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Author Topic: Out thrust offset..  (Read 11463 times)

Offline Joe Yau

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Out thrust offset..
« on: October 04, 2014, 03:46:29 PM »
Just the other day.. There were pretty much no wind at the field.  So I had to do the usual stepping back between the consecutive maneuvers.   After the flight, a flyer told me I should put some out thrust offset on the plane,  something like 2-3 1/3 degrees.  then I shouldn't have to step back.  I'm just wondering does this really work?  I'm just a bit worried that it might upset the current trim of the plane.

Thanks in advance.

Offline Gerald Arana

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Re: Out thrust offset..
« Reply #1 on: October 04, 2014, 06:13:20 PM »
Joe,

Don't do it! The reason for stepping back in dead (calm) air is to avoid hitting your own wake turbulence.  (From the wing tips and NOT the engine)

I have two crashes to my name that happened in calm air........I wasn't stepping back and NO amount of out thrust would've helped!

So now when it's calm, I sit and watch the other guys........ y1 y1

Cheers, Jerry

PS: One crash was a wipe out. The other is repaired and flying.

Offline Joe Yau

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Re: Out thrust offset..
« Reply #2 on: October 04, 2014, 09:31:16 PM »
Joe,

Don't do it! The reason for stepping back in dead (calm) air is to avoid hitting your own wake turbulence.  (From the wing tips and NOT the engine)

I have two crashes to my name that happened in calm air........I wasn't stepping back and NO amount of out thrust would've helped!

So now when it's calm, I sit and watch the other guys........ y1 y1

Cheers, Jerry

PS: One crash was a wipe out. The other is repaired and flying.


I wasn't going to.. at least till there is proof that it does work.   His theory was that the prop offset blast the wake to the  inside of the circle or something like that. but I could see maybe it might help the inboard wing tip's wake a little at best, but not the wake from the outboard tip.
« Last Edit: October 04, 2014, 10:45:47 PM by Joe Yau »

Offline Dan McEntee

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Re: Out thrust offset..
« Reply #3 on: October 04, 2014, 11:17:47 PM »
    I think Jerry is correct. You step back to get out of the turbulence the model makes. I think it it caused by both prop wash, and air disturbed by flap and elevator deflection, especially flap deflection, and it drifts outwards from the center of the circle. I follow the guide lines Bob Whitely spelled out in an article in Stunt News called "Things That Always Work."  I always put in 1 degree or so of down and right thrust. This is to counter act forces that the propeller creates on ALL model airplanes, not just c/l models. If you fly counter clockwise, torque tends to turn the airplane to the left or inward. In a free flight model or R/C model, it has the same effect. I don't get carried away with lots of offset because I think it makes the model turn right too hard and can actually crab, and when you give control inputs it can really make a model yaw quite a bit. If you really like the trim of your model, make any other adjustments very carefully and only one at a time, and make sure you can put things back the way they were if the new change doesn't work out.
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Offline Chris Wilson

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Re: Out thrust offset..
« Reply #4 on: October 05, 2014, 05:05:05 AM »
 His theory was that the prop offset blast the wake to the  inside of the circle.........

Unless the prop diameter approaches that of the wing span how could this be possible?

I thought that the wake was more caused by the thick sections in a stunt wing and the large hole it punches through the air.
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Offline Avaiojet

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Re: Out thrust offset..
« Reply #5 on: October 05, 2014, 06:48:50 AM »
Isn't there some world competiter who uses 3 degrees of offset?

Not sure, but I think I read this?
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Offline Steve Thomas

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Re: Out thrust offset..
« Reply #6 on: October 05, 2014, 06:59:17 PM »
Unless the prop diameter approaches that of the wing span how could this be possible?

I thought that the wake was more caused by the thick sections in a stunt wing and the large hole it punches through the air.

If it's anything like a full-size aircraft, then it's likely to be the tip vortices causing most of the trouble. They can be surprisingly powerful and persistent, and I learnt the scary way that they need to be respected. If you get a chance to watch airliners flying a final approach over water and the conditions are just right, the vortices (invisible) will hang around and gradually descend, until the surface suddenly erupts about a minute after the jet has passed.

Considering the lift our models generate in hard corners, there'd have to be some pretty strong circulation generated.

Offline Dan McEntee

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Re: Out thrust offset..
« Reply #7 on: October 05, 2014, 09:46:39 PM »
  If it was prop wash alone, or the air displaced by the model flying forward, you would run into it all the way around the circle flying straight and level in dead air. It takes control deflection and the wing increasing it's angle of attack in maneuvers to make the killer turbulence.
   The house I was born and raised in was under the approach to runway 24  at Lambert Field here in St. Louis. We lived pretty close to the airport and the airplanes were anywhere between 500 to 800 feet high as they flew over on approach for landing. On calm summer evenings, when a jet air liner or even a Connie or DC-7 flew over, you could count to ten and then hear the whirl wind of the wing tip vortices following behind the airplane. On the west side of the airport they used to have a public viewing area and if the conditions were right and the plane was low enough, you could feel them as they reached ground level. We used to take paper airplanes along to fly in the field next to the parking area, and try to time a toss with the passing of each plane and try to get the paper airplane in the vortice. They woud dissipate quite a bit by the time they reached the ground, but would still make a paper airplane do some funny stuff!
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Offline Brett Buck

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Re: Out thrust offset..
« Reply #8 on: October 05, 2014, 11:54:10 PM »
The primary effect of thrust line offset is pretty simple and you don't need to know very much if anything about spiral sliptreams, vortices, etc.

   Assume that the thrust is applied along a line, called the thrust vector, that is right along the engine shaft. If this line doesn't go through the CG (in all three dimensions), it applies a torque to the airplane. In this case, we are talking about yaw torque due to the engine offset to the right or left. It works in pitch too- adjusting the upthrust or downthrust changes the torque around pitch. If it goes straight through the CG, it applies not torque.
   
    The in-flight thrust averages about 2-2.25 lb, and probably goes as low as 1 lb and as much as 3-3.5 lbs on a typical stunt plane and more variation in the wind. The torque is the thrust times the distance the line misses the CG, so if your thrust line misses the CG by an inch, you might get 2 in-lb. Not a huge amount but not trivial, either.

   I think what you more-or-less want is the projected thrust line to go through the CG in the "plan view", to create no net yaw torque, and no variation in the torque when the thrust changes. That's why you want very little engine offset on airplanes with normal amounts of asymmetry, and a  lot of right thrust on airplanes like the All-American Sr. with grossly excessive asymmetry. The asymmetry sets the lateral CG location, because you have to adjust the tip weight until the CG lines up with the lateral CP, so it sets the "0-torque" offset angle.

    Up- or Down-thrust is more interesting. In most conventional layouts, the thrust line is already well above the CG, causing a nose-down pitch torque. Adding downthrust increases it, and you would need substantial up-thrust to get rid of this nose-down torque. I think you want a fair bit of nose-down torque to compensate for the nose-up torque from precession *with a conventional rotation*. With backwards rotation, you might need up thrust, or something like that, which may partially explain some of the odd characteristics  and trim settings of reverse-rotation electrics.

   Brett
   

Online Howard Rush

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Re: Out thrust offset..
« Reply #9 on: October 06, 2014, 01:01:40 PM »
The primary effect of thrust line offset is pretty simple and you don't need to know very much if anything about spiral sliptreams, vortices, etc.

This assumes that the thrust comes from a rocket that doesn't affect affect the nearby air.  Otherwise it's not so simple.
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Offline Joe Yau

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Re: Out thrust offset..
« Reply #10 on: October 08, 2014, 10:13:10 AM »
I think it it caused by both prop wash, and air disturbed by flap and elevator deflection, especially flap deflection, and it drifts outwards from the center of the circle.

Maybe that's how it works..  if the outboard tip's vortices's energy drifts outside of the circle, and the 3 degree offset pushes or diffused the energy from the inboard tip's vortices to the inside of the circle.. the plane is in clean air.     ;D D>K

Offline Joe Yau

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Re: Out thrust offset..
« Reply #11 on: October 08, 2014, 10:26:55 AM »
Unless the prop diameter approaches that of the wing span how could this be possible?

Well, the prop does pushes a lot of air.. (based on a .60 size plane with a 13" prop)   I think it is just the angle or the amount of offset it needs to be successfully effective, and without messy up the trim too badly.    probably have to do an experiment with smoke to plot what it does.

Offline Brett Buck

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Re: Out thrust offset..
« Reply #12 on: October 08, 2014, 01:05:28 PM »
This assumes that the thrust comes from a rocket that doesn't affect affect the nearby air.  Otherwise it's not so simple.

  Actually, I wasn't assuming that at all. I said the *primary* effect is simple, and it is. I agree the secondary (aerodynamic) effects aren't that simple. But so far I have yet to find a consequential effect of thrust line changes on stunt planes that cannot be explained by looking at where the thrust line goes WRT the CG.

    Brett

Offline Joe Yau

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Re: Out thrust offset..
« Reply #13 on: October 08, 2014, 06:17:53 PM »
Isn't there some world competiter who uses 3 degrees of offset?

Not sure, but I think I read this?

Would you know which country he is competing for? 

Offline Chris Wilson

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Re: Out thrust offset..
« Reply #14 on: October 09, 2014, 01:52:31 AM »
Would you know which country he is competing for? 
If I recall correctly - China.
Model had a radial cowl, no spinner and the wind was high so the three degrees wasn't visually noticed at all.
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Offline bob whitney

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Re: Out thrust offset..
« Reply #15 on: October 09, 2014, 08:55:07 AM »
if u can do a wing over or revese wingover without getting slack lines ,u dont need any more out thrust
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Offline Joe Yau

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Re: Out thrust offset..
« Reply #16 on: October 11, 2014, 12:12:39 PM »
If I recall correctly - China.
Model had a radial cowl, no spinner and the wind was high so the three degrees wasn't visually noticed at all.


So, it's not exactly to deal with the calm condition issue.

Offline Chris Wilson

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Re: Out thrust offset..
« Reply #17 on: October 11, 2014, 04:32:04 PM »

So, it's not exactly to deal with the calm condition issue.

My answer? No.

But I probably jumped in where I shouldn't have (my bad) and maybe Charles was referring to something else entirely.

Sorry. :-X
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Offline Joe Yau

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Re: Out thrust offset..
« Reply #18 on: October 13, 2014, 01:21:38 PM »
My answer? No.

But I probably jumped in where I shouldn't have (my bad) and maybe Charles was referring to something else entirely.

Sorry. :-X

Its o.k.,   Charles's post left open couple of questions.. I do appreciate your response on that.

Offline Steve Helmick

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Re: Out thrust offset..
« Reply #19 on: October 16, 2014, 04:19:11 PM »
I'm fairly certain that PW said 1.5 degrees in his planes. The only thing about that to surprise me about that was that it wasn't 1.500 degrees.  VD~ 

It might be worth reading what Frank Zaic said about engine offsets and the effect on circular airflow.  H^^ Steve
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Offline Joe Yau

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Re: Out thrust offset..
« Reply #20 on: October 18, 2014, 07:50:25 PM »
I'm fairly certain that PW said 1.5 degrees in his planes. The only thing about that to surprise me about that was that it wasn't 1.500 degrees.  VD~ 

So, was it more or less?

Offline Air Ministry .

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Re: Out thrust offset..
« Reply #21 on: October 23, 2014, 06:30:24 PM »
YES .

It depens on the square root of the hypotonuse of the nose moment devided by the chord / over the fin area with
the neutral factor from the LE shape interspersed with the angle from vertical of the TE , with a factor of .72 if its curved .
 n~ :! :##

Some People think this is Hogwash .



but the correct term is PROPWASH . n1 so it depens on the prop . >:D

Then Al Rabe invented ' P ' factor , as He thought things were getting to simple .







Gyroscopic Processions another thing altogether .

https://www.backcountrypilot.org/forum/precession-skywagon-with-a-three-blade-prop-5189

and tourgue reation to applied force in acceleration or due to applied load ,, so variable offset might be beast . >:D

EGAD .



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