Can you accurately calculate thrustlines - Page 2 - RC Groups
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Jan 26, 2013, 03:38 AM
vincent123's Avatar
Hmm, thinking about what you said.

It's just that since a long time now, I'm using symmetrical, on flying wings and tailed planes now, and since then I never experienced this effect anymore. I did not fool with the CG or anything, they just fly on a straight line, even when powering up (using a 800watt).

Struggling here .., isn't it so that a high lift wing (cambered), the effect is just plane visible, and with symmetrical, the max CL is lower and balances with the prop pulling force ?

Resulting on a wider speed range where your plane keeps flying in a straight line, without trim need ?
Last edited by vincent123; Jan 26, 2013 at 03:46 AM.
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Jan 26, 2013, 03:10 PM
B for Bruce
BMatthews's Avatar
You say you don't fool with the CG. But you really do when you choose to balance your model at some particular point based on your experience.

But move the CG ahead on one of your existing models and re-trim the elevator to achieve level flight at a cruise power setting and you'll suddenly find that the model wants to glide more steeply at low throttle and tends to nose up more strongly with power or during a hands off dive. Such behaviour to some folks suggests that the model needs downthrust to tame this issue. But the downthrust only helps with the nose up with adding power. Not so much the hands off dive behaviour. Restoring the CG to your original point would deal with all the issues rather than the one issue.

The max Cl of a cambered airfoil is typically higher than the max Cl of a symetrical airfoil. But the thing is that a model in cruise flight is operating at a lower Cl. And the two airfoils on otherwise identical models would both be at the same Cl when flying at the same speed. The difference woud be that the cambered airfoil can achieve the required Cl at a lower angle of attack. So the trick to useing a cambered airfoil in place of your symetrical airfoil would be to set up the wing's incidence angle to a slightly negative angle. The amount of this negative angle is the difference between the two airfoils' zero lift angles. If you do this and keep the CG in the same location your "new" design will fly very similarly to the original symetrical wing.

Where you would gain from such a swap is that the higher max Cl will let you fly more slowly at low throttle and in the glide. WHere it'll hurt is when you fly inverted or do outside looping maneuvers. That's because the positive camber shifts the lift curve up towards the positive side. When negatively loaded the airfoil has a lower max Cl than the symetrical option.
Feb 09, 2013, 11:44 PM
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Joel K. Scholz's Avatar
I thought you folks might like to see the first flight of the bird in question. It is extremely stable. I have not seen a hint of a tip stall. Aileron turns are very slow. V-tail is more responsive, but I find myself using both. I am painting the bird now.As you can see I added a subrudder. Initial glide test indicated it was needed.
Osprey Maiden Flight (6 min 38 sec)
Feb 10, 2013, 01:10 PM
vespa's Avatar
Nice work Joel, looks great and seems to fly very well! Try a little more rudder mix next time and see if the roll response improves.
Feb 17, 2013, 10:23 AM
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Joel K. Scholz's Avatar
Gentleman, thanks for your help. All that is left is to balance the folding prop?
osprey movie (9 min 9 sec)
Feb 17, 2013, 11:29 AM
vespa's Avatar
Wow! That looks great and sounds horrible! The turning and handling look perfect so you're either a damn good pilot or the design/setup is spot on. Nice work.
Feb 17, 2013, 12:25 PM
B for Bruce
BMatthews's Avatar
Yeah, there's some pretty seriously funky vibrations happening in the nose. You'll want to keep checking the EPP to make sure it's not breaking up. Or it may be that it's vibrating simply because the foam is that flexible.

For the wing area and weight I'm a little surprised that it's that fast a flyer. I'd have expected a little more floatyness. Perhaps with time and fine tuning the balance point it may trim out to fly a little more slowly.

It does make me want to consider my own soaring bird though. It looks good in the air.

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