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Motor Thrust Lines

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Tosh McCaber19/02/2020 11:33:58
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I have a question re Theory of the Flight! In particular- thrust line!

I have inherited a Clouds Fly motor glider. It comes pre-built with the motor mounted high up on a pylon behind the wing, thrust line being pretty well zero. When the plane is being thrown out on a launch, it immediately noses down, which has to be corrected by a fair amount of up elevator until it get further up, after which it seems to fly pretty smoothly without elevator correction.

My theory is that there is no correction for the motor being higher up on the pylon, and the motor is pushing the. Should the motor be thrustline be adjusted so that it is canted downwards towards the CG? Or is there something else at hand causing this phenomenon?

Any theories gladly accepted!

Thanks

Peter Miller19/02/2020 11:46:20
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The best advice I can give is buy Kermode's "Flight without Formulae" a very old book but so good that it is still in print.

When the trust line is a long way from the centre of drag it needs to be altered to allow for this.My best example was my Lake LA4 amphibian. Mine was control line but it still applied

The engine is high mounted on a pylon and pushing. The engine was pointing down,i.e up thrust.

When I applied power the nose went down, when I chopped the power it came up but then once the model had accelerated or decelerate it flew straight an level again.

This showed that the engine was well above the centre of drag but that it was balanced to suit in steady flight.

Kermode can explain and demonstrate this very simply

Denis Watkins19/02/2020 11:53:45
4168 forum posts
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Peter beat me to it

And without theory and just observation, as Peter suggests, both my pylon pushers are

Angled down pointing to the nose, thus pushing the wing down, but flys straight and level

Peter Miller19/02/2020 12:06:41
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In effect this is UP THRUST!!

Simon Chaddock19/02/2020 12:43:37
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It does depend on how far above the centre of drag the motor thrust line is.

If the pylon mounted motor is above the middle of the wing (many removable power pods are) it is simply not possible to set the motor thrust line to go through the centre of drag or it requires such a steep angle that the forward component of the thrust is reduced.

If this is the case you just have to live with the power on/nose down pitch effect. A solution may be to hand launch with reduced power and keep the plane level until the speed has built up before applying full power to climb away.

Don Fry19/02/2020 15:18:26
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I've got one of these. If you launch it full throttle, the motor pushes the nose down, and with no speed over the wing to provide lift, it nearly hits the ground, needing a good yank on the elevator to get the nose up. Very exiting. Once it's up to speed, it will climb well on power.

If, however, you launch it at about 30 % power, in goes away without fuss.

Tosh McCaber19/02/2020 17:25:45
24 forum posts
5 photos

Thanks for the explanations guys! I went onto the internet to order kermode's book. Hmm- £82.00 for the paperback edition, (but free delivery!!)- quite a bit for an "official" answer to the question!. However, I then went on to Amazon Marketplace- a much more reasonable £8.00, including postage, for a second hand"good" copy. So it's ordered! I shall sudy it at length- I like that sort of book!

Simon- the pusher prop is about 50mm behind the wing TE.

Don- I find the same thing! In fact, that's the reason for my question. I think that the motor should indeed have "upthrust" with the motor pointing down towards the nose- which would stabilise the model to a greater extent under full or low throttle?

Next question- how much upthrust??

Edited By Tosh McCaber on 19/02/2020 17:28:02

Peter Miller19/02/2020 18:23:04
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Posted by Tosh McCaber on 19/02/2020 17:25:45:

Thanks for the explanations guys! I went onto the internet to order kermode's book. Hmm- £82.00 for the paperback edition, (but free delivery!!)- quite a bit for an "official" answer to the question!. However, I then went on to Amazon Marketplace- a much more reasonable £8.00, including postage, for a second hand"good" copy. So it's ordered! I shall sudy it at length- I like that sort of book!

 

Edited By Tosh McCaber on 19/02/2020 17:28:02

Go to Amazon, Only about £10.

Edited By Peter Miller on 19/02/2020 18:24:03

Don Fry19/02/2020 18:46:00
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Posted by Tosh McCaber on 19/02/2020 17:25:45:

Thanks for the explanations guys! I went onto the internet to order kermode's book. Hmm- £82.00 for the paperback edition, (but free delivery!!)- quite a bit for an "official" answer to the question!. However, I then went on to Amazon Marketplace- a much more reasonable £8.00, including postage, for a second hand"good" copy. So it's ordered! I shall sudy it at length- I like that sort of book!

Simon- the pusher prop is about 50mm behind the wing TE.

Don- I find the same thing! In fact, that's the reason for my question. I think that the motor should indeed have "upthrust" with the motor pointing down towards the nose- which would stabilise the model to a greater extent under full or low throttle?

Next question- how much upthrust??

Edited By Tosh McCaber on 19/02/2020 17:28:02

Tosh, nowt wrong with it. If up put upthrust in, it will self loop when it gets up to flying speed. If you want a machine that is thrust neutral, get a Yak or similar aerobatic model. Moter in line with the wing.

Stuff like the cloud fly is a glider, with a moter, and fly it as designed. It's a nice little flyer, does twenty minutes on a knackered lipo. That's good if you have found some little victims (AKA visiting friends children) to teach the basics.

Simon Chaddock19/02/2020 20:13:55
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Tosh

I found this side view of a Clouds Fly fuselage and marked the likely centre of drag (horizontal line) and the CofG position (vertical line).

cloudsfly

To achieve a true neutral thrust line the motor would have to be angled down to where they cross. At such an angle you would loose nearly 1/3 of the horizontal thrust with the difference now pushing down on the wing so it would have to fly a bit faster to compensate.

You could try angling the motor to point down a bit as this would tend to reduce the pitch down effect on launching without effecting the flight performance too much. This is what is done on the similar layout but more powerful Bixler.

Gary Manuel19/02/2020 20:39:19
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The other problem with adding down thrust is that during normal flying, the propeller would not be cutting through the airflow at 90 degrees as intended. This would result in a loss of efficiency and an increase in propeller noise and vibration.

It may also reduce the rudder and elevator efficiency as the airflow over the control surfaces might be affected.

Edited By Gary Manuel on 19/02/2020 20:45:49

fly boy319/02/2020 21:03:05
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3628 forum posts
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Hope I do not cloud the issue, I fly ic and by just looking at the firewall you can see if up/down or left /right thrust has been used. Is there a way to spot it in this particular configuration. Thank you

Peter Miller19/02/2020 21:06:13
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I think that you will find that the thrust line does not have to intercept the centre of drag. Kermode will have that answer

PatMc19/02/2020 22:27:37
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Posted by Gary Manuel on 19/02/2020 20:39:19:

The other problem with adding down thrust is that during normal flying, the propeller would not be cutting through the airflow at 90 degrees as intended. This would result in a loss of efficiency and an increase in propeller noise and vibration.

It may also reduce the rudder and elevator efficiency as the airflow over the control surfaces might be affected.

Edited By Gary Manuel on 19/02/2020 20:45:49

The prop doesn't "cut" through the airflow, it drags the air in front of it from all angles then ejects it behind in a spiral flow. In essence the mean airflow pressure is exerted evenly over the prop.
The effect of the spiralling prop slipstream is well known & is the reason for engines/motors being mounted with sidethrust is some models & offset fins on many full size aircraft.

Peter Miller20/02/2020 08:31:15
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Posted by Peter Miller on 19/02/2020 21:06:13:

I think that you will find that the thrust line does not have to intercept the centre of drag. Kermode will have that answer

To expand a little and clarify . The angle is to balance the effect of the difference between the centre of drag and the centre of thrust.

Where the centre of drag is pulling the nose up it only needs a little down thrust to pull the nose down to counteract that leverage

Tosh McCaber21/02/2020 09:54:16
24 forum posts
5 photos

Thanks guys, for all the replies and interesting discussion and opinions on this.

I await Kermode's book with interest, Peter!

Tosh McCaber24/02/2020 08:50:37
24 forum posts
5 photos

Hi Peter,

I've just received Kermode's book, full of great information. It'll take me a while to digest it. I would be most obliged if you could point to the page(s) that you're referring to, relating to thrust line?

Thx

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