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Apr 05, 2010, 03:14 AM
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tuootal's Avatar
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Discussion

pylon motor thrust angle?


Gentlemen,

I have been trying to find an definite answer to question of thrust angle of motor for large scale glider when attaching motor to end of pylon. The normal practice in motor models is to point motor slightly (3-5 deg) down. In gliders with motor pylons I have seen both up, down and straight. And in some cases rather extreme angles.

I took the liberty to copy an pic from this wonderful page http://gprc.free.fr/construire/pylon.../pylonfixe.htm to describe one rather extreme example.

Questions :
- What should be the thrust angle for motor for 4-6m scale glider.
- Why?
- What are the pro's and con's of different solutions?

I guess that the question of motor direction (prop facing front or back) is irrelevant.

Tuomas
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Apr 05, 2010, 05:21 AM
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Tuomas,

I've not done this yet, but looked into it. The issue is how to prevent the model nosing-over as it starts its take-off run. Most motors I have seen point up by about 10 degrees relative to tailplane datum. Perhaps make it adjustable?

A small nosewheel (as in your photo) or drop-off dolly is also sometimes used to help with this nose-over tendency.

Are you thinking of sticking a pylon on your Petrel? Don't laugh! I seem to think Ken Wallis did exactly this when he owned one of the Petrels in the 1950's or 1960's. Wish I could find that photo.

Hope this helps,

Rog
Apr 05, 2010, 03:58 PM
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tuootal's Avatar
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Rog,

yes, as there is limited air-tow-events here I thought that it would make excellent addition to model. It seems (from the pics of various sources) that the common understanding should be straight or slightly up. As you commented, +10deg seems to be more-or-less common.

But I still would like to know the theory behind this. I think that the straight would look best as it is more or less scale like. Those pictures of prop pointing up look a bit like strange helicopter.

With Petrel in mind I think that dolly would be ideal anyway, as there is no wheels on that plane. Dolly prevents nosing over, I guess.

Tuomas
Apr 05, 2010, 09:30 PM
Horizon Hobby Team Member
sruelle's Avatar
The angle help a lot if you have a paved runnaway, the second thing that help a lot is to be able to adjust propely the speed of the motor to avoid generating too much thrust -> nose over. The wheel has to spin perfectly, a ball bearing is the good solution.

If like me you have a grass field, I guess the better is to be airborned by a short bungee.

I will try that shortly as weather will coorperate!

stephane

http://lesgpr.free.fr/construire/pro.../pylone-sr.htm

I think my angle is 15
Apr 28, 2010, 12:56 AM
Registered User
The objective of aligning the thrust line from the horizontal, to one that directs the propwash down over the tailplane, is to create a download on the tailplane under power to offset the nose down pitching caused by the motor being mounted high on the pylon above the fuselage. The angle is probably best resolved by experimentation as it will vary according to the installation on any particular pylon/glider setup. You will know when you have the correct angle because there will be no, or very little trim change between power on and power off. Back in the 20's & 30's when large biplane flying boats had the engines mounted on the struts between the wings, high above the hull, the designers resolved the pitch change problems associated with power changes by inclining the power nacelles down towards the tail so that variations in thrust from the props caused the propwash to vary the download on the tail. I believe I am right in saying that on the Supermarine Stranraer they got it dead right and there were no pitch changes with power changes.
Apr 28, 2010, 03:03 PM
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tuootal's Avatar
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Thanks for these comments. I think that I will make pylon vertical and initially motor pointing directly back (pusher). I will use dolly to prevent noseovers, and then start adjusting angle to check what is best.

Reasoning behind pusher arrangement is to allow using folding props without long 'nose' of pylon.

Pylon is already done, motor installation still pending.

Tuomas
Apr 28, 2010, 03:26 PM
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The SKY MAN's Avatar
Tuootal
What is the pylon going on?
please keep posting all of your findings as it will help many of us who haven't reached your stage yet.
please post motor esc power etc etc and that angle your searching for
Iain
Apr 28, 2010, 03:58 PM
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tuootal's Avatar
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heh, I have pylon, but the plane is still missing wings. It is the 5M Petrel 1:3,5 build here. It will take some time to get airborne, but it is getting there.

I will keep you updated.

On motor side I think that I will initially try rather modest setup. For Petrel which is approx 10kg plane I will try to use Turningy 50-series motor, something around 1000w range (like this). I know that this sounds ridicolously small, but this is assisting motor and I am not planning to fly 3D-hovering with it. I have 4,2kg grob which flies quite nicely with approx 500w.

I think that the motor performance should be as small as possible. Just to get up, nothing more. Reasoning is rather simple; even that modest turningy 50-motor weights 300g+ all the supporting hardware. If I have 300g sledgehammer at the end of motor pylon (20cm or so) it will be difficult to make it strong enough. And increasing power means more weight.

Tuomas


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