NP), Wing Loading, Wing Area and Stall Speed
rcplanes.online
It's been a while because I was waiting on parts but the edf, all servos, and control arms are installed. Just need a 50 amo esc before I can fly (thought I had one but I dont). I 3d printed r 60 missles for decoration. I'm going to fly it a couple of times and then decide weather I want to add flaps and gear or not. The current turtle deck is temporary until I have the esc in. It's big enough to be hard to get in frame while taking pictures
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Mr NCT
Site Moderator
Looks great!
telnar1236
Elite member
Looks amazing, good luck with the maiden flight!
Maiden didn't go great but it's still mostly intact. The edf im.using should put out around 1100 g of thrust in perfect conditions and the plane is around 900. After first attempt I put in cheater inlets but that still didn't work. I have the recommended battery and esc for the edf so is there any other ways to get more power?
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telnar1236
Elite member
I know the feeling. A lot of EDF advertising is for the unit outside of a duct and doesn't actually represent the thrust it will have once installed. Do you have some way of measuring the thrust? The first thing is to try making the cheater inlets bigger. You need a total inlet area that is larger than the area of the fan, and having plenty of extra area will help. Something else that could help would be fairing in the back of the shock cone. Having a point on the back will reduce the losses from that section of the duct quite a bit. Finally, I see something in the duct in front of the fan in one of your pictures. Not sure if it's possible to get rid of it, but anything in the duct hurts efficiency.
Another thing to check is if you're launching it fast enough. Delta wings can make a ton of lift at high angles of attack, but they also make a ton of drag, so make sure you're launching fast enough for the wing to start flying.
Beyond that, you might want to look at ways to reduce drag since that is what the thrust needs to overcome. Your scale cockpit is beautiful, but also probably pretty draggy, so you might consider a canopy you could put on in flight and take off for display. You could also maybe cut some drag on the bottom of the wings. But unfortunately foam board planes just have a ton of extra drag.
L Edge
Master member
L Edge
Master member
It is a shame it didn't go. You have one more option. That is, try it with a cheap landing gear. You can find out 2 important things. First, it will tell you if your wing is capable of lifting the plane in the air and secondly, the position of the CG. Can you do that?
Example, I designed DarkStar out of Adams Readi board, and at full throttle-T/W=1. Landing gear was installed and was unsure of CG(Calc programs differ) as well.
First run showed lift and aft CG. It wanted to flip nose high.
After numerous dry runs of moving battery forward, finally got it to lift off, fly level, and land. Notice it never went above a foot so how could I loose it. My CG was off by 2 and 1/8 inches(spline?). Imagine chucking it or going top speed and lifting off for it the first time. Been there, done that.
Here is the maiden flight in a 350 square ft area. Now have 22 flights on it.
So, I suggest anyone who designs an EDF or EDF's model either try using entry #9 rules or the suggestions listed above.
Example, I designed DarkStar out of Adams Readi board, and at full throttle-T/W=1. Landing gear was installed and was unsure of CG(Calc programs differ) as well.
First run showed lift and aft CG. It wanted to flip nose high.
After numerous dry runs of moving battery forward, finally got it to lift off, fly level, and land. Notice it never went above a foot so how could I loose it. My CG was off by 2 and 1/8 inches(spline?). Imagine chucking it or going top speed and lifting off for it the first time. Been there, done that.
Here is the maiden flight in a 350 square ft area. Now have 22 flights on it.
So, I suggest anyone who designs an EDF or EDF's model either try using entry #9 rules or the suggestions listed above.
I think the cg is correct because while it still had energy from my throw it flew it just doesn't have enough thrust to sustain flight. The missles fell of after the first throw so there is no extra drag under it. I have some weight saving I'm going to do as well as a narrowed exhaust cone. There is the wing spar that runs infront of the edf so I can't get rid of that. I'm going to add some plastic as a canopy to reduce drag in the cockpit too.
Got some plastic in place to reduce drag. It's a little ugly but it should work. Also reinstalled wings and elevators. I put battery and all electronics back on top and it weighs about 2/3 as much as before. Because it's now just one tube for the fusalage the intake area is also larger for the edf
