Motor, wing & tail on Thrust Line?

[John Carrodus]

Ad vantages or disadvantages of having the CL of the engine, main wing and tail all on the same line of thrust on a 40 powered stunt ship? See sketch below.

[Dave_Trible]

In your sketched layout, this should work pretty well.  In a more typical design the engine will be inverted.  This puts the bulk of the engine weight on the same side of the engine/thrust line as the landing gear and wheels,  likely to cause a vertical CG issue.  In that format the pendulum weight would cause the airplane to roll or bank to the left in level flight which would have to be trimmed by some means.  This is a primary reason why the wing centerline is typically 3/4" to 1" below the thrust line on a more typical design with inverted engine.  Profiles are better suited with either inline or only small offsets-maybe a little more where a heavy muffler is used.  On electrics battery stowage location can be pretty important for these reasons.

Dave

[Robert Zambelli]

Hi, Dave and John.
As I recall, Bob Baron and/or Bill Netzeband did some experimenting with inline designs.
Maybe someone here can enlighten us?

Bob Z.

[Dan McEntee]

  In line stunt designs have come around on the old record player in popularity for decades. One that comes to mind right off the bat is the SOG Primary Force ARF. I think a few top NATS flyers are flying in line designs. I built a Rabe style profile with upright engine mounting back in the late 1980s that was in line, because I had read some positive things about it back then. i think Dave Rees had a rather attractive design back in hios days with an upright mounted engine, similar to John's sketch, that may have been an inline model also. Tom Dixon has some designs in his plans lines also, his Charisma being one of them I think.
   Type at you later,
    Dan McEntee

[Dave Hull]

There is quite a bit of relevant discussion in the thread about the Avanti.

https://stunthanger.com/smf/stunt-design/stabs-and-the-'avanti'-for-ted/

I would also point to a classic beauty from Long Guyland called the Skylark. In addition to an in-line setup, it had some interesting construction methods.

Dave

[John Carrodus]

Many thanks guys. An interesting question in my mind was tail turbulence. Any additional thoughts on this? Or am I looking for hens teeth?

[Dave Hull]

John,

In my opinion, the turbulence issue (actually, I prefer to think of it as the stab flying in clean air) is as important a consideration as managing the vertical CG. Maybe you should build the tail as a testbed and try high, medium and low?

Another first order consideration might be the static pitching moment due to drag components from the wing, and the tail, holding the wheels and fin constant. The in-line layout would seem to have at first glance another advantage there--but the paucity of such designs suggests that other considerations are more important....

Dave

[Brent Williams]

Since around 2017 or so, I believe Paul Walker and Chris Cox have been competing primarily (possibly exclusively) with electric inline designs. Bob Hunt, Jim Aron, and Alan Reisinger have some recent competetive in-line designs also.   I would ask them about the advantages, disadvantages and differences in trim associated with in-line ships. 

Note, If you aren't using an electric setup featuring one of Igor's 15g 3-blade props, the observations and recommendations may be somewhat different for glow powered ships.

[Steve Helmick]

Disadvantage is that it causes some problems with pushrod/horn geometry, but that can be dealt with if you understand that the pushrods all need to be 90 deg. to the horn axis. Long horns will help prevent pushrods from conflicting with the stabilizer, for instance.

Advantage, from what I've heard, is that there is much less problem getting inside/outside turns of equal size. Less down elevator relative to flaps. I could see that it might be possible to get slightly smaller radius turns as a result...maybe.  Steve

[Paul Walker]

Rarely does one get a chance to do an experiment that simply changes a single variable. You can't build two "identical" planes unless they are from a FULLY tooled set of moulds. In the case of the test I did, that could be very difficult to do as some of the moulds would have to change.

What I did was take my OLD '97 Impact that is nothing more than a test bed now, and since the nose ring was long gone, it afforded me the opportunity. I built a motor adaptor that located the motor in the "plans" vertical location, and that same adaptor would also allow the motor to be moved down to the wing centerline vertical location.

To be sure the results would not be my wishful thinking, I recruited Chris Cox to fly it as well. I did not fly it until he was present for the test. It started by flying it in the "plans" location. We both thought is was still a good flying plane. Then the motor was changed to the inline location, and we both reflew it. I flew it first, and I usedmy best poker face not to influence Chris.
After he landed, he wanted to know WHY I didn't like it!!  Yes, I liked it. My poker face worked.

What did it do better: tracked better in level flight, tracked better in the wingover, round loops were easier, square corners more consistent. This carried through in all maneuvers. We agreed that there was not a single thing made worse by moving the motor inline.

Chris went home and started on a new inline plane, as did I.  Neither of us has build a non inline plane since.

Moving the motor to the inline position changes two variables, that being the thrust line, and the mass distribution is different.  It would be quite difficult to separate these two variables. But the result is such that it is of no concern to me to test more to find out.

As stated earler, this was done with a 16g 12 inch 3 blade prop. Heavier prop could make some difference.

[John Skukalek]

This is probably common knowledge (that I don't have) but where was the stabilizer on the OLD 97 Impact test bed airplane?

[BYU]

     I don't think there's any doubt that most "in-liners" are very "wafty" or funny-feeling around neutral, compared to regular stunt planes. I probably even saw Ted's Avanti flight way back when, well before we met. I was incrediby impressed with the Bob flew it, in fact, in many ways, my mind was set on what good stunt patterns looked like by the way Bob flew it.

    But - as anyone who read my first shot at a design column could see, the concept is fundamentally flawed. The idea is that since the airplane should turn the same inside and outside, it ought to be "symmetrical" and, of course, that means that you should put the engine, wing, and tail all in a line so that there is nothing that "favors" up or down. Great idea, unfortunately, unless you have drop gear, fin/rudder that looks like the tail on a largemouth bass (symmetrical from the side view), and contra-rotating propellor and engines, you have plenty of other assymetry already, so putting the engine, wing, and tail in line doesn't accomplish the  goal of making the airplane neutral in terms of up and down response. In fact, an in-liner will probably need quite a bit more downthrust and/or positive stab incidence, since you don't have the *advantage* of having the engine trying to nose you down to compensate for the precession like you did with a higher thrust line. I haven't thought it through whether or not the higher stab helps or hurts with this. I suspect it *probably* helps compensate, too, but I couldn't explain it in words.   Point being, the goal of equal/neutral inside/outside response is NOT accomplished by making an in-liner, in fact it probably hurts.

   But there is one whoppingly obvious (to me, anyway) disadvantage, and it's most evident in level or low-g flight. The turbulence off the wing in level flight streams almost straight out behind it. There's very little lift (just the weight) and it doesn't take much Cl to create it. Therefore the "downwash" is very minimal. So, bingo, the stab/elevator is running in the wing turbulence in level flight. And very tiny changes in the Cl from slight corrections at the controls move the turbulent area up and down around the stab. So not only is it in turbulence, slight corrections drastically change the turbulence, and thus the stab effectiveness. So you make a slight correction, or a slight perturbation comes along and loads or unloads the wing, and all of a suddenly your control input works 50% better or 50% worse, and you never have any idea what it's going to do next. Leading to that "vague" feeling.

     Of course, it clears itself up pretty well in harder maneuvers, and may be more equal in terms of stab effectiveness when you start getting to higher Cl as the downwash is moved away from the stab. But now, you have a situation where it works very weakly and unpredictably around neutral and really comes on strong in maneuvering. That might *sound* good, but I think that something that drastically changes charactersitics depending on the conditions is a formula for unpredictablity and inconsistency. Maybe you can practice a lot every day and get used to it, and used to how it's different in different conditions, but you are gonna have a bit of a time trying to just pick it up and fly it well the first time, or anytime the engine runs a little different, or when the air density changes, or the wind changes, or (etc).

   Never mind that almost everything that we have done in the past 20 years in terms of design and trim is geared towards making the airplane more positive and responsive around neutral. This arrangement has the precisely opposite effect.

   And the Avanti in particular has another, quite intentional, feature that encourages this "vague" feeling, and that's the old "stab bigger than elevator" trick. In fact, the plans show a rather exaggerated case, with the stab TE about 1/2" and the elevator about 3/16 (or something like that). That works on the same principle, creating turbulence that makes the stab less effective around neutral to "smooth it out". So in this case it's a sort of double whammy.

   The very last thing I think you want is a system where you move the handle a lot, and not much happens, then you move it a little more, and all of a sudden it "grabs" and really responds.


     Once again, a lot of people have been trying to *prevent* exatly this effect. Paul Walker started it, and I have subsequently ripped off the idea. We are actually rounding off the edges at the stab/elevator (and wing/flap) hinge line to reduce the turbulence and make the controls *more* effective around neutral than they would be otherwise. The 3 airplanes upon which I have done this have been *very very positive* around neutral and nonetheless track very well. With some remarkably aft CGs at time. Of course, I am also willing to build in or dial in the necessary positive incidence, and not feel "guilty" about it.


   So it's no wonder to me that the Avanti was a little tricky to fly. Even without knowing the historical record, you could pretty well predict that a very superior pilot (and make no mistake, Bob Baron was in general a very superior pilot) could fly it well, on some occasions - but probably not consistently from month to month. Which pretty much was what happened.

     Brett

[Brent Williams]

This is probably common knowledge (that I don't have) but where was the stabilizer on the OLD 97 Impact test bed airplane?

It was located at the rear of the airplane....

[GERALD WIMMER]

Hello
I think a lot of stunt and general control line deigns were inspired by WWII aircraft that had that classic low wing , higher thrust line and higher still tail.
How may old models had a Mustang of Spitfire somewhere in the design? They just looked right and the higher thrust line gave more ground clearance.
 Modern full size aerobatic and racing planes seem to have a mid wing, center thrust and almost inline tail like the Yak 55, Edge 360 or Lazer Z200 and a Monnett Sonrai Formula V racer .

Regards Gerald

[Dave Hull]

Another major design consideration for full-sized aircraft is the spar carry-thru. With a low wing, the pilot can sit on the spar. Mid-wing you have to get the cockpit forward or back, and that has a lot of other cascading problems. And, the plane has to do something useful, like carry a lot of fuel, ammunition, cargo, etc. That creates more design constraints on wing location.

Go look at any number of light and heavy commercial jets. They carry the spar(s) thru below the barrel of the fuselage so that the pressure "tube" remains structurally uncompromised. Then they do the best they can at wrapping a fairing around that intersection. It is one of my favorite areas to inspect/evaluate the design. After inlets, of course.

By extension, I would suggest that any of the unlimited aerobatic planes with mid-wing layouts (Edge 540, etc.) are doing it specifically because of performance. But the mid-wing layout is likely preferred to reduce roll-yaw coupling for axial maneuvers. And because the plane is so specialized and the flights are short the compromises are not an issue. The pilot who is highly motivated. Very little fuel storage. No baggage or frills. And notably, they don't "put slop in the elevator linkage."

For control line stunt, we can run the wing spars right thru the torso of the pilot (bust), and we don't need to worry the same way about roll-yaw coupling for axial maneuvers, so...that leaves us free to optimize something else. Like trying to get my PT-19 to stop hunting....

Dave