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Pusher thrust lines

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colin sayer04/09/2017 16:11:13
30 forum posts
3 photos

when fitting an electric motor or an edf in pusher mode at the rear of a model should the motor be fixed horizontally or the rear of the motor a few degrees up or down, I am doing a few repairs to a couple of old flying wings and unsure about thrust lines on them.thanking you in advance.

Denis Watkins04/09/2017 16:21:23
3559 forum posts
166 photos

I have no science for you Colin, but both my pushers appear about 15° upward pointing with the model upright

And appear to push the wing down, when the model is upright and flying.

Dave Bran04/09/2017 19:32:23
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1898 forum posts
5 photos

Most of my large collection of pusher wings (many own design) have no offset thrust at all in any direction. A couple have minor up thrust, that is the prop driver tip centre is higher than the shaft entry to the motor housing, pushing thrust down on the trailing edge (as behind CG).

However, there are so many variables involved that the very best way is to allow for quick change, pick a day with a fair breeze, and launch into wind on part throttle, JUST enough to feel it might fly, and see effect.

I fly mode 2 and am quite happy launching reverse discus across my front left handed from the port wing, so have right hand on major Tx controls to "fly" the plane immediately.

onetenor04/09/2017 21:26:31
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1889 forum posts

I alway give mine a couple of deg upthrust. The equivalent of downthrust on a tractor engine /. motor model.

Erfolg05/09/2017 14:12:09
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11211 forum posts
1108 photos

I have a few pusher aircraft

covered.jpg

A pushycat

Flying wing

Twin Delta

v2.jpg

A Canard

Dornier 335, strictly a push me pull you type, or axial thrust theory type.

If you spoke to BEB, now doubt he would talk about, component drag, pitching moment and thrust lines. Just a simple sketch (called a FBD by engineers) would put into context all the forces and enable a simple numeric assessement to be made if necessary, generally just looking at the problem supplies a pretty close answer.

However, I am an ex engineer, and in the case of the Pushycat and the Do335, have had the answer provided. Which is no down thrust or upthrust.

However in the case of the flying wing, the wing drag is pretty much dominant drag force, with a fair pitching moment. This required upthrust, just, only a few degrees. There is a similar issue with the canard. But without it, I just meant I held in a bit of up on the stick.

Perhaps it is worth noting, that all my thrust lines are pretty much along the fus and wing axis (when viewed in side elevation). If the motor were very much higher or lower, the turning moment would be far more significant, requiring for comfort either up or down thrust. We must remember that all the thrust lines are pretty much a compromise, never being correct in an absolute sense, just one air speed, probably cruise.

Simon Chaddock05/09/2017 15:07:45
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5357 forum posts
2807 photos

As Erfolg points out its all about thrust and drag lines regardless whether its push or pull.

My favoured pusher layout is a 'pylon' configuration.

Complete Oct 10

This puts the thrust and drag lines pretty close together so no down or up is necessary however as the heavy battery is well below the thrust line there is a noticeable pitch down inertia effect resulting from any rapid increase in speed.

With the above plane all that happens is the inertia pitch down simply counters the normal speed pitch up until the drag begins to match the thrust. In other words apply full power in slow flight it simply speeds up remaining level before gradually entering a full zoom climb and all without touching the elevator. wink 2

ron evans05/09/2017 16:06:12
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374 forum posts
22 photos

On some of my pusher wings, even after moving the CG as far back as was comfortable there was still a pitch up on full power.

I've found that adding downthrust helps a lot, the same affect as downthrust on a tractor motor.

Downthrust on a pusher tends to lift the tail just like down elevator, and that pushes the nose down.

Ron

colin sayer05/09/2017 16:06:23
30 forum posts
3 photos

thanks to you all for taking the time to reply, it would appear that the favoured method is to not use any up,down or side thrust at all so that is what I will stick to,with a little up elevator.

Erfolg05/09/2017 17:32:36
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11211 forum posts
1108 photos

I have just read Rons post.

Which started me thinking, all my models have the motors(hence the thrust line), either at, or above the CG in the side view. For stability the CG is always in front of the neutral point. In my case there is a nose down pitch due to camber, with a nose up pitch to an increase in wing lift, when power is inputted.

In my case the pitch up from the wing is greater than the nose down due to camber, in some cases I need some up thrust to balance the forces, ideally so that no stick input is needed. In the case of the wing, before I put in the upthrust, I would have to feed in down elevator, to prevent a pitch up.

Although not strictly part of the discussion, with the wing, I initially had the CG a long way back, and obtained an absolutely fabulous glide. There was a price, as I was operating very near the stall, when I turned the motor on, I often stalled the model, due to pitch up, where upon it would enter an absolutely flat spin. No amount of stick waggling, power on or off did anything, until alighted the ground just like a Frisbee.

But at the end of the day, I am not convinced that it matters, just set "0", fly the model. If the model pitches up from steady flight when power is switched on, upthrust is required. If it pitches down, the opposite. Although it is a compromise, dependant on throttle and power available settings. Just recognising most things about our models are transitory, the steady state view of a system, that allows an instantaneous calculation is just that.

In this day and age, you can do what I had programmed into my Sagitta, that was down elevator, via the TX on throttle up. Then again I am putting 400w into a 2m glider. If the power is low, you may notice nothing. Now I do not bother, I just hold as much down in as I think is needed, it does not look as pretty and controlled, who cares?

onetenor05/09/2017 17:43:26
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1889 forum posts

Just to be awkward what about planes with motor on pylon above the wing ala the old Veron Velox

Erfolg05/09/2017 18:35:42
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11211 forum posts
1108 photos

I am not sure if in principle it is a challenge.

I think that the Velox was a pusher , with the motor high up, to the rear of the CG.

Looking at things simply. If the motor is low down as in a typical trainer, we all know or accept down thrust is required. Motor now moved up above the CG (side on), then up thrust is required. Move the motor know behind the CG, at the same level, down thrust. If it is possible, the first position of motor . but now behind the CG up thrust.

As we all know, we can contrive situations where my generalisation is not true. Yet as a simple model of balancing forces. I expect the the Velox to have had down thrust, if it is configured as i think.

In many ways although Vic Smeed, it is a position for the motor that from some aspects seem less than great. As the motor thrust can play a massive part in trimming, power on and off.

bert baker05/09/2017 19:14:57
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1282 forum posts
276 photos

I have just purchased a secondhand Irvine Xite, its my first pusher plane and we'll twitchy on half to full power, am interested in this Thread as should motor tilt up or down

It pitches up and down violently , putting lipo as Farr up front tamed it a little but sure it is a thrust line issue

 

Edited By bert baker on 05/09/2017 19:21:52

Erfolg06/09/2017 18:11:36
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11211 forum posts
1108 photos

Whilst driving today along an emtyish motorway, I contemplated my flying wing and its apparent need for up thrust.

The penny finally dropped, when I recognised how insensitive the thrust line can be.

When I produced the body for the glider, I just cut of the body at 90 degrees.

However when I positioned the wing dowels, I assumed that the body ideally should be in line with the air flow. To achieve this on a glider, which spends a lot of its time just a degree or so from the stall (supposably the best L/D ratio) I set the 0-0 line very positive, probably about 8 degrees, for the wing.

I real terms the motor was possibly at a down thrust relationship. Which proved unhelpful. I then packed the motor mount to provide apparent up thrust. In reality it possibly became aligned with the 0-0 line.

Yet at the end of the day, it was not the thrust line that was my main problem, it was the rearward CG. The thrust line just initiated a problem due to the CG position.

By the end of my 60 ish mile journey, having considered all of my models, as long as the motor is roughly inline with the 0-0 line, it does not matter to any great extent. it is the CG that is far more significant

Edited By Erfolg on 06/09/2017 18:11:54

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