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Old Oct 18, 2014, 12:40 AM
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greenseaships's Avatar
Scottsdale, Arizona
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Build Log
A-7 CORSAIR: My very first EDF - EXTRA ugly!

OK, so I've been in the hobby since 2007 and have resisted EDFs almost with a passion. Pusher jet guy for life, right?

Well, the local Jet Rally fly-in is coming up next month and they don't allow pusher jets. Finally after so many years, I have been pushed over the edge. I'm here to introduce what I hope will be my very first scratchbuilt (or either for that matter) EDF jet.

This is a concept for construction that has been banging away in my head for years. It finally took the perfect alignment of creative drive, spare time, and deadline to make something happen.

It is a box-profile fuselage A-7E Corsair II. Some specs;

Overall Length: 48 inches.
Wingspan: 39 inches.
AUW: TBD (fuselage with motor, fan unit and ESC) weighed in at 19 oz.

Some details about the construction;

I use a special kind of light weight acid-free foam core board I get from my picture frame business as the primary build foam. It's lightweight, yet firm and takes to spraypaint and bending because it's sided in paper. It is about 3/16-inch thick.

The aircraft is to be powered by an EDF-motor unit that came out of an E-Flite F-86. Someone in my club gave it to me a while back. The E-Flite Sabre is about 55oz AUW and I'm hoping to keep this baby under 40 for sure!

Control will be by elevon alone. No ailerons or flaps. Nothing electrical in the wings at all in fact!

Wheels will be of the tiny "belly roller" type. Obviously, asphalt only takeoffs!

As this is my first EDF, I made some newbie mistakes I'm already aware of.

Chief among them is the motor exhaust efficiency. For example, not wanting to demand a lot of nose weight, I moved the motor/fan unit all the way up to the trailing edge of the wings. That resulted in a fan exhaust duct about 15-inches long! And it's only about 1/4-1/2 narrower in diameter between the tail end and the fan end of the duct. Finally, the steerable nose wheel gear is going to interfere with the immediate air flow inside the nose intake scoop.

THAT handicap I plan to offset by adding large cheater chutes in the sides of the fuselage under the wing, immediately forward of the fan unit.

SO- including nose steering and rudder, we'll have 4 servos in this thing. A 4S 3000+ mah lipo. The wing- while re-enforced with plywood spar- is very lightweight. Who knows- maybe I can keep her to within 35 ounces AUW.
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Old Oct 18, 2014, 06:47 AM
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good power pick just watch full th run time u should use carbon fiber spar
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Old Oct 18, 2014, 08:51 AM
EDF Jet Jam 2015 , May 28-31
Kevin Cox's Avatar
St. Louis Intl, Missouri, United States
Joined Jan 1997
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Great first effort!
A few things that standout, if you don't mind some constructive criticism

1 )The thrust/exhaust tube works best if it runs to the rear edge.
2 )The thrust tube should taper. The amount of taper depends on the fan swept area (83-90%)
3 )The fan swept area (FSA) is used for the intake as well. I don't see an intake tube, and that is fine but you will need to have a bell mouth (intake ring mounted to the fan to maximize the power.
4) The intake opening looks to small...back to the FSA, so a cheater hole may be required.

Not correcting these items will not make the project a failure....but correcting them should make it more successful.
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Old Oct 18, 2014, 02:10 PM
EDF rules... :)
AirX's Avatar
Joined Nov 1999
13,647 Posts
Following the build.
I have done some exploratory work on building a large scale version for 100-110 sized fan.

Cheers,
Eric B.

BTW, I usually use the aft end of the wing to locate my fans, and your right it will get the batts in a more favorable position to CG the aircraft.
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Last edited by AirX; Oct 18, 2014 at 02:13 PM. Reason: Added BTW
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Old Oct 19, 2014, 01:21 AM
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Scottsdale, Arizona
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Thanks, all!

As a matter of fact, yes, I'm adding a great deal more air flow to the fan unit in the form of those large cheater openings I mentioned in post 1.

I've tried to illustrate here where the openings will be in relation to the fan unit.

@Kevin Cox- Re: Suggestion 1-

What if I cut away the top and bottom panels of the 'box' so by default I'm putting the end of the exhaust tube at the very end of the fuselage, BUT I kept the side panels the way they are for a more scale appearance (from the side view)

Would that at least be helping matters?
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Old Oct 20, 2014, 02:54 AM
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Scottsdale, Arizona
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Disappointed.

The kind of batteries this motor is gonna want pushes my AUW to at least 45oz. Getting awfully darn close to the AUW of that original F-86 Saber. (Although I must point out that mine is a bigger model)

But on the up side, I installed the elevon servos and wing-mounting lumber tonight. She's almost got all her parts now.

AND I plugged in and fired up the motor. It was the first time I've ever run an EDF. Ever.
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Old Oct 20, 2014, 07:31 AM
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I've ever run an EDF. Ever watch out
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Old Oct 20, 2014, 12:16 PM
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Compared to the prop planes I usually fly, I wasn't prepared for the marked vibration through the airframe! Is that normal? Not the kind of vibration you can see, but when holding the model, I felt my fingers might start to go numb. I did notice that after I got above about 25% throttle, the vibration reduced.
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Old Oct 20, 2014, 01:01 PM
DELTAS RULE
corsair nut's Avatar
tehachapi, CA
Joined Jan 2006
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I normally don't have any vibes, but sometimes it takes some balancing and "clocking" of the blade/spinner. if its a lot of vibes, I suggest you attempt to balance before running it anymore. I always test run and set them up before installing them
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Old Oct 20, 2014, 01:41 PM
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Scottsdale, Arizona
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One thing I think of is the building material.

This is rigid paper-covered foam board. Some of you may have encountered the stuff at Michael's or something. I think it may be more sensitive to vibes than the normal foams you guys are used to.

If it turns out there's nothing wrong with the fan balancing, should I do something like wrapping the RX in foam? What about the servos?

I wish you guys could feel for yourselves. It's possible I'm making too much out of it, but vibration is a hard thing to put in terms of cold hard data and numbers. Just, ya know... a feeling.
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Old Oct 20, 2014, 04:32 PM
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EDF's run at full throttle most of the time that stop's vibration
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Old Oct 20, 2014, 04:55 PM
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Interesting point. I just ordered my 4S lipos in the mail last night. So I did the test run on 3S. Obviously I'm not able to reach the higher throttle settings, so I didn't get to see that.
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Old Oct 20, 2014, 05:31 PM
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Quote:
Originally Posted by greenseaships View Post
Compared to the prop planes I usually fly, I wasn't prepared for the marked vibration through the airframe! Is that normal? .
.
.
Vibrations/loud noise = unbalanced fan.

What I do (roughly): Mark the motor/shaft versus fan position.
Then rotate fan (next blade) to motor position and run the fan.
When I find it running smooth (minimum vibration). I'm finished.
(Kind of dynamic balancing).

You may also put motor (rotation part) and fan into a prop balancer
and balance it static.

Key to successful EDF Flying: Balancing fan / bench testing max thrust
and fine tuning COG, adjust throws
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Old Oct 20, 2014, 06:32 PM
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EDIT-

Having had enough time to think about Floey's description, I see that he was already explaining it in detail. SORRY AND THANKS, FLOEY!
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Last edited by greenseaships; Oct 21, 2014 at 01:06 AM.
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Old Oct 21, 2014, 03:46 AM
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Quote:
Originally Posted by greenseaships View Post
SORRY AND THANKS, FLOEY!
No big deal To describe it is easy it may be difficult to achieve ...

To do a true dynamic balancing You need expensive equipment
or use a lot of time (trial and error )
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