Gentlemen,

I am perplexed.
What should the engine offset be for a Full Body stunter?

Mr. Pat Johnson says he prefers 2 degree offset.

The Brodak plans state 0/0 (no engine offset).

Mr. Wayne Willey recommends that 0/0 is the engine offset.

At one time Mr Brett Buck recommend some offset.

What engine offset is recommended for a Profile stunter?

Not sure what engine offset is the best for the Chas Parrott Ryan.

Any additional suggestions?
           1.0 degree?
           1.5 degree?
           2.5 degree?
           etc......

Thanks for your advice.

T Michael Jennings     
Knoxville, TN.       

Somebody told me recently that Paul's Impact article says 2 deg. right thrust, but I just looked and found it says 0-0 on rudder and engine. His PW-51 Mustang did use right thrust, I'm pretty sure.

I learned a long time ago (but then forgot) that a little engine offset will very much speed restoration of lost line tension...if that ever happens to you.  You still trim the model to fly tangent to the circle with LO guides. This will put the LO's a bit far forward when the motor/engine stops, but with the speed reduction, the LO's are really too far back anyway. Remember that the faster the model goes, the farther back the LO guide should be, per Line 2 & Line 3 programs, so when in glide mode, they should move forward, but don't. So, I'm with Patrick. And RJ Whitely, OBTW.  Steve

Remember that the faster the model goes, the farther back the LO guide should be, per Line 2 & Line 3 programs

I wouldn't think so.  If there's no wind and the programs don't consider aerodynamic side forces on the airplane, the line shape is independent of speed.  Both line drag and "centrifugal" force are proportional to speed squared.  There's a difference in drag from Reynolds number, but it's wee.

Remember that the faster the model goes, the farther back the LO guide should be, per Line 2 & Line 3 programs...

I finally downloaded Line III.  It says the lines go forward as the speed increases.  How come is that, Bob?   

That's not it.  Drag is proportional to speed squared, too. Does line drag coefficient decrease with Reynolds number?  Looks like it might, but I can't find any free data on the Web with sufficient resolution.

He must have determined centrifugal force increases with speed greater than line drag.

Aha.  I found it, and yes, he did.  Indeed Pete's paper was in the Line III folder.  (It's also at http://www.fesselflug.ch/download/pdf/2011/Leinezug/linedrag2003.pdf ).  It includes Reynolds number of lines, which looks like the reason for line angle going down with speed.  It's a really cool paper.  It also discusses whipping and the effect of wind.  You also included a .pdf file of his paper on atmosphere, written for modelers and similarly wonderful.  

For Pete's sake, Pete, read Pete's paper. 

For Pete's sake, Pete, read Pete's paper. 

Why, does it redfine the basic laws of physics?

I did mention that the explanation above is the Most Simple Terms / Example. And besides, everything eventually is reduced back to the most simple of explanations. Such as (you move the leadouts further froward as speed increases) This is a case that starts with simple physics then a whole bunch of external factors get tossed in with equally numerous and complex formula to identify it all to get right back to the very simple answer of "you move the leadouts further froward as speed increases".

Now Y'all can get tangled up in the minutia of the why if's and what then's and the calculation of Pi to the 10,238th decimal place, but I have enough of that at work to get all wound up about it with model airplanes. Besides the frictional coefficient of stainless steel wire in air, at 3,231.683497 ft. altitude above MSL, at 43.821 % humidity, and 76.821 degrees F., and how it impacts the engine offset and lead out position on a toy airplane really belongs in the Engineering section of the forum. 

Aha.  I found it, and yes, he did.  Indeed Pete's paper was in the Line III folder.  (It's also at http://www.fesselflug.ch/download/pdf/2011/Leinezug/linedrag2003.pdf ).  It includes Reynolds number of lines, which looks like the reason for line angle going down with speed.  It's a really cool paper.  It also discusses whipping and the effect of wind.  You also included a .pdf file of his paper on atmosphere, written for modelers and similarly wonderful.  

Hang in there T Michael!

When you look at the aerodynamics, mainly of the wing, the outboard wing flys faster than the inboard wing.  Wild Bill had the formula someplace.  It depends solely on the wingspan  related to the line length.  So on a 48 in. plane  with equal panels the aerodynamic center of the wing is about .44 in. to the right of the centerline.  On a 60 in. plane it would be about .6 in.  This gives you a couple(unbalanced force) between the thrust line and the center of drag, on a full-bodied stunter that is always turning the  plane out of the circle.   Profile planes lose some of this couple because the width of the fuselage subtracts from the wing aero offset, reducing the force.

On a profile you can get the couple back by aiming the thrustline so it crosses the wing centerline close to the quarter chord of the wing.  Then the force layout is almost identical to a full-bodied plane.  Adding washers under the front engine bolts works, but is hard on the case and motor mounts.  I've had much better success cutting the angle into the motor mounts, usually about 3/32 in. in the about 1.5 in. where the motor bolts up.  Make sure to angle the bolt holes to match and the bolts will love you.

You can add to the effect by making the outboard wing a little larger(a few square inches).  Some folks do this by enlarging the outboard flap.  Then you need to balance that by adding either more tip weight, and/or moving the leadouts back a bit.  Other folks eliminate the couple by making the inboard wing a few square inches larger by making it half an inch or so longer. That simplifies some trim problems but introduces a few also.

The bottom line is no matter what you do the plane is assymmetric when it is flying because the outboard wing flies further than the in board wing.  All the trimming and design tricks are aimed at minimizing this effect on the plane in maneuvers, but in the end they are only perfect for one speed and one attitude.  So just get it close to some happy medium.

One advancement you can't use in PA yet is Spectra lines.  They weigh about 1/6 of what stainless steel lines weigh.  The low weight almost completely eliminates any effects from the bow in the lines swinging around as the plane maneuvers.  The weight of the lines usually shows up in the maneuvers with turns of a tight radius.  The plane slows, the backwards bow in the lines kind of rolls down the lines, usually just in time to jerk the plane as the next corner is starting.

Combat planes don't weigh much and have relatively fat lines.  That's why their lines bow back so much. Use Line III.  It's the bees' knees.

Look, Steve, I'm a busy man.  I can't be reading the questions I answer.  Good trick, you rascal.

Gentlemen,

I am sure glad that issue is cleared up!!!!

Thanks,

T Michael Jennings         
Knoxville, TN