Ligbaer
50 Percenter
im pretty sure for pushers you wanna point the thrust line towards the cg (it works when my pushers don't fly straight)
Anybody else scratch building tonight?
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Brian B
Elite member
What do you do if the thrust line is perpendicular to the CG?
I've never fully wrapped my head around this one, but I don't think it is pusher or puller related, as much as position fore and aft. And prop-wash over the stabilizer is probably a big factor too, at least it was on my Macchi M5. My Navigator has 4 degrees upthrust, maybe to help "pull" off the water. But I'd say the design in question could maybe use downthrust.
SSgt Duramax
Junior Member
I purposefully modified a plane where the thrust angle was perpendicular to the CG in the wing and i still needed down thrust. Maybe they meant the CG of the actual plane, but all of the articles I read said the wing.
Monte.C
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Yep that's why I said down thrust.
Not just pushers. If the prop were ahead of the CG it would be pulling from the general area of the CG instead of pushing toward.
Monte.C
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By "perpendicular to the CG" I can only think you mean the prop is directly above the CG...
It isn't really about pusher/tractor exactly. You're right about the fore/aft thing. I think of it like this:
With a high-mounted prop, it's like the whole fuselage is exerting all it's drag well below the thrust line. Naturally when you give it some gas, that thrust wants to torque your plane about the pitch axis in a down-pitch direction.
With a prop mounted forward of the CG, some up-thrust will act to raise the nose and counteract the forward-pitch torqueing. With a prop behind the CG, down-thrust will act to push the tail down, counteracting the forward-pitch.
I just picture the fuse with its mass and its drag just hanging there, and I push the throttle stick and of course the plane wants to pitch down. Now if I move the prop fore or aft, and change its angle in my head, what will it do to the result?
JasonK
Participation Award Recipient
if the thrust vector of the motor is directly through the CG (of the whole craft, all 3 dimensions) - the thrust will only push/pull (torque from the prop might cause other issues). however if that thrust vector line does not go straight through the CG, it will impart rotational energy to the craft. if the line is above, it will cause nose down tendencies, if below, nose up tendencies [this is thrust effects only]. (left/right will cause yaw).
then you have the impact of the torque and other stuff, like the CG being infront of the NP, which can cause you to want the thrust line to be slightly off the CG to help account for some other effects that happen when under power.
then you have the impact of the torque and other stuff, like the CG being infront of the NP, which can cause you to want the thrust line to be slightly off the CG to help account for some other effects that happen when under power.
Monte.C
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I really think we can consider CG and center of mass the same thing here. Or the same thing, period.
(?)And when we say to point the prop at the CG, it doesn't necessarily mean directly at it, but towards it. Angle the prop a bit in that direction.
It's also about the center of drag. I haven't figured out the relationship between center of drag and CG.
Timmy
Legendary member
Monte.C
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I think we're confusing this.
The CG is a point. There can be nothing perpendicular to it. We might be saying "directly through" the CG.
If the plane has all the other parts besides just the wing, then the CG of the wing itself is meaningless. We might as well be talking about the CG of the motor in the nose. This is all in relation to the CG of the plane.
Monte.C
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Sort of.
You aren't taking into account the direction of the thrust line in relation to all the flight surfaces.
If your high-mounted prop is only a little ways forward of the CG, you sure don't want it to point directly away from the CG, or it would be pointed at a severely high angle upward. Then when you give it some gas you'll just do quick backflips. Likewise with a prop mounted just a little aft of the CG. A few degrees of up- or down-thrust is needed. NOT to point straight away from or straight toward the CG.
SSgt Duramax
Junior Member
Depending on how your plane is situated, the center of mass could be anywhere. I think when we refer to center of gravity, what we really mean is longitudal balance in relation to the center of lift of the wing. Since after all that is effectively what the plane is "sitting on" in the air, assuming the stabilizers aren't providing any meaningful lift.I really think we can consider CG and center of mass the same thing here. Or the same thing, period.
And when we say to point the prop at the CG, it doesn't necessarily mean directly at it, but towards it. Angle the prop a bit in that direction.
It's also about the center of drag. I haven't figured out the relationship between center of drag and CG.
Oh well, semantics. The way this article explains it, if the thrustline is through to the "CG" of the wing, there should be no issues.
https://www.flitetest.com/articles/Motor_angles_for_pusher_planes
I think it is a good article. But on your favorite plane in my hangar, my daughters pelican, I intentionally added about a half an inch to the fuselage, so I could mount the motor in such a fashion as to not need downthrust, because I was too lazy to experiment with getting the perfect thrust angle. It did not work. I still needed downthrust. Easily solved with a pair of pliers since it is an aluminum L angle mount, but none the less.
However, keep in mind that 70-80% of the weight of the plane was below the center of lift of the wing (what we call the center of gravity) so the center of mass (or true center of gravity) was actually below the wing. So I would assume that even though the center of thrust was aimed right through the center of gravity/lift on the wing, there were other forces acting on the fuselage such as drag creating a moment around that center of lift and making the plane pitch downwards which needed to be solved with a couple of degrees of downthrust.
However, on a plane like this, I wouldn't expect an issue.
Monte.C
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It's all good. I don't want to break my brain on this any more tonight, or start losing friends in the process, we're talking circles around something most of us already have some idea about. One thing's for sure, please don't say "perpendicular to the CG" any more. It's a geometrically nonsensical phrase.Depending on how your plane is situated, the center of mass could be anywhere. I think when we refer to center of gravity, what we really mean is longitudal balance in relation to the center of lift of the wing. Since after all that is effectively what the plane is "sitting on" in the air, assuming the stabilizers aren't providing any meaningful lift.
Oh well, semantics. The way this article explains it, if the thrustline is through to the "CG" of the wing, there should be no issues.
https://www.flitetest.com/articles/Motor_angles_for_pusher_planes
I think it is a good article. But on your favorite plane in my hangar, my daughters pelican, I intentionally added about a half an inch to the fuselage, so I could mount the motor in such a fashion as to not need downthrust, because I was too lazy to experiment with getting the perfect thrust angle. It did not work. I still needed downthrust. Easily solved with a pair of pliers since it is an aluminum L angle mount, but none the less.
However, keep in mind that 70-80% of the weight of the plane was below the center of lift of the wing (what we call the center of gravity) so the center of mass (or true center of gravity) was actually below the wing. So I would assume that even though the center of thrust was aimed right through the center of gravity/lift on the wing, there were other forces acting on the fuselage such as drag creating a moment around that center of lift and making the plane pitch downwards which needed to be solved with a couple of degrees of downthrust.
However, on a plane like this, I wouldn't expect an issue.
But whatever keeps the plane in the air.
BlockerAviation
Legendary member
I'd put the power pod right on the wing and have no thrust angel at all. That would make it fly more neutral
SSgt Duramax
Junior Member
I just said that because someone else did. When I thought about it after you said something, I was like "he is right, that doesn't make any sense."
I'm the same way. I was about to dig out an old statics and dynamics text book, then I thought, you know what, it doesn't matter. We all misuse words and use some that aren't technically correct, so who cares.
At the end of the day, we are going to do what I did, and real world engineers and other folks would do in this scenario, which is exactly what I did. If said thrust angle doesn't work, even though it "shooduv" they will just tweak it until it does.
I suspect you're right. If you think about the moments acting around the center of lift of the wing, the craft below the wing is creating drag and basically causing it to rotate in a fashion where there would be downward pitch, and with the motor oriented above the wing, it would also cause a rotation around that moment in the same direction. Pitching the engine upward would help negate the plane wanting to naturally nose down.
Edit. Getting somewhere on the jet. All i need to do is glue the wings together, servos, and the canopy/nose area. Never flew a swept wing or a jet before. We shall see. Undercamber wing tips may be my saving grace.
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Matthewdupreez
Legendary member
Hey guys. . I do believe I'm about to start on an Rc paraglider, full scale 😜... Will let you know when or if this idea pans out
Brian B
Elite member
Whoa I started sh-t with a quick reply using the word "perpendicular"! Sorry, poor choice. Any then others got sucked into the fray! 😔 Totally correct you can't be perpendicular to a point (kicking myself). Overall, If the thrust line is above the center of mass/gravity/drag/(fill in the blank), it induced a moment/torque/couple, which must be countered somehow, or compromised with the lesser of evils (smallest moment arm). Up or down or none depends on plane configuration, so let us know how the initial "slight" downthrust works. End of string. 😃
Matthewdupreez
Legendary member
JasonK
Participation Award Recipient
so your talking about the other stuff indicated - like thrust line vs NP at the end of the block of text?
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