Thrust Line?

[Ron S]

I've been trying diffrent thrust lines on my motor. It seems like the less down thrust,and right thrust I have the better the plane hovers,and flys. Is zero thrust what you really need in a foamy? Ron:confused:

[Chuck George]

I can't tell you about 3d foam planes, but I can tell you my 30 years
of building balsa and glass planes -- not trainers.
Everyone that I have builded over all those years have been "0" out wing, tail and motor. 99% -- for the 1st flight either flew with no trim adj. or one or 2 clicks of trim.

I can see that for 3d hover you would want "0" trim

hope this help

Chuck

[torqueroll]

I've been trying diffrent thrust lines on my motor. It seems like the less down thrust,and right thrust I have the better the plane hovers,and flys. Is zero thrust what you really need in a foamy? Ron:confused:

We fly foamies regularly and have experimented with all different thrust lines. The best thrust line for hovering is upthrust and right thrust. You must achieve "0" trim on the elevator and "0" trim on the rudder in the hover. The only way you can do this is to have up and right. When the plane is prop hanging it will hang in a slightly nose forward attititude. If you have trim on the elevator then the elevator is loaded.
Have a look at some of our indoor video. These planes are set up with up and right.
http://www.marcs.org.au/marcs/Videos/Friday%20Night%20Fun.wmv
http://www.marcs.org.au/marcs/Videos/FridayNightFun23-09-05.wmv

This is the way we set up all planes for IMAC and Pattern

Regards Barrie

[Chuck George]

This may help RonS -- good info for hover.

Pattern I alway set everthing to "0"


Chuck

[GallopingGhost]

I too have been in and out of RC for almost 30 years and have probably built and flown 12 to 15 "original" designs. Drawing from my past success with fun fly and sport aerobatic designs 0-0 flys best for me with a couple of degrees of right thrust depending on the motor and prop. I had a recent example of up thrust with a Flat Out 580 and didn't like it since it seems to cause trim changes with throttle setttings. Besides up thrust is down thrust when inverted. None is the same either way up. Now keep in mind I don't consider myself a flat wing expert and I do prefer airfoils! But Foamies are fun and if you bang the sticks around enough you my find what works best for you.

[BMJR]

For 3D planes the only reason you have a thrust offset is to correct for design/setup problems.
A common misconseption is that you need right offset to counter torque. This isn't true as torque only effects the roll axis.
You use right offset ONLY to compensate for spiral slipstream effect. For a 3D aircraft there really is no other reason for a left or right motor offset.
This is a good reason to design planes with a lot of vertical stab/rudder below the thrust line so that spiral slipstream is equal or near equal so that no right thrust is required.

Up or down thrust isn't typically needed either. In most cases people use up thrust to correct for a poorly balanced model.
If the planes wing and stab are lined up properly (0-0) then the motor should also be set at "0".
If you have it setup "0-0-0" and you find yourself holding up elevator deflection to keep it still in a hover, then chaces are it is bottom heavy.
Take the model with the battery unpluged and hang it by the prop shaft. In most cases you can simply grip it with your fingers and get a good idea. Suspending by a string will give a better indication.
Either way note which way the plane is leaning. Correct the CG by moving components so that the plane hangs vertically and a 0 thrust setting should work great.
This "bottom heavy" condition is common on the typical 3D foamy model because all the equip is typically mounted on the underside of the model. Good for looks, but not so good for a properly balanced model.

[50+AirYears]

The type and amount of thrust offset depends on the design and purpose of the model. Fly the model, start with whatever the plans or assembly instructions say as a starting point, start flying the plane the way you want, and then try small variations in offsets till you get what you want out of it.

[Ron S]

Thanks guys! Ron

[Chuck George]

RonS,

you now have 110 years of modeling, testing and re testing. I new this would be a good thread. Wonder if the Wright Bros. had all this advise. Bet they did not. You have the best of both worlds. -- world wide 21st century hi-tech and the ability to gather info from people that have been there done that.

Chuck

[50+AirYears]

They really didn't have a lot of good advice to go on. In fact, one of their sources on airfoils died a bit before they built their wind tunnel and found out a lot of it was wrong. They had to develop the science we still use today to design propellors. After the screw propellor had been around over 100 years, they found there was no design science. It was mostly TLAR (that looks about right). Their main source of advice was themselves. Bruises from things that weren't (W)right were a good teacher. Like Edison said, you learn lots of ways not to do something before you learn out how to do it.

[Ron S]

You have that right! The pile of wrecked foam is piling up! Ron:)

[DickCorby]

I always put down and right thrust in my planes.

In my travels with my Son in law to Imac contests this year, a lot of the pilots set the motor 0-0 and use transmitter trims and mixes to overcome the thrust changes with power change.

I'm still of the old school where you trim a plane to fly straight by itself, then interrupt it's flight with the sticks once in a while.

Worked fo me since 1968.

[DickCorby]

<quote>For 3D planes the only reason you have a thrust offset is to correct for design/setup problems.
A common misconseption is that you need right offset to counter torque. This isn't true as torque only effects the roll axis.</quote>

If that is the case, then why does a plane jump to the left when you apply power on the ground. It can't be rolling on the roll axis if the wheels are on the ground.

I think you will find that the spinning prop affects more than the roll axis, at least that is what I was taught. "For every action there is an equal and opposite reaction" Part of the action of the spinning prop does contribute to the roll axis, the rest applies to the Yaw axis, and the pitch axis. Not sure what the percentage is.

Until the Rudder begins to "Fly", there is a torque effect that causes the plane to go left, even with thrust built in. Without right thrust, the plane will always have a tendency to yaw and roll left. Without down thrust, the plane will tend to pull toward the canopy under power.

Using a computer radio, you can do mixing and so forth to overcome these problems, but for the simple transmitters, right and down thrust are a wonderful thing.

[loker]

I always try to make my planes 0,0. I don't have much experience, but I can hover all day long, and much more, and 0,0 works real well for me, except when I try a reverse torque roll, things seem out of whack, but I'm flying 14 oz planes outside, the effects of changing the thrust line may be more apparent in precision indoor flying, which I will do for the first time today! yay. and also, I don't measure it or anything, just eyeball the firewall and glue it. maybe after flying inside I will have to add some right thrust or something, anyway, I guess it depends on what you're doing. I have no idea how those ETOC guys have their planes set up as far as thrust lines go, maybe ask one of them, I think they have Q&A forum in rc universe, that's your best bet, ask an expert, cause there's a lot of aerodynamics involved, and just assuming something that may seem obvious may lead you to the wrong answer.

PS I did read somewhere that the air from the prop rotates around the whole airplane, and ends up hitting the rudder on the left side by the time it gets back there, which would cause a left turn, so a little right thrust might help, but hovering requires so many inputs every second from the pilot, what's the point in right thrust if you're constantly adjusting anyway.

[BMJR]

<quote>For 3D planes the only reason you have a thrust offset is to correct for design/setup problems.
A common misconseption is that you need right offset to counter torque. This isn't true as torque only effects the roll axis.</quote>

If that is the case, then why does a plane jump to the left when you apply power on the ground. It can't be rolling on the roll axis if the wheels are on the ground.

As I posted above the cause is spiral slipstream effect.
The typical aerobatic design has a lot more stab/rudder above the thrust line than below. Therefore when the helix of air comming of the prop gets to the tail, the result is that the air pushes on the left side of the stab/rudder on the top. And this causes the left yaw on take off.

Another thing that will cause a left yaw is P-factor although it's effect is minimal compared to spiral slipstream. On takeoff the prop produces more thrust on the downward portion of rotation because the blade is hitting the air at a slightly higher angle of attack.

When you are flying inverted at high angle of attack it will cause the plane to yaw to the right.

Scale aerobatic models will need some amount of right thrust or a programmable mix as you mentioned. A bit of up or down offset (or mix) might also be needed to fine tune the pitch axis.
This is simply to compensate for the differences in airframe drag above and below the thrust line, and the tall vertical stab/rudder above the thrust line which is why you need the right thrust or "thro-rud mix" (spiral slipstream effect).

[DickCorby]

Well that means over the years a lot of RC pilots and Builders that have advised thrust on the engine, and explained why, are very wrong.

glad to have that cleared up now, and a basic law of physics taken out of the loop as well.

[fabricator]

I am a relative newbie and just built a Mountain Models Magpie, the supplied motor mount has 5 degrees of downthrust built in, to a newbie 5 degrees looks like the thing is pointing down at 45 degrees:eek: it looks like a lot but it flies ok, I think I'm going to shim some out of it to see what happens, this is a very informative thread, thats why I love this place.:D

[qban_flyer]

If you have it setup "0-0-0" and you find yourself holding up elevator deflection to keep it still in a hover, then chaces are it is bottom heavy.
Take the model with the battery unpluged and hang it by the prop shaft. In most cases you can simply grip it with your fingers and get a good idea. Suspending by a string will give a better indication.
Either way note which way the plane is leaning. Correct the CG by moving components so that the plane hangs vertically and a 0 thrust setting should work great.
This "bottom heavy" condition is common on the typical 3D foamy model because all the equip is typically mounted on the underside of the model. Good for looks, but not so good for a properly balanced model.

Thank you.

I could have not said it as good myself. I always place the components where they belong, not where they may make the plane "look" best, and have used the sting balancing method for ages.

Learned that from the late Jim Conlan, a stickler for perfection if I ever met one. Glad to see he was right after all.

[torqueroll]

Thank you.

I could have not said it as good myself. I always place the components where they belong, not where they may make the plane "look" best, and have used the sting balancing method for ages.

Learned that from the late Jim Conlan, a stickler for perfection if I ever met one. Glad to see he was right after all.

I have just completed a full balanced foamie base on the Ikarus edge 540. It has 4 edge top vertical fuselages, it has 4 fins and elevator and rudder are the same size. This foamie has no down or upthrust as the gear is also balanced around the plane. It does have right thrust.
Rolling harriers are a dream and it hovers and torque rolls all day. It stands on the tail and takes off vertical and I have on occasion landed it vertically.

[torqueroll]

I have just completed a full balanced foamie base on the Ikarus edge 540. It has 4 edge top vertical fuselages, it has 4 fins and elevator and rudder are the same size. This foamie has no down or upthrust as the gear is also balanced around the plane. It does have right thrust.
Rolling harriers are a dream and it hovers and torque rolls all day. It stands on the tail and takes off vertical and I have on occasion landed it vertically.

Another photo of the balanced foamie

[qban_flyer]

Another photo of the balanced foamie

Now that's an interesting concept. Looks very good too!

I have been toying around with the idea of a stabilizer the way you did yours, but I am taking it a step further, which in the end may or may not fly. And if it flies it might be rather hard to control because of orientation issues.

How could that be you may ask? It will have crossing wings just as your stab does. Two wings crossing at the thrust line with two ailerons each! No I won't be able to land it as "normal" plane, but since it will be a 3D foamie type, I'll catch it in mid air just as I do now with the ones I fly.

That is if I manage to tell which side is which:confused:, which surface is doing what:confused: before gravity takes over and it goes up in a splash of Blucor! :eek:

I'm still in the planning stages (been at it for 16 or so months).

[BMJR]

Now that's an interesting concept. Looks very good too!

I have been toying around with the idea of a stabilizer the way you did yours, but I am taking it a step further, which in the end may or may not fly. And if it flies it might be rather hard to control because of orientation issues.

How could that be you may ask? It will have crossing wings just as your stab does. Two wings crossing at the thrust line with two ailerons each! No I won't be able to land it as "normal" plane, but since it will be a 3D foamie type, I'll catch it in mid air just as I do now with the ones I fly.

That is if I manage to tell which side is which:confused:, which surface is doing what:confused: before gravity takes over and it goes up in a splash of Blucor! :eek:


I think I know what you're talking about with that concept. I have done some experiments with these type aircraft and they fly well. They may fly a bit odd to some pilots due to the planes ability to pitch or yaw equally as well. It will seem unstable in yaw until you get used to it.
It helps to have a slighty longer tail moment than a "normal" configured aircraft.
Below is a pic of bungee launched free flight model I put together to test straight line tracking of the design.
I have the wings set so it rolls slowly in flight and it flies straight and level when launched.
A powered rc model will fly great. The problem is orientation and landing as you mention, which is probably why you don't see much stuff like this in the air.
As a visual aid you could color one wing different from the rest and possibly add some design or color to the opposite side for reference.

I am a relative newbie and just built a Mountain Models Magpie, the supplied motor mount has 5 degrees of downthrust built in, to a newbie 5 degrees looks like the thing is pointing down at 45 degrees:eek: it looks like a lot but it flies ok, I think I'm going to shim some out of it to see what happens, this is a very informative thread, thats why I love this place.:D
The magpie doesn't have a symmetrical airfoil section, and most likely has a few degrees of positive incidence set in the wing.
This is typical of trainer type aircraft so that it has a self correcting tendency.
With this type aircraft the downthrust is needed to compensate for the upward pitching moment caused by speed increase (power input). The faster this type design is flown the more it wants to pitch up. If it had no down thrust offset it would probably loop with power input.
The setup for trainer type aircraft and 3D aerobatic aircraft are a lot different.

[fabricator]

Thanks for the info BMJR.

[qban_flyer]

I think I know what you're talking about with that concept. I have done some experiments with these type aircraft and they fly well. They may fly a bit odd to some pilots due to the planes ability to pitch or yaw equally as well. It will seem unstable in yaw until you get used to it.
It helps to have a slighty longer tail moment than a "normal" configured aircraft.
Below is a pic of bungee launched free flight model I put together to test straight line tracking of the design.
I have the wings set so it rolls slowly in flight and it flies straight and level when launched.
A powered rc model will fly great. The problem is orientation and landing as you mention, which is probably why you don't see much stuff like this in the air.
As a visual aid you could color one wing different from the rest and possibly add some design or color to the opposite side for reference.

It's interesting that both of us are thinking along the same track.

I have had the idea in my head for quite some time, but back then electrics did not exist and planes were made out balsa wood. When electrics came into being, most everything was still being made out of balsa, and there was not enough power to get a simple monoplane airborne, much less one with two wings at 90 degrees from each other.

When I began to mess around with Blucor and outrunners, the idea popped back into the radar screen once again. I am still messing with a simple way to have two full sized wings and stabilizers intersecting at the thrust line with a motor up front.

Lately I've been thinking about making the tail feathers of a composite material of thin liteply reinforced balsa so that I could attach four carbon fiber skids to be able to land it on its tail. If I can hover a 3D foamie, I can hover that thing backwards onto the tarmac. Talk about the ultimate VTOL R/C model!

I'll be posting photos when the thing is nearing completion. Next ideas I may PM them to you rather than hijack a thread.

Take care

[Ron S]

Guys,I tried the hanging string trick,and got the plane hanging plumb! It really helped the hovering(it's almost effortless now!) I was also running a 11X4.7 prop and getting alot of prop wash! I went to a 10X4.7(I think that's what it is?) and it really started to handle well. The bigger prop cuts down on the rpm needed to hover,but seems to induce alot of tourque(P-factor). I wish I had tried this before now! I've been trying to hover consistantly for 1 1/2 years! Now I can get the plane real close to me and even walk around it (when I don't stumble) while hovering! Thanks for all the info. Ron:D

[BMJR]

Glad that helped you.
Vertical balance can make a huge difference in the way a plane flies. Most planes are simply setup with a for/aft CG to find the neutral balance, and the vertical distribution of components is ignored. It is well worth the time and possible modifications to get the vertical balance on or as near the thrust line as possible.
3D flying is hard enough without having to fight a poorly balanced model.;)
The prop size is another thing that can effect flight performance as you have found out.
On most of my electric models I try to use the biggest prop that I can practically use to get the best efficiency.
However, a large prop means a higher gyroscopic force that the plane has to counter during pitching and yawing manevers.
On some planes it may be impractical to use large props because of this force, and a much smaller prop will be needed. In most cases going to a much smaller diameter prop will result in a more maneuverable aircraft. Loops will be a little tighter, spins better, faster tumbling rate, etc.
If efficiency is not a main concern then you're probably better off selecting the smallest prop that will still provide adequate blast over the controls.
You can tell when you go too small because the contols will become less effective in a hover.

qban,
I'd like to hear how your design turns out. I've got one of my own drawn up but haven't had the time to put it all together yet.

Regards