Design > Stunt design
engine, wing, elevator-stab center line; why not in a straight line?
Charles Hofacker:
There are numerous comments on the subject of symmetry but I don't get the "why". I was browsing Aerofred free plans and it seems most of our c/l models have the wing c/l below the engine c/l and the stab c/l above the engine c/l to varying degrees as far back as I could see plans. Would somebody jump in here and explain why the asymmetry is used? I did see a few designs with the wing and stab c/l aligned and at least one design had all three c/l aligned but these seem to be the exception. We seem to have evolved to a configuration but why?
Dan McEntee:
--- Quote from: Charles Hofacker on September 12, 2020, 08:20:52 AM ---There are numerous comments on the subject of symmetry but I don't get the "why". I was browsing Aerofred free plans and it seems most of our c/l models have the wing c/l below the engine c/l and the stab c/l above the engine c/l to varying degrees as far back as I could see plans. Would somebody jump in here and explain why the asymmetry is used? I did see a few designs with the wing and stab c/l aligned and at least one design had all three c/l aligned but these seem to be the exception. We seem to have evolved to a configuration but why?
--- End quote ---
When all three components are all lined up, that is called an in line configuration and has been around for quite a while, and it tends to come and go. I believe, but not 100% sure, that Paul Walker has gone back to it on his current model as have a few others. I have built a few in line models over the years going back to 1990 or so.. There is an aerodynamic reason for both set ups that I'm positive that Brett, Bob Hunt or Howard and some others can elaborate on in great detail that I would be sure to screw up if I tried, I just know from being in this event for a lot of years and reading stuff in Stunt News and on the forums. I'll just say that this is one subject that you can stay up all night ling while drinking a lot of beer and arguing about. They both work, and both are subject to how accurate your alignment and over all construction is.
Type at you later,
Dan McEntee
Jim Svitko:
As Dan said, both configurations work. I have built and flown both and I am not able to see a difference in performance. But, I have not yet built the same plane with each configuration to make a valid comparison.
One item that can be factor here is the vertical CG position. This might not matter very much but it can be a problem, depending on configuration.
If you mount the engine inverted and everything is in-line (thrust line, wing, stab), you will need a longer landing gear for prop clearance. The power train CG is below the wing chord. The longer landing gear will also pull the vertical CG down. There might not be enough structural weight above the wing chord (fuselage, vertical tail) to compensate. Your leadout exit location will have to move down as well. Depending on where the vertical CG ends up, you could have a situation where the wings will not be level in flight.
When I saw my first KA10, I noticed that the leadouts exited the wing below the wing chord line. If my memory is correct here, the KA10 is an in-line design. When I asked about the leadouts, I was told that it was necessary due to the vertical CG position.
You can make some calculations to get things very close to keeping the vertical CG on the wing chord so you do not have to make any leadout position change.
Ken Culbertson:
--- Quote from: Motorman on September 12, 2020, 10:50:54 AM ---the thrust line is up to give clearance to the cylinder head and the tail is up to eliminate the Z bend in the push rod.
Motorman 8)
--- End quote ---
It may be as simple as that. You center the wing on the 3" side sheet, put the elevator on the top in the back(Sunk in of course) and place the motor mounts to the top in the front and you have just laid out 75% of the stunt ships designed in the last 50 years.
Ken
Dave_Trible:
--- Quote from: Jim Svitko on September 12, 2020, 01:39:46 PM ---As Dan said, both configurations work. I have built and flown both and I am not able to see a difference in performance. But, I have not yet built the same plane with each configuration to make a valid comparison.
One item that can be factor here is the vertical CG position. This might not matter very much but it can be a problem, depending on configuration.
If you mount the engine inverted and everything is in-line (thrust line, wing, stab), you will need a longer landing gear for prop clearance. The power train CG is below the wing chord. The longer landing gear will also pull the vertical CG down. There might not be enough structural weight above the wing chord (fuselage, vertical tail) to compensate. Your leadout exit location will have to move down as well. Depending on where the vertical CG ends up, you could have a situation where the wings will not be level in flight.
When I saw my first KA10, I noticed that the leadouts exited the wing below the wing chord line. If my memory is correct here, the KA10 is an in-line design. When I asked about the leadouts, I was told that it was necessary due to the vertical CG position.
You can make some calculations to get things very close to keeping the vertical CG on the wing chord so you do not have to make any leadout position change.
--- End quote ---
Jim I think you have hit upon 98% of why we do this-the vertical CG position. The bulk of the engine, pipe or muffler, landing gear with wheels and what else is attached (pants, spats) will all fall below the tether point in the vertical plane and that may cause the airplane to bank inwards slightly. It can be trimmed out but will still vary with flying speed and then maybe different yet overhead. All that rests above the tether is a small portion of top fuselage and maybe the stab. Also in-line setups can look unusual and might have less curb appeal. A good representation to look at would be the Baron Avanti. If it weren't sort of jet-styled it would be hard to be made to look good. Most full scale monoplane airplanes are low wingers and we are used to looking at them. Another reason many are like that in both model and full scale is to shorten (lighten, stiffen) the landing gear. In any event there should be moderation when designing a stunt airplane in this respect. When the wing centerline goes too far south the inside/outside turn rate goes asymmetric, the stab placed too high, or V tailed as well. These can be trimmed out with handle bias or control system adjustments but it's just easier to color inside the lines......
Dave
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