Engine offset: is it still in style?

[frank mccune]

        Hi All:

        Do people still add engine offset to their planes to get more line tension?  If so, How much?

                                                                                                                Tia,

                                                                                                                Frank McCune

[Dan McEntee]

   I don't ever use the exaggerated offset that was always on the profile plans. I go by Bob Whitely's list of "Things That Always Work." And that is 1 degree right thrust, and one degree down thrust, and that's all. RJ's list was published in Stunt News several years ago. I started practicing that and started to get models that flew right off the board with very little trimming needed to do the pattern. The last two stunt models I have been able to build, a Shark.45 and a profile stunt/scale model of Dusty that is basically a kit bashed SIG Twister, both flew a nice complete pattern on their maiden flight.
   Type at you later,
   Dan McEntee

[Steve Helmick]

I asked Paul Walker about right thrust, and he said 2 degrees.

When I flew the Twister, it came with none. I put some in, and found that the main benefit was that WHEN the lines got loose, they got tight again MUCH quicker. That there can save your plane, so I think it's a fine idea. Put it in when building and trim around it. I expect the LO's would be further forward than without the right thrust.

I even thought about what happens when the engine stops, but if you look at the LO position calculator ("Line2", etc.), the reduced speed of the plane in glide mode would suggest (ideally) shifting the LO's forward as it slows. Since that's not happening at present, there's no reason to worry about it.

Now, think about rudder offset. Problem is that as the models slows at various points in the tricks, it loses effectiveness. That be bad!

About downthrust...I don't believe PW uses any. However, in my experience, "cross-controls" are a way to gain stability. I think that is what it is. Cross control.  Steve

[Motorman]

I think the trick is to have line tension with the plane flying straight. This means smooth maneuvers and maintain velocity. If you're crabbing nose out you'll loose line tension when you use the elevator in both directions.

MM

[Steve Helmick]

That's true. But by moving the LO's forward, you can compensate for the right thrust and get back to flying tangent or whatever you think works the best. The right thrust will still help get the lines tight quicker when stuff starts going wrong. Saves glue. Steve   

[Keith Renecle]

Hi Frank,

It all comes down to overall trim balance if you want to get a C/L stunt plane to fly really well. It is not just engine offset and whatever you adjust effects something else. There have many articles on this forum and also the Stuka Stunt forum on how to achieve this total setup. One thing is for sure, that the old story of a C/L plane that has to fly at a dead tangent to the circle is not really true because the fuselage will then be generating some lift towards the circle, or negative lift. Getting good line tension everywhere in the pattern is a real challenge and is also an ongoing process to get right in all conditions. Paul Walker's trim chart that came out many years ago was always the go-to article to keep handy, but then then Paul retired and did a great job of expanding on this article in fine detail in his latest articles in Stunt News. You can also find them on the Flying Lines website here:

http://www.flyinglines.org/walkerstunt.html

Keith R

[frank mccune]

     Hello All:

     Thanks to all for all of the answers submitted.

     A very big thanks to Keith for posting the PW articles.  I put them on my favourites list for a reference source.



                                                                                                                 The best to all.


                                                                                                                 Frank

[Mike Alimov]

There are two ways to obtain line tension on a CL airplane: from centrifugal force (F = m*V^2/r) or by aerodynamic means (engine offset, fuselage lift, etc).  The centrifugal force can be significant, and it grows with the weight of the model and its speed.  But - it gets reduced at the top of the circle by the weight of the airplane (m*g), and even more so when the plane heads into the wind (the wind speed gets subtracted from the model speed). 
So a couple of years ago I picked up a Strega ARF, and equipped it with Saito 72 (a monster of en engine, swinging a 15" prop), being all but sure that I've satisfied my need for overhead line tension.  The flying weight came close to 70 oz, and it flies 5.2 sec lap on full 70' lines.  This is about as big and as fast as I can handle.  The result?  Going into 18 mph wind overhead, line tension was nearing the Netzeband wall (I had to pull on the lines to fly the overhead .   A couple degrees of engine offset made things much better.
I'm a believer now.

[Phil Spillman]

Hi Frank, From my point of view use as little if any right thrust on a profile as you may get away with. I prefer to simply use thicker stock for the rudder and airfoil it on the inside, side toward the pilot, so that you have a natural pointing of the rear toward the inside of the circle which thus points the front of the plane toward the outside! I don't ever use rudder offset anymore! Also an adjustable weight box for tip weight helps in trimming as well!

Tally Ho!

Phil Spillman

[Avaiojet]

This, probably original, Barnstormer plan calls for 5 degrees of "outside" thrust. Plans call for clockwise flight also.

I cropped the plans to show the description text of the amount of offset. Hope it's readable?

[frank mccune]

      Hello Charles:

      Thank you for the Barnstormer post.  I grew up with engine and rudder offset as a requirement for control line airplanes.  I have not seen it mentioned since I cam back to model airplanes in 2013.  That is why I was under the impression  that it was no longer required in CL planes.  It seems to not to be mentioned in construction articles that I have read.  Perhaps It is I who is missing something. Lol

                                                                                                                Stay well my friend,

                                                                                                                Frank

[EddyR]

Frank 
  The Barnstormer is a 1950 design. That was the thinking back then. The plans were for the mass market who more than likely not build the model to contest standards. The engine mounted with all that offset would pull hard but it also made it difficult to trim out. No engine or rudder offset works much better today if the model is built straight and trimmed correct. I remember building Ringmasters as a kid with lots of engine and rudder offset. Yes they flew but not anywhere as nice as I can get them to fly today.
EddyR

[Avaiojet]

I read someplace that fuselage offset was used on some world class airplanes. I think I read that right here in this Forum.

I kinda remember who, but I'm not sure? When I have some time I'll look for it.

I offset the fuselage one degree on my Stuka Tank Buster and the ARGO 2, based on these suggestions.

I also have one degree of offset on the engine, based on the same suggestions.

Both these models have adjustable leadouts and weight boxes, plus adjustable elevators.

Much of this, offset, depends on the design of the model AND what the model is.

Full blown stunt or profile R-1. Different models, apples and oranges.

Charles

[Mark Scarborough]

Charles, you are suggesting that the fuselage have angular outthrust in it? I dont recall anyone saying that, at least not anyone who has won any significant contests.
the Engine offset, yes, its pretty common, one to one and a half degrees typically, but the fuse and the wings should be perpendicular to each other, as should be the stab.

as to rudder offset, its very out of style in competition airplanes as is airfoiled rudders. the problem with airfoils and offset is that as the speed varies, so does the influence of the rudder causing weird trim issues...

[Target]

My c/l mentor says too much offset, especially rudder makes the plane sometimes lose tension in overhead eights, but pull harder in level flight. He says too much of a good thing causes too much crab, slowing the plane a bit and also making it draggy up on the top of the circle, where gravity has it's greatest effect. He says too use minimal rudder offset and about 2° right thrust to try to tone for equal tension everywhere in the pattern.
So far, he hasn't been wrong that I can see.
R,
Chris