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Nov 22, 2015, 04:50 PM
German Engineering.......
HugePanic's Avatar
Thread OP

Longer intake duct with sharp inlet


I am currently designing a MIG-21.
The edf is about 500mm from the intake. The intake tube is allways way bigger than FSA. Edf is 70mm

The intake itself is as sharp as i can get it, about 0.5mm.

Will the sharp intake reduce the thrust? I hope the long intake with a significant reductiob in area will straighten the airflow.

Are there aby experiences with long an sharp intakes?

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Nov 22, 2015, 07:19 PM
Classic jets rule
AIR SALLY's Avatar
I have made some long ones for a 70mm not to have a sharp LE on inlet ducts. Step back from scale and radius the LE.also some fans work better than others in long ducts. So you might have to try different ones to find best performance.
Nov 22, 2015, 08:03 PM
Mike Warren's Avatar
I have and have flown the K&A Models F-100 90mm that has both a long inlet duct and fairly sharp inlet lips. Probably .7mm radius & I'm sure it was not totally optimum but the plane flew just fine. Showed it at E-Jets (picture) a few years ago.
Now it was not a 70mm, it was an Aeronaut TF2000 on 8s, plenty of power to brute force the air into the intake. That only really applies to static because I think once flying a sharp inlet becomes much less critical.
Good luck,
Nov 23, 2015, 12:45 AM
German Engineering.......
HugePanic's Avatar
Thread OP
Thank you.

Static thrust is no problem, since I plan to start with a bungee rubber.

I just donīt want to waste any/much of the thrust.

I think i will do some bench testīs with a cardboard tube...
Nov 23, 2015, 05:20 AM
Registered User
Gordon's Avatar
I guess the nearest intake to your 70mm MiG 21 will be that of my 70mm Lightning. The intake geometry is described here:

The model would have looked silly with a radiused intake lip. The thrust on 500W or so was fine and the bungee-launched model model was lots of fun. Heck!! That was 10 years ago!

Nov 23, 2015, 07:01 AM
Registered User
You won't have any problems with the sharp intake.

My F-100 flies fine.
70mm fan, long intake tube, 700-800 watts.
Plenty of power for top speed of about 90 mph.
Nov 23, 2015, 09:51 AM
German Engineering.......
HugePanic's Avatar
Thread OP
Thanks for confirmation!

I would have hated the rounded intake a lot...

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