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Old Jul 12, 2010, 02:14 PM
Rangers Lead the Way
Joined Mar 2010
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Cheater Holes: when are they OK?

Flew my first EDF, a twin 55 Mig 29 from HC Hobby / Arttech. Flies OK, but not that fast. Has long intake ramps with motors positioned 2/3 towards the back and no cheaters. I also have a Eflite F-16 with similar motor placement and no cheaters. Haven't flown it yet.

It seems both applications could use cheater holes. My question is when and where these are appropriate from a performance perspective. I am not concerning myself with FOD and cosmetic considerations for the moment. This is purely a performance question. Do they always help power, or can they be a detriment. Can I always be sure they will add power, or is it a process of trial and error. Should they have a certain shape or be cut at a certain angle to the fan disc? Is their purpose to add power at all times, or just to get air to the EDF in high AOA situations.

On a similar topic, most ARF foamies I see place the EDF 2/3 the way back in the nacelle, thus resulting in a long intake and short efflux. Although I understand this is done for CG purposes, am I correct when I believe this is exactly the reverse of what should be the case from an ideal power perspective (EDF positioned 1/3 back from intake so you have short intake and long thrust tube area)?

Finally, is there some magic formula for thrust tube size?

Any help / knowledge / opinions are appreciated.
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Old Jul 12, 2010, 08:31 PM
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bmiller's Avatar
Newport Beach, CA
Joined May 2002
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Typically cheaters will increase low speed thrust(climbout) but top speed suffers.
Long SMOOTH constant diameter intake with a short exit is much better for performance.
Long exits suck more power than long intakes.
Yes there is a magic formula for thrust tube size. 90% of the FSA (fan swept area)is generally a great compromise between speed and climb. On a typical 55mm fan try an exit of 45-48mm. should be close enough.
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Old Jul 12, 2010, 09:26 PM
deltas are cool
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Old Jul 12, 2010, 10:00 PM
Pro Horder
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upland CA
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Only if the real jet had them!
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Old Jul 12, 2010, 11:31 PM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Uden Volkel, Netherlands
Joined Dec 2003
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Mig21 has them
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Old Jul 12, 2010, 11:57 PM
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Redmond, WA, USA
Joined Oct 2000
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What did you expect from the Soviets?
More seriously, the F-84s had something fairly similar to cheaters in the way of auxilary blow-in air intakes below the cockpit.
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Old Jul 13, 2010, 12:02 AM
Team EJF >>>WHOOSH>>>
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Alot of modern fighters also have variable geometry intake/exhuast...
BUT THEY'RE NOT CHEATERS!!! lol
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Old Jul 13, 2010, 12:03 AM
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Finland
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Yep, Mig-21 has a door on both sides which stays open until dynamic pressure on intake duct is strong enough to close the doors. So basically they help the engine have enough air while starting and taxing.

But the doors are not very big and they do not open much, so in EDF model their usage would not give much help.

I worked with Mig-21 during my military service and in ruggedness its way ahead of western planes.. (read:solid block of aluminium alloy..)
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Old Jul 13, 2010, 05:29 AM
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Canberra Australia
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You can see them in action in this video at 1.28. Not to be confused with the airbrakes at 2.00.

Soviet MiG-21 Aerobatic Team (2 min 54 sec)
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Old Jul 13, 2010, 09:37 AM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Uden Volkel, Netherlands
Joined Dec 2003
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http://www.afbcaslav.cz/med/foto/tec...1/mig21_03.jpg
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Old Jul 13, 2010, 11:42 AM
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TANGOSIERRAROM's Avatar
Beck Row Mildenhall
Joined Mar 2009
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An EDF propulsion system should be thought of as one entity, (intake, fan unit, thrust tube.) To make it as efficient as possible the intake airspeed needs to be quite close to the maximum level flight airspeed so as not to induce lots of drag and the efflux speed should be significantly higher. Usually this works out at around 90-110% FSA for the intake and 80-90% FSA for the thrust tube exit diameter. Most 55mm fans work well with 100% FSA intake, and an 85%-90% FSA outlets.

Make sure the intake is rounded unless you are going very fast, make sure the exit is a sharp cut edge to get good separation.

Cheater holes tend to limit top speed because they cause turbulence in the intake and also because they increase the intake area above the optimum. Blow in doors for take off and slow speed work might be useful but the weight penalty is seldom worth it.

Fan position within the duct is ideal at no more than 4-5 diameters of the exit diameter from the exit point so about 7" for a 55mm if it is a lot further then the thrust tube is better built as a 100% FSA tube and then a tapered tube for the last portion.

To go faster check the thrust tube exit diameter if already at 85% with a typical GWS or Hyperflow fan you will probably not gain a great deal by going to 80% as these fans struggle with back pressure once you go below 85% some of the other fans will work as low as 75%. You would be better looking at the motor and considering either more cells if it will take it, or a higher kV motor on the same number of cells.
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Last edited by TANGOSIERRAROM; Jul 13, 2010 at 11:48 AM.
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Old Jul 13, 2010, 12:29 PM
Rangers Lead the Way
Joined Mar 2010
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Thanks for the technical explanation Tangosierra.
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Old Jul 13, 2010, 01:57 PM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Uden Volkel, Netherlands
Joined Dec 2003
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Quote:
Originally Posted by TANGOSIERRAROM View Post
Cheater holes tend to limit top speed because they cause turbulence in the intake and also because they increase the intake area above the optimum. Blow in doors for take off and slow speed work might be useful but the weight penalty is seldom worth it.

How much weight penalty for doors? just a few hinges right? The Mig21 has only a few hinges and springs. The doors can be opened with an index finger (been there, done that)
I think many larger edf jets I have seen struggling last year to ROG need a different approach on the ducting designs. What is the use of restricted FSA for top end speed while it is barely able to take off from grass? I have seen Sparks and Elektras needing more runway on grass as a turbine jet.
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Old Jul 13, 2010, 02:49 PM
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Steve C's Avatar
United States, CA, Lodi
Joined Feb 2001
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Cheater holes make for short takeoff runs, but I have seen them hurt climb performance. Look at the Lander MiG 17 and how much better it climbs with some ducting in the new version.

Steve C
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Old Jul 13, 2010, 02:59 PM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Uden Volkel, Netherlands
Joined Dec 2003
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Then close the holes after take off. Much more effective as variable nozle right?
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