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Sport Planes (Formerly I/C & Gas Conversion) Discuss I/C or Gas Conversions, Aerobatic Planes and Sport Aircraft

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Old 05-09-2013, 12:01 AM   #1
CNY_Dave
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Default Thrust line question

Gas to E conversion of a '70s clipped wing cub, designed as an aerobatics trainer.

Instructions say no down-thrust or side-thrust needed. I had just a little right thrust on it and oh boy did that mess things up, so I want to make sure I interpret the no down-thrust question properly.

This plane has a symmetric wing, and the wing leading-edge point to trailing-edge point is level when the tail (Hstab) is level.

Now, I have the motor shaft mounted square to the firewall, yielding zero left/right thrust (tail to shaft string test verified), but the firewall is not vertical when the tail is level. With the tail (and wing) level, the motor has a couple degrees 'down thrust'.

I would like to check my assumption that for level flight the plane will need at least a few degrees angle of incidence on the wing (of course it does, I even feel silly typing that), so a couple/few degrees of downthrust relative to tail and wing level (line through leading edge and trailing edge of the symmetric wing is level), so with the plane in level flight there is essentially no downthrust, and all is well.

As far as I can tell the gas mount I took off adds no down thrust relative to the firewall, that is the mounting surface for the motor is at right angles to the firewall mounting surface.

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Old 05-09-2013, 07:27 PM   #2
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Originally Posted by CNY_Dave View Post
Gas to E conversion of a '70s clipped wing cub, designed as an aerobatics trainer.

Instructions say no down-thrust or side-thrust needed. I had just a little right thrust on it and oh boy did that mess things up, so I want to make sure I interpret the no down-thrust question properly.

This plane has a symmetric wing, and the wing leading-edge point to trailing-edge point is level when the tail (Hstab) is level.

Now, I have the motor shaft mounted square to the firewall, yielding zero left/right thrust (tail to shaft string test verified), but the firewall is not vertical when the tail is level. With the tail (and wing) level, the motor has a couple degrees 'down thrust'.

I would like to check my assumption that for level flight the plane will need at least a few degrees angle of incidence on the wing (of course it does, I even feel silly typing that), so a couple/few degrees of downthrust relative to tail and wing level (line through leading edge and trailing edge of the symmetric wing is level), so with the plane in level flight there is essentially no downthrust, and all is well.

As far as I can tell the gas mount I took off adds no down thrust relative to the firewall, that is the mounting surface for the motor is at right angles to the firewall mounting surface.
A Couple of degrees of down thrust is a good thing, it will help to load the main wing and give you a nice solid Flight, Your good to go, test fly the plane, if it wants to porpous up and down when flying, give it a little more down thrust, Hope that helps, Chellie

I may be getting Older, But I Refuse to grow Up I am Having to much Fun to Grow Up LOL
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Old 05-09-2013, 07:55 PM   #3
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Originally Posted by CNY_Dave View Post
Gas to E conversion of a '70s clipped wing cub, designed as an aerobatics trainer.

Instructions say no down-thrust or side-thrust needed. I had just a little right thrust on it and oh boy did that mess things up, so I want to make sure I interpret the no down-thrust question properly.
I always start with the designer recommended for thrust lines. With a fully symmetrical wing - you should NOT have down or right thrust.

That said wing position and decalage (the angle of the wing to horizontal stab) do matter in thrust angles too.

But as you discovered started where recommended is smart - as the designer should have figured that all out for you.

Originally Posted by CNY_Dave View Post
Now, I have the motor shaft mounted square to the firewall, yielding zero left/right thrust (tail to shaft string test verified), but the firewall is not vertical when the tail is level. With the tail (and wing) level, the motor has a couple degrees 'down thrust'.
That is certainly odd. Zero down should have the firewall perpendicular to the datum line. If the firewall is angled - that sure indicates they anticipated down thrust to be used.

Originally Posted by CNY_Dave View Post
This plane has a symmetric wing, and the wing leading-edge point to trailing-edge point is level when the tail (Hstab) is level.
That indicates there the decalage is Zero/Zero - again expected from an aerobatic airplane.

Originally Posted by CNY_Dave View Post
Now, I have the motor shaft mounted square to the firewall, yielding zero left/right thrust (tail to shaft string test verified), but the firewall is not vertical when the tail is level. With the tail (and wing) level, the motor has a couple degrees 'down thrust'.
Correct - but odd when they say they don't want down thrust.

Originally Posted by CNY_Dave View Post
I would like to check my assumption that for level flight the plane will need at least a few degrees angle of incidence on the wing (of course it does, I even feel silly typing that), so a couple/few degrees of downthrust relative to tail and wing level (line through leading edge and trailing edge of the symmetric wing is level), so with the plane in level flight there is essentially no downthrust, and all is well.
No you do not want to mess with the decalage unless there is a reason to do so (flight issues) so I don't think you want to do this.

Originally Posted by CNY_Dave View Post
As far as I can tell the gas mount I took off adds no down thrust relative to the firewall, that is the mounting surface for the motor is at right angles to the firewall mounting surface.
Sounds like you have all the angles covered.

Remember MANY airplanes do not benefit from any right or down thrust. If this was a regular flat bottom wing Cub you would want BOTH but since this is intended to be aerobatic you likely don't want any.

High wing does yield some oddities with aerobatics - and VERY few airplane fly perfectly with no adjustments in any flight conditions. That is very hard to do.

A few planes come mighty close - the Slick from 3DHS and the Visonaire from Horizon are two airplanes I few that essentially did everything and anything you told them too with zero unwanted behavior. Those are both airplanes designed to be very neutral.

I say fly it and less see what happens. You can start to mess with things (decalage, thrust angles, etc) when you have a reason to need to do that.

Mike
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Old 05-09-2013, 08:22 PM   #4
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Cub = motor is slung low compared to center of drag. This means applying power would tend to pull the nose up if there is no down-thrust. so a couple of degrees down is expected.

All the rest in the above post is correct.

If it was a "Pattern" design the motor shaft would be in line with the wing and tail planes and there should be no need for down-thrust.

Where the motor is in relation to center of drag (usually the wing is the biggest drag contributor) almost always determines the up/down thrust needed with a symmetrical wing.
Flat bottom or other "high lift" wings we expect to need a little down-thrust even with the motor in line with center of drag.

You can fly with the thrust line off a little (and just changing the prop on some planes means needing a different thrust line) and determine the adjustments needed by how the plane reacts to throttle... if it is trimmed correctly for glide.
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Old 05-09-2013, 08:45 PM   #5
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Well...... I get where you are going - and generally agree.

Down thrust on models is an interesting thing - related only to RC. On full scale airplanes it is generally not used as you want the airplane to climb under power and descend under low power. When the angle full scale engines most of the time it is with multi engine where they are trying to get a behavior for engine out characteristics.

With RC airplanes we all want them to do nothing but "go faster, go slower" with the power and use the elevator to climb/descend. That is a bit odd candidly.

At any rate I am big on the designer figuring it all out - and just telling you. But I am baffled by the down thrust built into the firewall. Most good plans specific down thrust is built into the firewall.

I do agree with what you say....high wing and to a degree low wing airplanes both have issues in aerobatics. The best I have flown have wings and stabs right on the datum line.

At any rate - go fly it - see what it does.
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Old 05-09-2013, 11:20 PM   #6
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You will find some interesting thrust angles on full scale too...
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Old 05-10-2013, 12:32 AM   #7
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What I can say is with just a little right-thrust the results were... interesting.

It flew pretty straight but on a left turn the nose would point up and it needed tons of rudder, I thought at first the plane had horrific adverse yaw.

But, I flew some right turns and some turns with the power off, right turns with power and the nose dropped, turns with no power and it was well-behaved, so i figured i had fat-fingered the thrust line.

If the weather cooperates early next week I should get to try it set to perp to the firewall, i.e. no left/right thrust, and whatever down thrust is set by the firewall.

Thanks all.

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Old 05-10-2013, 12:36 AM   #8
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On a few yes.... Again mostly multi engine.
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Old 05-10-2013, 08:02 AM   #9
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Down thrust is dependant on the layout. With the high wing and (relatively) low thrust line of the Cub I'd expect a little downthrust to be required.

To check for down thrust fly the plane level at WOT and chop the power. If it dives you need more downthrust. If it climbs you have too much downthrust. if it keeps going pretty much straight only starting to descend gently as it slows down then you have it right.

I have never yet found a single prop plane that didn't benefit from a little right thrust. Right thrust has nothing to do with wing airfoil. It's to do with the various asymmetric effects of the spinning prop (P-factor, spiral slipstream, torque, gyroscopic precession) which combined tend to cause yaw and roll to the left. Right thrust helps counteract most of these effects.

To check for correct right thrust pull the plane into a vertical up-line. If it yaws off left then it needs more right thrust, if it yaws right it's got too much right thrust. if it continues straight up then you have it about right.
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Old 05-10-2013, 01:28 PM   #10
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Originally Posted by JetPlaneFlyer View Post
I have never yet found a single prop plane that didn't benefit from a little right thrust. Right thrust has nothing to do with wing airfoil. It's to do with the various asymmetric effects of the spinning prop (P-factor, spiral slipstream, torque, gyroscopic precession) which combined tend to cause yaw and roll to the left. Right thrust helps counteract most of these effects.

To check for correct right thrust pull the plane into a vertical up-line. If it yaws off left then it needs more right thrust, if it yaws right it's got too much right thrust. if it continues straight up then you have it about right.
I have had a bunch that don't benefit at all. I watched Ben Fisher (owner of 3DHS) argue with a guy about motor thrust on his models. The guy was taking him to task it got pretty comical as Ben explained why his models were set with zero/zero.

They are some of the truest best flying planes I have owned - and I never felt the need to question his recommendation for no thrust offset.

At any rate - I when I buy models I expect thrust, decalage, CG range and airfoil design to be all done by the designers and I expect that to be right. The "dive" test only works for example if the CG is correct - it skews the result of that test. Many variables on these planes.

I have had some that were not - it is sure a pain to fix other peoples mistakes.

Mike
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Old 05-10-2013, 02:23 PM   #11
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The wing design I was mentioning since I think airfoil type affects how much alpha it'll need at level flight.

The firewall has no right thrust built in, and the manual says to add none, I erroneously automatically added some.

If the ill handling was indeed caused by that small amount of right thrust I will not be shocked, but will be somewhat surprised such a small amount could cause such mayhem. Of course since it is overpowered compared to the gas engine required, and is swinging a 13 inch prop vs the 10 or 11 with the gas engine, any error in the thrust line will be magnified.

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Old 05-10-2013, 04:51 PM   #12
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Originally Posted by rcers View Post
I have had a bunch that don't benefit at all. I watched Ben Fisher (owner of 3DHS) argue with a guy about motor thrust on his models. The guy was taking him to task it got pretty comical as Ben explained why his models were set with zero/zero.
Mike,

I think you might have got the stances reversed in that particular argument, or maybe they were discussing down thrust rather than right thrust? Anyway, All of Ben Fisher's (3DHS) models have 2 or 3 degrees of right thrust. Also by spooky and ironic coincidence Ben Fisher recently wrote a great article on why RC models need right thrust

Here is the article: http://www.3drcforums.com/content.ph...d-Prop-Effects

And here is Ben's summary:
So, to sum up:
  • Props are thrust producers, but they also impart a lot of other forces into our airplanes, like gyroscopic force and many others.
  • These forces are strongest when the prop RPM (engine power) is high.
  • At high airspeeds, the tail of the plane counteracts these prop forces effectively, but not at low speeds.
  • Therefore, at high power at low airspeed (like takeoff and hovering) we feel prop forces most.
  • The net effect of these forces is usually to make our aircraft turn left.
  • To take some work off of our left thumb, we mount our power system and prop at an angle pointing right so that at high power it balances the left turning force with a right turning force.
  • This angle is a compromise based upon the need for the airplane to handle well at a variety of airspeeds.
  • Any decent quality aerobatic ARF already has an adequate right thrust angle, changing it is only for advanced pilots.
  • Applying the classic test to check right thrust can be difficult on a modern 3D airplane since our planes power right past the limits used in the classic test.
  • So, we might need to experiment to find perfection, and this is easiest done by experimenting first with rudder to throttle mixes before taking your plane apart.
As for your models that don't benefit from right thrust... Have you actually tried any right thrust on them?
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Old 05-10-2013, 05:54 PM   #13
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All I can do - is tell you what he said 2-3 years ago to a guy but perhaps it was ONLY down thrust he was arguing about.

I need to go look - I sure don't recall much if any right thrust but guess there is some there....
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Old 05-10-2013, 11:47 PM   #14
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I am a big believer in most rules work most of the time, but getting out there and testing what works best is the only way to actually know what's best.

Toe in/out on a taildragger is a great example, there are so many variables
(camber, cg, traction, wheel diameter, wheel profile, suspension flex, camber change with flex, all the same for the tail wheel plus amount of caster)
anyone who says they KNOW in/out/neutral will be best for YOUR plane is a fool.

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Old 05-11-2013, 08:45 AM   #15
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Originally Posted by CNY_Dave View Post
Of course since it is overpowered compared to the gas engine required, and is swinging a 13 inch prop vs the 10 or 11 with the gas engine, any error in the thrust line will be magnified.
Your problem might be lack of directional stability, the way you describe it wanting to yaw in turns sounds like that might be part of the problem. Reason is that tractor props are de-stabilising and the bigger and more powerful the prop/motor the more de-stabilising they are. The fix is usually to increase the size of the vertical stabiliser. To test if this is the issue you can tape foam extensions to it.

You can see this same issue illustrated in some full size planes. the Spitfire is a good example as it had a long operational history and was progressively upgraded with ever more powerful engines. The vertical stab got bigger with each engine/prop upgrade, compare the vertical stabs on these two Spits:
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Old 05-12-2013, 10:36 PM   #16
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They had to do the same thing with the Mustang. The early ones with the turtle deck had no dorsal fin. Had to put it on the bubble canopy version to make up for the lost fuselage side area.

Gord.
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Old 05-12-2013, 11:22 PM   #17
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any airplane is a set of compromises bolted together. change one and it may change em all.
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Old 05-13-2013, 01:09 AM   #18
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If the weather gets better I'm set to test with no r/l thrust.

If it is a prop effect would it tend to yaw left?

I may stick a 12 or even an 11 on there for that flight- this plane is lighter and a lot less draggy than my trainer which did ok with a 12x6.

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Old 05-13-2013, 02:19 AM   #19
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The prop throws a swirling air stream and generally this will cause a tendency to turn left. (assuming a normal prop) The shape of the fuselage can affect this.

Some designers work hard to minimize the need for thrust offset. Some take into account the expected prop and power, and design the needed offset into the firewall.
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Old 05-13-2013, 05:38 PM   #20
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Yeah, mine had a severe yaw right under power, with just a tad of right thrust, so I don't think it will be worse with no right thrust, but I will be cautious.

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Old 05-17-2013, 02:51 PM   #21
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Got in 2 quick flights as the wind was picking up.

Much improved with neutral L/R thrust, and I tested it with an 11x8 prop in place of the 13x8. The 11x8 makes it fly like a gas plane, I think 12x6 or 12x8 will be the sweet spot, and I'll save the 13x8 for when I'm a bit better.

The gas prop on it was 10x6...

It is still a bit twitchy in pitch, but it's an aerobatic trainer and my DX5e only has a tiny bit of expo (turned on via the secret method).

The Vstab needs a bit of speed to be effective, so it has even more drama on takeoff than my Fokker D-VII as there always seems to be a crosswind and the runway itself has a little side-slope.

I have found it is very easy to get the speed up beyond what I really want, my trainer was draggy and of course the bipe is, this is my first 'clean' plane.

I've gotten the landing speed a bit more under control, but it was starting to get gusty and I didn't want to do a bunch of slow-speed and stall and landing practice.

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Old 05-17-2013, 04:30 PM   #22
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Once you have it trimmed and have a nice calm day try the vertical upline test to check for correct right thrust.
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