OT airplanes, asymmetric wings, off set engine?

Current practice seems to be symmetric wings, no enigne off set. Many of the OT airplanes have asymmetric wings with the inboard side longer. Should these airplanes also have engine off set? Many of them did. The MARS didn't. Would it fly better and/or be easier to trim with or without engine off set?

Hi Ron,
In my understanding of the old plane design, the Old Time (pre '53) pattern only had one "square" manuever. That being the square rectangle (loop). The effects of too much yaw was not nearly as noticable when compared to the modern pattern. The originators of stunt had to deal with underpowered planes that couldn't always do the entire OTS pattern so they resorted to a lot of assymetry, rudder off set, and engine off set to get line tension under less than optimum conditions.
With the means to adjust tip weight and lead out exits, etc., much of the "off set" is no longer needed.
On the MARS I am building, I reduced the rudders' off sets slightly, and have all the adjustable features I could add.
I do make sure that my engine is set with no "inset" just to be sure!
Bill <><

Ron, Bill,

(Not knocking you, Bill, but I still credit panel area offset as very useful.BTW, it ISN'T just the "Olde Tyme Areoplaines" that use/used it. <g> Equal panel wings is a choice, involving acceptance of tinkering flap areas, tipweight, leadout rake, etc. It CAN be made to work. )

I've convinced myself that the dynamics of our kind of flying gain when we reduce as many variables as possible, first, then trim out any stray factors. Such strays always seem to be there, and as Len often says, no two stunters fly identically the same.

My view of the dynamic picture requires a certain amount of panel offset -- actually, by moving the fuselage centrline slightly outboard from the center of the physical wing. I estimate this amount.

Other things that I use with reasonable success and good repeatability across very different looking stunters are more internal and detailed. I'm one of the nuts interested in bellcrank and control horn response -- it's NOT symmetrical when done as usual.

Thrust offset is odd -- depending on fuselage and line lengths, there is a 'natural' offset of 1.5º to 2º with a front engine on a straight centerline fuselage -- IF the fuselage flies perpendicular to the imaginary line from flier to CG. The path of the flight circle curves "in" both fore and aft of the CG -- the straight fuselage doesn't.

Consider trigonometry when thinking about built-in engine offset. Remember?

Cosine 0º is 1.000.
Cosine of 5º is 0.9962, and
Sine 5º is 0.0872.

So? If total engine offset -- 2º 'natural' plus 3º built-in -- is 5º, the 'forward component' of thrust is still 99.62% of total thrust.

If a gust, goof or other disaster causes a sudden loss of speed and line tension, we still have (Sine 5º =) 0.0872 times thrust to pull the nose away from the flier... that is 8.72% of thrust acting to restore taut lines at a cost of 0.38% from 'forward' thrust.

So, built-in offset IS sort of a belt-AND-suspenders approach. However, it only involves engine thrust, not the aerodynamic or senior-moment problems that put the plane in danger to begin with. When all else has stopped being helpful, this factor remains -- whatever the model's attitude. It works the same way as long as the engine is running. I have detected no problems involving gyro precession or other influences from these slight offsets...

Wow, this is good Lou!

Yesterday at the flying field a fellow flyer remarked on the "coolness" of a tank adjuster on my Brodak Zero. It dawned on me then......once we're done trimming a model, we really don't need much of these trimming devices anymore! I'm sure this is not a new discovery, but can you come up with a mathematical presentation of this thought?

Remember this that the only thing that gives line tension in the overheads is off set.

What?

See Ya

what do you not understand?

WHAT AGAIN??????

Ok on last time. Its not only early stunters that use asymmetrical wings and engine off set. SV 22 for instance. Keeping line tension in mind it calls for engine off set and also has a asymmetrical wing.
I think a better question would be point of teather. If you think it doesn't matter do the math. Or you could just think about it from an engineering stand point. The mass of the engine is always trying to line up with the point of teather.

Actually the model's center of mass is trying to line up with the tethered point, not just the engine. Most leadouts are positioned near the CG of the model to avoid much yaw in either direction. Obviously yawing of the nose to the inside of the circle can be exciting.

The major contributor of line tension overhead is speed of the model. That small thrust component isn't adequate to hold an average 40 oz. model out on the end of the lines when it's overhead.

Let's use Lou's numbers for a minute. Assume that your favorite stunt motor will generate 5 lbs. of static thrust. If the offset will vector 9% of the thrust away from the flyer then there is approx. 7.2 oz of thrust to hold that 40 oz. plane over your head!

Guess what will happen?

So if we need generate 40 oz of thrust outward then we need more offset or more thrust... a motor that generate about 27 lbs. of static thrust would work!

Speed may kill but it keeps CL models from falling on our heads

Well I think I'll just keep pointing my engines out. It has worked sense 1965 can't argue with success.

Sparky, and Dick Fowler,

Thanks for the kind words and thoughtful responses !! I agree with DF's remarks about the usual QUANTITY of outward vector, and add a few thoughts:

First, the model, going over the top, still has quite a bit of its velocity. The MAIN force keeping it out there is STILL "centrifugal." That is not an 'Omigosh!!' situation -- it is a slight forward speed loss (with consequent, not so slight loss in CF, or pull, which makes us nervous...)

Second, DF is RIGHT that the center of mass -- we usually think of it as longitudinal CG -- will line up as best it can with the pull force. I design and trim to have the fuselage centerline as tangent to the flight path circle AT the CG as possible. Now, think about how the pull force reaches the model. The lines sag aft of (and slightly below) a theoretical straight line from the center to that CG. Our lines are, in effect, limp tension elements. They can only pull, not push in any direction. The share of pull force on each line stays centered within that line's diameter.

-- The angle the lines make with the imaginary line as they get to the tip guides CAN be estimated. Once inside the wing, the lines aren't exposed to air drag anymore. I place leadout guides to meet the lines at the wingtip so that the center of the net pull force aims AT the CG. (It also reduces wear, leadouts nearly 'float free' in the guides.)

-- Adjustable leadouts mainly change where the the model yaws to line up 'pull'with the CG. The lines don't so much push the leadouts fore or aft, as aim the net pull force ahead of or behind the CG. That creates a moment yawing the model, which is resolved as a 'couple'(torques match, yawing stops) with the fin/rudder effect (straight fuselage) and any other yaw stabilizing forces present.

Finally, about the quantity of the outward vector... I think of prop thrust as an elastic tow rope, in effect. Speed stabilizes where the engine is mixture, pipe, load, etc., limited from winding higher. Stunters are clean forms with little profile and induced drag in level flight. The ACTUAL thrust applied to the model in cruising flight (e.g., between maneuvers) is ONLY as much as needed to maintain steady speed. We know that the prop pulls a lot more at launch, so if the model rapidly slows in the air, we 'stretch' my imaginary rubber tow rope. Like towing a rowboat that suddenly catches a big glob of seaweed?

-- As prop thrust applied to the model lessens from takeoff acceleration to where it settles in at 'applied thrust = total drag,' slowing the model should put us back at the same applied thrust we had when the model accelerated through that speed on takeoff. Instead of a forward applied prop thrust of 8 to 12oz (level cruise) we may be back to from 16 to 50 oz. The outward 9% of that still isn't much, but if the model is otherwise beyond any of our usual control efforts, that vector IS still there.

As I said, it's more like belt AND suspenders...

Regarding Lou's explanation of the fact that we fly in a circular path results in a small amount of engine out thrust, assuming that the model flys tangent to the pilot. The same thing applies to rudder offset as well. Even a rudder with no offset built in will have appoximately twice the effective offset as the engine due to the longer moment arm. My rough calculations find that a model flying in a circle with no offset rudder is equivalent to a model flying in a straight line with the entire rudder offset about 3/16 of an inch. Certainly enough to keep a well trimmed model out where it belongs! And if the model flys not quite tangent, you get more of one to compensate for less of the other, a good deal!

I can see that point sort of, but the plane is not just hanging up there overhead, it got there with momentum from airspeed. It would seem that only a little off set thrust would be needed, say just enough to yaw it away from the pilot. It would seem to me that if you look at the force vectors from the bell cranks point of view you could'nt discount the foward momentum that is already in play. There are force vectors that aid the small amount of off set thrust. Thats probably why you can run a straight rudder with just enough trim to get through the overhead maneuvers where yaw really comes into play. Just my own opinion/observation.

SoH (disclaimer: spelling/grammer)

>Remember this that the only thing that gives line tension in
>the overheads is off set.

WHAT!!!!?????????

I have not used engine or rudder offset in my last four ships, and I have MORE than enough line tension. Its all about centrifical force and horsepower. 67 ounce @ 60 MPH on 65 foot lines + piped Jett .50 = line tension.

Engine offset.

Assuming no peculiar trimming, Larry's numbers equate to about 3.6 g's (approx 13.7lbs of pull in level flight) That leaves him about 2.6 g's at the top of his wingover. (about 10 lbs. of pull).

Judging from the lap times that people post, most guys seem comfortable when the plane generates at least 3 g's in level flight.

It really is a much bigger number than can be generated by engine offset.

OK. Good stuff! But, is engine offset more beneficial to models with asymetric wings than for models with symetric wings? I suspect that the virtual center of drag (if there is such a thing...) on a plane with asymetric wings will be towards the longer panel relative to the centerline of the fuselage. If so, an engine mounted on the fuselage centerline without offset would create a yawing moment towards the center of the circle. Somewhat like a twin engine airplane with one engine out, only to a lesser extent... Is it desirable to try to get the "thrust vector" to align with the "virtual drag center"? Are there larger factors? like: torque, gyroscopic forces, "P" factor, etc., etc.?

Most of the plans for current, HI-ZOOT stunt planes show zero engine (or rudder) off-set.

I think that engine off-set is used more on littler models...1/2a's like lots, .15's some.

My own experience is that I have quit using any at all....

See Ya

Ron,

BTW, DHutch is right. With a model of typical proportions, the same flight circle curvature effect gives 'natural' fin/rudder offset about double the 'natural' engine offset effect for a straight fuselage. Back to t'other stuf.

Lift varies from inboard tip to outboard tip. The inboard tip is at least several wingspans 'out' from the center of the flight path. Lift varies with area and with velocity squared (and Lift Coefficient.) Wing drag (Induced drag) is the result of lift, and grows or shrinks with lift. Without going into calc, with d(this)/d(that) etc., reason it out this way...

The inboard tip flies a shorter path; lap distance is 2*pi*R(inb).
Outboard tip flies a longer path: 2*pi*R(outb), right?

Unless you got a 4th dimensional model, the tips complete a full lap in the same time. So the inboard tip flies slower, and the outboard tip faster, than the speed at the fuselage center. Any problem?

Across the span, speed increases smoothly, but not linearly. As a result, lift, and associated induced drag, increase the same way. At some point across the span, there is equal lift to the inboard side and to the outboard side. Since lift and induced drag are linked, that point is the center of wing lift AND (induced) drag.

I put the fuselage and CG right there, spanwise. Fuselage drag, tail drag, thrust all line up where wing lift and drag are centered. Static balance the model in roll for flight conditions, too, so panel or tip weight won't add a disturbance.

Honk into a corner, and the largest forces are lift, induced drag, and line pull. They center at the CG; so do weight, drag and thrust fuselage and tail forces. No roll from more lift on one side than the other. No yaw from more drag one side than the other.

This can only work as advertised for a single flight condition. Other factors mess with it, like engine torque (it takes more effort to turn the prop as drag slows the model however slightly), gyroscopic precession (with higher RPM and often heavier props, this is a real thing.)

BUT -- we started out with a good approximation of centering the large forces at the CG. Deviations, and disturbances they cause, are nearly mirror images of each other for upright/inside maneuvers and inverted/outside maneuvers.

Strong correcting forces are available, a bit much for this post, but mostly involving the lines, and where they aim 'pull.' If we started 'centered,' deviation (maneuvering) disturbances are about equal both ways from clean trimmed. Because pull is usually strong when we maneuver hard, the model's slight shifts in roll and yaw should escape notice. After all, a proper corner lasts at most 1/3rd of a second. A lot happens quickly -- gross deviations will show; slight ones won't, particularly when the model tracks out of a corner clean, without bobbing and weaving.

That last part is up to the guy holding the handle, more than it is to the airplane. Timing. Finesse. Placement. Confident comfort with THAT model. ...The results of good and ample practice. No shortcuts!

And finally, there will be final touches to groom your "fit" with the model to perfection. There's always some trimming to do. Centering the model's forces well just offers hope for a reasonably close start point.

Because R increases linearly from inboard tip to outboard tip, it is not clear to me that lift should not increase linearly from inboard to outboard tip.

>Because R increases linearly from inboard tip to outboard
>tip, it is not clear to me that lift should not increase
>linearly from inboard to outboard tip.
>
>Jim

I agree.... I think that lift increases linearly and drag increases by the square of the velocity from inboard to outboard. So maybe the old timers' thought that the because lift was increasing from inboard to outboard that the shorter outboard wing would "balance" the lift forces over the length of the wing. Don't know just thinking out load.

I vaguely think that lift and drag are proportional. If so, then lift would increase as the square of the increase in velocity as well.

Jim and Dick,

Lift does not increase linearly with radius, because radius determines the speed, or velocity. Velocity DOES increase linearly from inboard to outboard tip.

Lift calculates from the following factors:

rho (Air density)
S (Surface area, wing area)
C(L) (Lift Coefficient, depends on the airfoil and angle of attack)
V(squared) (Velocity squared)

The traditional Lift equation, in Excel-type notation, would be:

L = C(L)*(rho/2)*S*V^2

A linear change is first order, V^2 is a second order term. That's why Lift increases non-linearly.

The same equation finds drag, if you use C(D) - Drag coefficient - instead of C(L). Getting a value for C(D) ain't easy.

However, for induced drag, the drag that producing lift creates, there IS a relationship:

C(D(i))= ((C(L)^2)/(pi*AspectRatio)

So, as I mentioned somewhere in here before, we can find a number that must exist for C(L) by juggling the basic lift equation.

We know the other terms: Velocity (ft/sec), Lift = Weight(lbs) in level flight, Wing area(sq ft), and a standard value for rho in US dimensions of 0.002378(DON'T ask what they call this!)

With the C(L) value, we use the equation relating C(L) and C(D(i)). The only other thing we need to do that is the wing's aspect ratio. For anyone who didn't know that term, it is the ratio of the average chord of the wing to its span:

AR = B/C (B - breadth, span; C - chord)
or if we have an odd shaped wing:

AR = B^2/ S (S - surface area, as we used above.)

Heck with all the hoopla on engine off set. Maybe we should point them in 1 degree like speed planes? But for now I'll keep pointing them out 2 degree's

1) With a radius of sixty feet, the tension of the model just equals the weight of the model at about thirty miles per hour. So, in a wing over the tension is zero at the top, any less speed and the model comes in. Thus, speed is the primary source for tension, however, when tension is low the engine offset would provide a model outward force to fly the model to line tension. Remember models were designed to do circles, kept lite, and flown with engines that did not have adequate thrust to always keep speed through a figure, low line tension was often.

2) With a radius of sixty feet and a speed of about fifty five to sixty mile per hour there is adequate tension to hold the model out. However, with a four foot wing span the inboard wing tip is producing about fourteen per cent less lift than the outboard wing tip. This lift difference is spread across the wing according to the square of the wing section speed . This causes the center of the wing lift to be closer to the outboard tip, hence the need was felt to have need of wing to body offset, which also helped in low tension conditions.

3) The basic difference between now and then is engines with better power. As stated somewhere the average fox produced about thirty three ounces of thrust at stunt speed, other were less. This limited the size, weight, line length, and drag of the model. Present model power makes these less of problem, so thicker airfoils, bigger models, longer lines, better wing lift. Better power give ok line tension, so no need for engine offset. Better power with bigger models and longer line length lessen the wing lift difference effect to only need small or no wing to body offset.

Well the man who has built more airplanes than most any two combined uses Engine off set. You all know who that is I hope. The reason being all this arodynamic math has squat to do with control line.

Sparky,

We all get out of our hobby what we look for in it. I can gently disagree about it all not meaning doodly-squat for CL, because I enjoy looking into that side of it. If you don't, fine, you are getting what you prefer.

Few if any of us are being paid to do this. Most of us "invest" a lot, rather than make $$ on what we do. That's part, IMHO, of what a hobby is -- something we do that is worth what it costs to do it.

As long as it pleases us, we're both doing great.

Last post on this subject.
I am not posting these messages to tick anyone off. I am just trying to let people under stand the art and how it has changed. Or NOT..
When I was a boy my dad owned a hobby shop and I built my ##### off. Flew and flew. I use to build planes for Tom Warden. I built the fist two Futura's Super tiger 46 Profiles 60 inch wing span 1968.He was working for Testors and We really had wood! Not like today..
So keep flying .

So, Sparky...

Whos the dude who has built twice as many planes as any two of us???

Inquiring minds want to know!

Well lets say he's on the f2b team this year.

SNIP

>Profiles 60 inch wing span 1968.He was working for Testors
>and We really had wood! Not like today..
> So keep flying .

I don't know...... today we have Viagra!

Dick:

....and today we also have electric starters. It'd be fun to see a vote on which makes our lives better.

Jeff W.