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Oct 28, 2020, 08:10 PM
Slow Flight
Thread OP

My Corsair did not fly- not sure why.

Dear Builders,
Just finished my mini profile Corsair and it did not fly.
I must start mentioning that it was really windy .

weight : 47 grams
receiver : V911
Motor: 820 with geat ( i have read that thrust for this motor is 65grams)
Motor downthrust angle= about 3 degrees
Wing: KFM2 with step at 50%
WS =400mm
w chore=105mm

Batteries: 1s/ 150mAh and 200mAh

1. doesnot take off.
2.Not sure where is the CG on the KFM airfoil.
3. seems under power even at hand lauch, seem like floating with no clear direction and goes down.
4. Tends to turn upside seems that vertical center of gravity is above the wings...not stable .

Attemps to solve issue , none of them worked
- move battery behing LE under canopy
-hand lauch
-use a fresh charged battery

My own thougth of potential problems:
-Underpower, motor 820 does not have a thrust of 65grams
-receiver too high on the plane moves vertical center of gravity to high making it very unstable.
-get new batteries
-KFM2 airfoil do not help on this small model, a regular airfoil mat be better
-Motor should not have a 3 degrees of down thrust, should be just zero thrust angle
-Elevator too small
-Plane must flyonly in zero wind

I have a couple of bad video clips not sure if they provide any info since it did not fly .

Any comment is greatly appreciated.


Last edited by herreraa; Oct 28, 2020 at 09:02 PM. Reason: missing photos
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Oct 29, 2020, 07:38 AM
Hals und Beinbruch !!!!
lw_hughes's Avatar
I’ve never seen an airfoil like that. I think that’s your prob.
Oct 29, 2020, 07:50 AM
Slow Flight
Thread OP
Originally Posted by lw_hughes
Iíve never seen an airfoil like that. I think thatís your prob.
There is no much difference with a flat airfoil. But i will keep it in mind if i need to rebuild.
Oct 29, 2020, 08:07 AM
Slow Flight
Thread OP
this is the motor used.
Oct 29, 2020, 08:40 AM
Registered User
AntiArf's Avatar
I'd look at where the balance point is. Corsair's are difficult to balance, without everything forward, where the board may need to be fully at the nose.
Oct 29, 2020, 09:32 AM
Slow Flight
Thread OP
Originally Posted by AntiArf
I'd look at where the balance point is. Corsair's are difficult to balance, without everything forward, where the board may need to be fully at the nose.
Yes that is a problem i do not know CG, i moved the battery to front and to back and i could not make it fly. But i will test it again.
Oct 29, 2020, 10:35 AM
Registered User
perttime's Avatar
What exactly happened when you tried to fly it?

Jus eyeballing it, my first thought was that it is heavy or underpowered.
Oct 29, 2020, 10:53 AM
Hals und Beinbruch !!!!
lw_hughes's Avatar
Flat plate airfoil yes, but you have a huge step down on top of your wing.
Oct 29, 2020, 11:27 AM
Or current resident
glewis's Avatar
As AntiARF said, sounds like it's tail heavy. The test glide is your friend. If it glides, it'll fly under power. Make sure control surfaces are centered. Adjust CG until you get a decent glide. Was just looking at the pics and both ailerons are up slightly? That will pitch the nose up and would cause the problem too. Correct that first, then test glide.
Oct 29, 2020, 11:42 AM
Slow Flight
Thread OP
Originally Posted by perttime
What exactly happened when you tried to fly it?

Jus eyeballing it, my first thought was that it is heavy or underpowered.
Sorry about the upside video but this is the only one I have. It takes off just few inches just becuase was against strong wind. But i have no manufacturer information about thrust of this motor/gear to confirm is under power. Video has motor at full throttle.

If hand launched, it floats a bit going doing at full throttle.

corsair profile not taking off (0 min 15 sec)
Last edited by herreraa; Oct 29, 2020 at 11:50 AM.
Oct 29, 2020, 01:35 PM
Registered User
perttime's Avatar
Originally Posted by lw_hughes
Flat plate airfoil yes, but you have a huge step down on top of your wing.
That isn't necessarily a problem in itself. It is a Kline-Fogleman profile. Proven to work as a wing profile.
Oct 29, 2020, 10:36 PM
Registered User
Concur with glewis.

There are plenty of folks here who’ll love to sort out what’s keeping your Corsair down, but there’s not enough info to go on in the video.

Any way you could post a video of a few gentle tosses over high grass?
Oct 30, 2020, 12:19 AM
saucerguy's Avatar
Adjust your H-stab 1.5 degrees down, with the leading edge of it pointed downward or pitch your wing upwards. There are two main forces in play along lift, the angle of attack and the airfoil. Yours being a symmetrical airfoil you can only use pitch and in this case, it's the same thickness throughout the plane, it won't be the best along air displacement so it's not going to be as responsive compared to a tapered one, it certainly will not be creating lift on it's own regardless of it's velocity and I think these two things fundamentally are why this plane will not take off from the ground.
Oct 30, 2020, 01:06 AM
Registered User
I use exactly the same power system on several models, and I think it should be sufficient. 47g is a bit marginal for that span, and it would benefit for sure from being lighter. Make absolutely sure that both your batteries and the connection to the brick are sound. Is there another model that you are very familiar with that uses the same batteries? Just make sure that you get full power with that one.

Testing a new model in such a wind is not recommended - you need everything in your favour. Early morning is usually good, at least where I live!

I think if I were using that brick I would try to find a way of mounting it horizontally just under the wing. Not sure weight above that level is what you want, as you also suspect.

As has already been suggested, it's a really good idea to do glide tests in calm conditions to check CG. Apply pieces of blu-tac where required to get it to glide the way you want.

You don't mention right thrust - is there any?

Persevere - if nothing else you are going to learn from the experience .
Oct 30, 2020, 03:21 PM
Maltese sparrow
andrewhalst's Avatar
The geared motor combo should be suitable, but the propellor supplied with it is poor. Most of them are very much out of balance ( add tape to the back of the lighter Blade ) and I have heard several people I trust saying the pitch is too low.
I have not tried it myself, but they suggest hot water and twisting more pitch into the prop. The pitch must be the same on both blades, of course
I bought several “packs “ of these motors and also the 720 motors. They all use the same gears and are supplied with the same prop. So I believe the prop is designed with low pitch to suit the smaller motors.
So balance and repitch the prop, or change to another prop that suits the 820 motor better.
Have a look at
Who suggest a gws5043 for max thrust, but point out it exceeds 2amps ( should be OK with a 911 brick). GWS 5030 for less than 2amp . They do also sell the prop you have , but they may have sorted the balance and pitch issues. Perhaps this is where you got the thrust Figure from?
When the power is marginal, the down thrust is not necessary and is using some power in pulling the plane out of the sky.
I would also use a simple flat airfoil or curved plate, but in all honesty don’t know about the KFM airfoil in this size and speed of plane.
As many people have said - test glide. It’s what king size beds are for!
Last edited by andrewhalst; Oct 30, 2020 at 03:23 PM. Reason: Smiley not working

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