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Old Jan 20, 2013, 08:49 AM
Jim in the Desert
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United States, NM, Las Cruces
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Should the plane take off before applying elevator?

I am learning with a GWS slow stick. Last week, with just enough power to get it moving down the field, after a while it would take off gently on its own without me touching the elevator.

I spent a day doing some rebuilding and making things a bit straighter and paid more attention to balance at the CG, and now with just enough power to make it roll it gets the tail up and rolls and rolls down the field without lifting off. When I touch the elevator a bit, it gently takes off.

Which is the right behavior for the slow stick? is it the same for all planes?

When the plane is in flight, should I need any elevator for it to fly level?

Thanks...
Jim
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Old Jan 20, 2013, 09:43 AM
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It should be trimmed so that it flies level at a cruise speed, say half throttle.
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Old Jan 20, 2013, 10:49 AM
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There are several factors involved. If the plane is trimmed for level flight at one throttle setting in the air, increasing throttle will cause it to lift, decreasing to descend. On or near the ground, you also have ground effect, so when it reaches the speed that achieves level flight in the air, that effect often gives a bit extra lift, so it will begin to rise. More speed, and it should easily lift without any control input. This is actually the best way to do ROG, if you have enough runway, as yanking it up with the elevator is the fastest way to a stall and crash; too much up, too soon, and airspeed drops quickly, so it isn't flying anymore. Exceptions are aircraft with lots of thrust relative to weight, like 3D airplanes.
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Old Jan 20, 2013, 02:15 PM
buyer of the farm
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You moved your CG forward a bit? You just increased the speed at which it will lift off without elevator. No harm done at all. If you liked it better before, move the CG to where it was.

It's all good. Once you get the CG to where it WILL FLY, it's all basically a preference call. I look at stall behavior and behavior at speed. You're looking for a stall that doesn't put the plane into the "dive of death" for 20 feet. The further forward your CG is, the longer the dive to regain flying speed. If you have a brushless motor you can just hit the jets when the nose starts to drop and just drag the plane out of the condition, but without power you're going to do the whole dive before you're flying again. The further back you move the CG the less altitude you lose in the dive after a stall.

This gets balanced by the plane getting twitchier and twitchier at "high" speed and becoming less stable in the wind. If you find the plane getting scary in those conditions, moving the CG forward will help make the plane more stable.

It's all a dance between the two conditions and we all set the CG differently according to our comfort level. In my case the CG is forward of where I would have it for a sailplane. Why? Because I am obscenely overpowered. All I have to do is point the nose up and hit the jets and I'm outta there, stall or no stall.

Experiment and choose. That's about all anyone can say.
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Old Jan 20, 2013, 04:33 PM
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Quote:
Originally Posted by cloud_9 View Post
I am learning with a GWS slow stick. Last week, with just enough power to get it moving down the field, after a while it would take off gently on its own without me touching the elevator.

I spent a day doing some rebuilding and making things a bit straighter and paid more attention to balance at the CG, and now with just enough power to make it roll it gets the tail up and rolls and rolls down the field without lifting off. When I touch the elevator a bit, it gently takes off.

Which is the right behavior for the slow stick? is it the same for all planes?
It is the same for all planes, and either way is an acceptable way to take off. The critical factor is not whether or not you have to give it up elevator to take off, but that when it does leave the ground its airspeed is faster than its stall speed. Just be aware that in its current configuration if you apply up elevator before the plane has accelerated to sufficient flying speed, it will stall.

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Originally Posted by cloud_9 View Post
When the plane is in flight, should I need any elevator for it to fly level?
That depends upon how fast you want it to fly. If you want it to fly level without the need for any elevator then you have to adjust the throttle to attain level flight. To fly level at a higher speed, increase the throttle and add some down elevator trim until the airplane is once again flying level. If you want to fly level at a lower speed, decrease the throttle and add some up elevator trim until the airplane is once again flying level.

Larry
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Old Jan 20, 2013, 05:19 PM
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The faster a plane goes, the more it will climb.
My real Cessna needs to be trimmed for climb, then once at altitude I have to trim it again and then any power increase or decrease, I have to retrim it and when I'm power off, coming into land, it gets trimmed again. Constantly retriming, except in a level flight cruze.

But on my toy planes, I take them up a bit, level the flight at medium cruze speed and trim them there, because that's where I do most my flying.
If I cut the power, it comes down and if I add more power, it climbs.
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Old Jan 20, 2013, 06:37 PM
buyer of the farm
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You know, thinking about it I'd rather have to give elevator to take off. That's because on landing you want your plane to "stick" to the ground and not hop back off again with a little wind gust or a bump.

Needing to give a bit of elevator to take off says that it will tend to stay down after touchdown.
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Old Jan 20, 2013, 08:03 PM
Jim in the Desert
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Boy, what great answers, thanks much. I think I just learned a lot more than I expected. So you can actually trim it while flying.

I really want the plane to float, but also find myself battling the wind, so I'll experiment with the CG more forward or back. Earlier the battery was under the wing, now the battery is in front of the wing (same battery).

Earlier, my constant crashing shattered the front few inches of the CF fuse shaft and twisted the motor mount. I just kept bending it back and it somehow kept hanging on to the shards of the shaft.

This time, I sized a basswood square stick to slide in the fuse, and cut a 1.5" section of the old CF shaft, epoxied it to the stick, then stuck the stick into the new fuse shaft without epoxy with about .5" of stick showing between the motor section and the full fuse. The motor mount is screwed to the 1.5" shaft and stick. So if crashing damages anything, it will be the short section with the motor and the stick, which I can slide out of the new fuselage, then just make a new motor section to slide in. This has the effect of moving the motor about 2" forward of where the motor was in the stock position. I think I have the wing farther forward too because I have to put the battery in front of the wing to get it to balance. So yes, I have moved the CG forward and the plane is 2" longer.

Unfortunately I snapped the antenna in half on my old JR 72mhz and it will take a week for the replacement to arrive so no testing for a while.

Jim
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Old Jan 21, 2013, 12:29 AM
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4 1/8" back from the leading edge of the wing is a good starting point for the CG, no matter where the wing sits on the fuse.
Shimming the trailing edge up can help on windy days too. But as it lowers the angle of attack, it increases stall speed. 1/4" is what I use, but you may want to start with thinner shims on a calm day to note the changes in handling.
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Old Jan 21, 2013, 06:16 AM
Jim in the Desert
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4 1/8" back from the leading edge of the wing is a good starting point for the CG, no matter where the wing sits on the fuse.
Shimming the trailing edge up can help on windy days too. But as it lowers the angle of attack, it increases stall speed. 1/4" is what I use, but you may want to start with thinner shims on a calm day to note the changes in handling.
So no matter where the wing is on the slow stick fuse, try to move the battery and stuff around so it balances at 4 1/8" back from the leading edge?

I was using 10 cm, from a youtube video, which is closer to the leading edge by a half inch. If I use 4.125, then I am more nose heavy than I though. That should make a difference, thanks. I put a couple dots of epoxy on the 4.125 marks.

Is there an optimal place for the wing? How does handling change when you move it forward or toward the tail? I see SS with the wing in quite different places.
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Old Jan 21, 2013, 06:45 AM
buyer of the farm
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Originally Posted by Ron H View Post
Shimming the trailing edge up can help on windy days too. But as it lowers the angle of attack, it increases stall speed. 1/4" is what I use, but you may want to start with thinner shims on a calm day to note the changes in handling.
Lots of people use two front or two rear wing mounts to make the wing flat with the fuselage. I've liked my SS so much I've just left it alone, but that configuration would give you a lot better wind penetration and keep the nose from pitching up as much with increased speed.
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Old Jan 21, 2013, 08:11 AM
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Is there an optimal place for the wing? How does handling change when you move it forward or toward the tail? I see SS with the wing in quite different places.
It really depends on your flying style. Farther forward makes the plane more docile, farther back makes it more responsive.
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Old Jan 21, 2013, 09:38 AM
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I was using 10 cm, from a youtube video, which is closer to the leading edge by a half inch.
10cm is 3.94 inches, so not a half, but ~1/8 inches closer, which is only about 5% difference.
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Old Jan 21, 2013, 01:07 PM
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Shimming the trailing edge up can help on windy days too. But as it lowers the angle of attack, it increases stall speed.
Shimming the trailing edge lowers the incidence angle of the wing, not the angle of attack. Shimming the trailing edge only changes the airplane's trim so that the airplane will need more power to fly level and level cruise speed will be higher. Shimming the trailing edge will not increase the stall speed and it will not effect the airplane's 'floatiness', both of which are determined by wing loading.

Larry
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Old Jan 21, 2013, 07:54 PM
Jim in the Desert
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Thanks everyone for all your help on this. Seems like a pretty deep subject. Hopefully what I've learned here will suffice to get the slow stick floating around...

Jim
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