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Old Jul 31, 2015, 03:19 PM
BashaBolla is offline
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BashaBolla
United Kingdom, England, Derby
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Help!
Advice needed please - thrust line problem ??

Hi

I have a HobbyTopGun L39 Albatross.
90mm 12 blade ChangeSun fan running on 8s
Plane is balanced with correct C of G .
All wing and tail incidences appear correct.
I have flown this plane quite a few times....it is a great flyer ..lots of power but generally stable and predictable- with just one problem/issue :-
The issue I have is that when moving to full throttle the plane will climb quite rapidly( much more so than would be expected from increased airspeed)
This nose up attitude is easily curtailed by throttling back ....to about 1/2 throttle at which point the aircraft resumes hands off level flight but I cannot fly at high throttle settings without constantly feeding in down elevator to keep the plane level.
I have dived the aircraft at 1/2 throttle to see if increased speed causes the nose up attitude. This does not appear to be the case ..increased speed in a dive does not seem to affect the flying characteristics.
The only thing that seems to cause the nose up/ climb attitude is going to high throttle .
I have checked the thrust line of the motor and it appears to be all ok. The efflux tube is standard for the plane ( i.e. its a foamy and has no modifications from standard to the fan fittings or the efflux tube )
I have considered mixing in some down elevator when the throttle is opened but my radio is Futaba 7 which is a bit rubbish when it comes to programming mixes ..and I would need only a very small exponential down trim change to come into play only from about mid throttle up ( model is already quite elevator sensitive )..so I have shelved this idea in the hope there is another solution.
I am baffled as to why this is happening....something to do with the efflux pipe shape on the L39 perhaps ?
Can anyone shed any light on what might be the cause of this and what I can do to remedy it ?
Thanks !
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Old Jul 31, 2015, 03:35 PM
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Sounds like it has something to do with the angle in which the exiting exhaust is pointing. My guess would be that the angle is pointed slightly upwards so when more thrust is applied (greater throttle) it causes the rear of the plane to push downward which in turn causes the nose of the plane to raise upward. Or how about mixing error? Does any of the control surfaces move at all when applying full throttle?
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Old Jul 31, 2015, 06:32 PM
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Quote:
Originally Posted by YokohamaSoGo View Post
Sounds like it has something to do with the angle in which the exiting exhaust is pointing. My guess would be that the angle is pointed slightly upwards so when more thrust is applied (greater throttle) it causes the rear of the plane to push downward which in turn causes the nose of the plane to raise upward. Or how about mixing error? Does any of the control surfaces move at all when applying full throttle?
This sounds like the correct analysis to me.

Remember that in the full-size world of aviation, pitch for speed, throttle for altitude.

In the model aviation world we reverse this so throttle controls air speed and elevator controls altitude.

The reason why model airplanes are reversed is to lessen the workload on the pilot so the elevator doesn't need to be re-trimmed every time the throttle position is changed.

In propeller-driven models, it's simply a matter of adding down-thrust.

But, as you correctly point out, in jet models, the thrust line is pushing through the model from the back to the front. So, if the thrust line is pushing below the vertical center of gravity, then the nose will rise. If it's pushing above the vertical CG, then the nose will be forced down.

In fighter jet models, it's best to have the thrust line exit straight out the back of the model.

You could mix elevator to the throttle trim, but there will be a slight delay until the airspeed catches up with the throttle setting.

You could make a new two-piece tail pipe that alters the thrust line.

Or, you could simply put a vane off the back of the tail pipe that redirects the efflux so it's inline with the fuselage center line.
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Old Jul 31, 2015, 06:50 PM
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My father and I have the same model and also experienced the pitch up with power. If you look carefully at the upper portion of the exhaust it has an upward curve which is what causes the change in thrust line from the rest of the tube. We simply glued some plastic sheet (about the size of a credit card) to make the curved section run straight out the back rather than curve up.
Problem solved. No other mixing or changes needed.
Cheers
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Last edited by 4stripes; Jul 31, 2015 at 06:58 PM.
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Old Jul 31, 2015, 06:57 PM
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Here is a picture of the installed plastic card.
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Last edited by 4stripes; Jul 31, 2015 at 07:09 PM.
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Old Jul 31, 2015, 08:57 PM
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Essentially, the thrust lines should pass through the balance point. Usually we think of the balance point in one dimension only when in reality, the BP is located in two dimensions, both the longitudinal and the vertical. If one built a plane with as symmetrical wings, the location of the BP would be defined in three dimensions.

If the thrust line passes through a point above the BP, a turning moment is generated. This moment is mathematically equal to the thrust in lbs multiplied by the distance, in inches, above the BP. This turning moment will drive the nose of the plane downward. More thrust, more turning moment. The opposite holds true if the thrust line is directed to a point below the BP. In this case, the turning moment will force the nose of the plane upward.

An easy fix has been previously described, that of using a bit of card stock to alter the thrust line. This would be a trial and error effort. A more complex solution would be driving a movable vane with a servo/ gyro setup. The elegant solution would be to calculate the BP in two dimensions and adjust the thrust line accordingly.
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Old Jul 31, 2015, 09:01 PM
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The model is designed correctly with the proper thrust line. Unfortunately the last bit of the exhaust was rounded upwards near the top.
This model doesn't need any change.
Shame it isn't available anymore. Super model.
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Old Aug 01, 2015, 02:34 AM
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BashaBolla
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Hi All
Many thanks for your comments/advice.
I am glad to hear that this problem is not unknown.... I will give the plastic insert a go ( all makes sense to me )
Special thanks to 4stripes ( and thanks for the photos ..very useful !) . I have to agree that it is a shame that the model is no longer available . I love mine so much I managed to snap up a spare which I am keeping in storage in case anything happens to the current one ! .
Have a look at the JepeJets website. Jepe mentioned a while ago that he is re introducing the L39 ( same model as the HobbyTop Gun one but in pure white foam ) ..and it looks like its on the way !
Richard
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Old Aug 01, 2015, 09:16 AM
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http://www.jepejets.com/jepeshop/fas...-albatros.html
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Old Aug 02, 2015, 02:17 AM
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Can anyone shed any light on what might be the cause of this and what I can do to remedy it ?

Interesting to hear that, the angled exhaust is one of the reasons. I fly mine on hot 10S set up with no characteristic you mention. I redesigned my exhaust as in the pic. attached.
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