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pusher thrust lines

Old 11-07-2009, 05:34 AM
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Default pusher thrust lines

Hi all, need some more advice on thrust lines. If the twin pusher motors on my Arado 234 are under the wing, would I expect it to pitch up on acceleration? The reason I ask is that it seems to be an issue with pusher motors on the low wing Me 262, which has neutral thrust angles and it seemed to effect my above fuse mounted edf on the He 162, which is angled about 2 degrees down.The Arado is a high/shoulder mounted wing, and the motors are basically aligned with the centreline of the fuse. I have neutral thrust angles on both motors. It does not appear at this early stage to have any bad habits as such, I just need to know so I can incorporate it into the trimming. Cheers...
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Old 11-07-2009, 08:29 AM
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There are a couple of factors that effect pitch up/down due to thrust line..

The first issue is due to mass. Figure out where the plane's centre of mass is.. That’s the overall CG both longitudinally and vertically. Now look at the thrust line, does it pass over below or right through this point?
Ideally it should pass through the mass centre. If it's above (like the He162) you will get a nose down pitch when the plane accelerates. If it's below (like the Me 262) you will get a nose up pitch during acceleration.

There is another effect due to drag. You need to figure out where the planes centre of drag is.. this is not easy but generally if the plane has high wings then the centre of drag will be at a point just below the wing, if low wing then the drag will be just above the wing and if mid wing the drag will act roughly at the wing position. Again compare the thrust line to this position. Thrust line higher than drag gives pitch down, lower than the drag gives pitch up, in line with the drag is ideal. Unlike the mass effect described above the drag acts all the time even when the plane is at constant speed so you don’t need to be accelerating to notice that effect, you just need to have power on.

You can see on some high powered full size planes the efforts the designers go to align thrust lines with the centre of mass. This is most obvious on the zero length launch rocket assisted fighter jets. Take a look at the offset thrust nozzle on the rocket booster of this F104 and note how it's thrust line passes through the planes centre of mass: [media][/media]The Arado should be ok because the motors are quite central on the fuselage so the thrust line will pass close to both the mass and drag centres.


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Old 11-07-2009, 11:40 PM
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Default Neutral handling

Hi Steve, thanks for that. Followed your advice, and the thrust lines and centre of drag do seem to be symmetrical to the engine pods. It will make swapping it to edf in the future much simpler.The Arado 234 does appear to have quite pure lines, and very little in the way of deficits, and literally flew out of my hand once the cog was right. Not even much of a hint of torque from the twin pusher props. Will try counter rotating them anyway. All the best, cheers.....
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