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Thrust angle on 3D profile plane?

Old 04-07-2008, 09:09 PM
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Fly Time
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Default Thrust angle on 3D profile plane?

I am currently putting together my first 3D capable airplane. It's a Ikarus Pitts Special Shock Flyer (flat foamie), and will weigh about 8 ounces all-up. It came with a brushed motor and gearbox, but for about the cost of a brushed ESC, I can get a brushless motor and ESC, so that is what I've done.

Anyway, the assembly instructions say nothing about thrust angle. I know the rule of thumb for other RC airplanes is 2-3 degrees right thrust and possibly some downthrust too depending on the airframe. Does the same hold true for this type of plane?
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Old 04-07-2008, 09:18 PM
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pburt1975
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FT, I believe right thrust is going to be required regardless of the air frame.

For an average every day kind of plane, 2-3 deg of down and right is used

For 3d, just right thrust is used (if any at all). Adding down thrust can be done as well, but I've heard it affects inverted flight characteristics.
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Old 04-21-2008, 09:30 PM
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I maidened my Ikarus Pitts about a week ago, and man did it ever fly strangely! For one thing, it is underpowered on a 2 cell lipo, so that is going to have to change. But it also refused to turn left using ailerons. I could coax a turn out of her using just the rudder, but that was about all. Left aileron just rolled the plane into a knife edge, which it held great with no rudder input! Even full up elevator in knife edge would barely turn her to the left. Right aileron caused it to spiral down. And for some reason, inverted flight was much better than upright flight. What an odd bird! I'm lucky I brought her home in one piece.

I've got a new $12 outrunner ordered that should give me a little more prop speed on 2 cells. If that doesn't work, I'll go with 3 cells. And I am going to remove the right thrust altogether. I think it was the right thrust that was preventing me from turning left.

Not sure about down thrust. I didn't build any into the mount, and will probably leave it that way for the next flight, but if anything I might need a little up-thrust.
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Old 04-22-2008, 05:09 AM
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CHELLIE
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Originally Posted by Fly Time View Post
I maidened my Ikarus Pitts about a week ago, and man did it ever fly strangely! For one thing, it is underpowered on a 2 cell lipo, so that is going to have to change. But it also refused to turn left using ailerons. I could coax a turn out of her using just the rudder, but that was about all. Left aileron just rolled the plane into a knife edge, which it held great with no rudder input! Even full up elevator in knife edge would barely turn her to the left. Right aileron caused it to spiral down. And for some reason, inverted flight was much better than upright flight. What an odd bird! I'm lucky I brought her home in one piece.

I've got a new $12 outrunner ordered that should give me a little more prop speed on 2 cells. If that doesn't work, I'll go with 3 cells. And I am going to remove the right thrust altogether. I think it was the right thrust that was preventing me from turning left.

Not sure about down thrust. I didn't build any into the mount, and will probably leave it that way for the next flight, but if anything I might need a little up-thrust.


Hi Fly Time Normally the profile planes need a lot of down thrust to stabilize them 3 to 5 degrees, or they will wobble all over the place Normally No Right or left thrust is needed, just Down thrust, here is a build of my Mavrick Cap Foam Profile plane, they fly great with 3 cells, you need the thrust to keep them straightened out flying a 2408-21 Towerpro motor and a 7x6 sf prop work great, with a 1300 to 1600 mah lipo 3 cell, here is my build on the Mavrick and a video of it, Enjoy, Chellie

http://www.wattflyer.com/forums/showthread.php?t=30651


[media]http://youtube.com/watch?v=wz5ilV8jQRM&feature=user[/media]
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Old 04-22-2008, 06:54 AM
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Originally Posted by CHELLIE View Post
Hi Fly Time Normally the profile planes need a lot of down thrust to stabilize them 3 to 5 degrees, or they will wobble all over the place Normally No Right or left thrust is needed, just Down thrust, here is a build of my Mavrick Cap Foam Profile plane, they fly great with 3 cells, you need the thrust to keep them straightened out flying a 2408-21 Towerpro motor and a 7x6 sf prop work great, with a 1300 to 1600 mah lipo 3 cell, here is my build on the Mavrick and a video of it, Enjoy, Chellie
Thanks Chellie. First I'm going to take the right thrust out and try it again (with a stronger motor too). If it's wobbly as you suspect it may be, then I'll know exactly what it needs. I'll let you know how it turns out.
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Old 04-22-2008, 07:07 AM
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Originally Posted by Fly Time View Post
Thanks Chellie. First I'm going to take the right thrust out and try it again (with a stronger motor too). If it's wobbly as you suspect it may be, then I'll know exactly what it needs. I'll let you know how it turns out.
Hi Again here is a little example of the down thrust I have on my plane, Take care, Chellie


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Old 04-22-2008, 07:35 PM
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Default Long rambling reply

I'm trying to get my head around the effects of downthrust on a flat foamie.

For right thrust, it makes sense to me that if there is about as much vertical surface area (fuselage and tail) below the thrust line as there is above the thrust line, then right thrust would make the plane fly worse, not better. When air corkscrews around the airframe off the prop, it pushes against both sides of the plane equally. That's not true of most airplanes, since the tail surface sticks up into this flow of air, whereas the flow of air under the fuselage dosen't have anything to push against in the opposite direction. Based on the way my plane flew, I think I've proven that right thrust is not neccessary with this design

But downthrust is different. On Chellie's plane, I can see that the thrust line is on center with the wing. The Pitts is the same, though obviously there are 2 wings and the center of thrust is about centered between them. That takes away thrust/drag differential as the reason for having downthrust. (don't know if that is the right term, but what I'm referring to is when drag created by the wing is not in line with the thrust created by the prop, which is one reason why downthrust is more important on a high wing plane. The drag from the wing acts as a lever against the forward thrust to pitch the fuselage up.)

So absent the effect of thrust/drag differential, my first thought is that downthrust would just pull the plane nose down, which would be bad. But on second thought (and by Chellie's observation), maybe there is more to it than that.

Here's my theory: 1) downthrust pulls the nose down. 2) to keep the plane from diving, some up elevator must be trimmed in, pushing the tail down (and levering the nose up) to compensate for the prop pulling the nose down. 3) this causes the wing to move through the air at a positive angle of attack, creating lift.

So maybe that's it: A flat wing must have a positive angle of attack to generate lift, and the downthrust forces offset by downforce on the stab force the wing to fly at a positive angle of attack.

Sorry to be such a geek about this stuff, but it's in my nature to try to understand it. Comments???
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Old 04-23-2008, 04:40 AM
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Hi Fly Time Sounds good I just know that my first profile Mavrick plane, would porpoise/wobble all over the place, I added some down thrust, about 2 percent, and it helped some, but wanted to climb with throttle, Sooo i added more down thrust about 5 percent, Perfect, flies like an Arrow now I guess not having any airfoil, you need to load the wing more, thats just my guess, all I know is that it works and I tried different things too, I had about 3 percent of right thrust, but had to add a lot of left trim, did not need it, Take care and have fun, Chellie
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Old 04-24-2008, 08:21 AM
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I visualise the situation like this. A propeller produces thrust which has a vector, and that vector points slightly up and to the left from the direction of the shaft. So, what you need to do is line up that slightly off-center thrust vector with the centerline of the plane. With different props and motors the vector can be different. On my Parkzone Cessna there is about 20 degrees of downthrust and maybe 10 degrees of right - it looks broken, but it flies good that way.

Here is some information about trimming the thrust vector:
http://www.scootworks.com/rdrc/trim.html

On a slight weird note - I have built 3 profile planes and all of them at one point or another in the instructions, specifically said NOT to put any down thrust or right thrust on the motor. Not sure why they said that, but I built it the way the instructions said, and I don't notice any weirdness. In fact, my Yak-55 pulls down a little bit if you jam the throttle up, and downthrust would have made that worse. Still scratchin my head on that one, but you know - fly it however it flies right.

There is information here about why a propeller does not produce a thrust vector that is perfectly in line with the shaft. It's about boats, but some of the physics is basically the same. Ignore the part about the surface of the water disrupting the thrust 'cone'.
http://continuouswave.com/whaler/reference/prop3.html
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Old 04-24-2008, 07:37 PM
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Originally Posted by jasmine2501 View Post
I visualise the situation like this. A propeller produces thrust which has a vector, and that vector points slightly up and to the left from the direction of the shaft...
That doesn't sound quite right. Or maybe it's just an oversimplification. A prop pushes air straight back. It's only when you attach it to an airframe and allow it to pull that airframe through air that things get complex. Thrust is offset by drag and gravity is offset by lift. 4 equal and offsetting forces for a plane in level flight, but add to that the effect of torque, prop wash over the airframe and varying airspeed and angle of attack and then all of a sudden the plane will not fly straight through the air on its own. Adjusting the angle of the prop relative to the airframe is how we counteract those forces, and it makes sense that different airframes and power systems will require different thrust angles to bring all these forces into equilibrium.

Thanks for sharing that your profile planes have no right or downthrust. I'll bet they fly upside down just as well as right side up, which is precisely how I would like mine to fly. Though if it means it will be squirley in flight, then maybe I'll settle for a plane the flys upright best.

I'm going to re-build my motor mount so that I can easily adjust thrust angles. Then I'll take it out to the field and spend some time experimenting.
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Old 04-25-2008, 03:21 AM
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Somebody gave me a detailed explanation of it one day and from what I remember it has something to do with the prop producing a slight bit more 'push' on the down stroke, relative to the plane... so yeah maybe it does have something to do with the fact that it's mounted on a plane.

On my Cessna, with all the down and right thrust - if you power it up and put it in front of your face, you can feel that the wind is blowing straight back over the tail... so the net effect is a straight thrustline, but it sure looks weird...
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Old 04-26-2008, 05:20 PM
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http://www.alphatrainer.com/handouts/ac61-23c.pdf
Starting at about page 17, there's a discussion about the turning tendencies caused by 4 main factors: torque, spiral slipstream from the propeller, gyroscopic precession, and P-effect.

IMHO: In current 3D designs; the less need for down-thrust is due to a closer alignment of the motor thrust-line, CG and aerodynamic center-of-drag (e.g. wing). The less need for right-thrust is due to (1) larger amounts of side-area located ahead of the CG; and (2) behind the CG, a more equal distribution of side-area (particularly vertical fin) above and below the thrust-line.

Unless we go to counter-rotating props, we'll always need "pilot skill" to compensate for gyroscopic precession and P-effect (and crooked airplanes).

Last edited by gyrocptr; 04-26-2008 at 08:08 PM.
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Old 04-26-2008, 07:01 PM
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There are no crooked airplanes - crooked politicians... welll...

It is the P-factor I was talking about, just couldn't remember what it was called. Gyroscopic effects are noticeable on little foamies too - swinging all that mass around at the front, with the tiny weight of the plane behind it, and it kinda wants to stay facing the same direction sometimes and will do squirrelly things if you turn too fast.
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Old 04-27-2008, 09:26 PM
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0/0 maybe a little right but unless a bad desighn or model built out of sqaure no down. A flat plate
will hunt at high speeds, but then you dont really 3d at high speeds which is what makes them such great 3D planes .

This is just the way Ive always done them and Ive had some great 3dr's . Any thrust angles are going to screw you up in High Alpha. Like a hover that always wants to pull foward from the down thrust. It will always act nose heavy. Just fly the plane nose heavy if you want to pattern .
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Old 04-28-2008, 02:32 AM
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I fly my small electric stuff all @ 0/0 as stated above down or right thrust will not help in high alpha flight. I have had a foamy bipe (Snoopysolution) that did not turn well on ailerons, it needed lots of rudder. It did hover and harrier really well but did not do knife edge spins as there was not enough rudder to counter all the side area it had in front of the wings center of lift. I did cut a bigger rudder and ailerons but the bigger rudder caused a ton of adverse roll. Went back to monoplanes.
In glow (all 4stroke) I use right thrust but never add down, if required will fly with a throttle driven trim offset or cut off the stab and reset.

Last edited by flashburn; 04-28-2008 at 02:33 AM. Reason: arg
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Old 05-01-2008, 12:03 AM
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I think I got this plane just about dialed in now Thanks to everyone for all the help!

First of all, I got a new motor for it from HeadsUp: TP 2410-09Y It turns the prop a little faster and gives me okay thrust with 2 cells. I definately could use more thrust though, so I might still need to invest in a small 3 cell lipo. I've got a couple of 1300's that weigh about 105 grams, so maybe they will work.

I re-built the mount so that I could shim in any thrust angle I need. I test flew it this afternoon with zero right/zero down. I kept it nose heavy and I thought it flew pretty good. I'll play around with it some more, but it is already SO MUCH better than before! This will be a fun little plane with a few more volts through the motor.
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