Design > Stunt design
engine, wing, elevator-stab center line; why not in a straight line?
Dan McEntee:
--- 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 ---
The Sakitumi is an in line design that flies quite well and mine has a carbon tube for a push rod with no off sets in either end of the push rod.
Type at you later,
Dan McEntee
Dan McEntee:
--- Quote from: Motorman on September 12, 2020, 09:24:34 PM ---Ain't got flaps.
--- End quote ---
So? The model that got me out of beginner, through intermediate and won an advanced trophy or two was an in line design, had flaps, and dave brown pushrod with no zig zag bends.
Type at you later,
Dan McEntee
Igor Burger:
It was covered several times here or on stuka ... short answer is that drag under the thrust line helps beat prop precession pushing nose up (therefore it does not work with pusher props) ... the long answer is really long, because there are many other variables in he formula, like motor offset, elevator coincidence, L/G drag, p-factor, down wash ... so better stay with that short answer VD~
Steve Berry:
I'm going to really throw in a monkey wrench and add another couple of variables. Built-up fuse vs profile for inline designs, and then electric vs glow.
Years ago, I took the wing from an old Sterling Mustang, cut off the flaps, and built a plane that we dubbed "The Drone." It was in line and only 2" thick. 2" x 36" profile fuse, motor mounted on centerline, along with stab on centerline, and 2 little rudders on either end of the stab. I was tired of breaking off the rudder if I landed inverted (whether through engine quitting at wrong time, or pilot error) and so built a plane that was symmetrical top to bottom with nothing sticking above the fuse. Turned into the best flying, best tracking plane I've ever had. Equal turns, solid line tension, and would do anything I asked of it.
Next, with electric, there's no cylinder hanging down, so the only thing to worry about would be weight of landing gear. This assumes you mount the battery directly inline with the motor.
Now, let the arguments really begin.
Ted Fancher:
Apropos of nothing I have a unique story re "in-line" stunt ships. The events occurred at the 1980 Nats and starred Bob Baron and his Avanti II in line designed stunt ship with which he finished 4th that year...obviously pilot and plane flying exceptionally well. I, on the other hand, managed a 5th place finish just behind Bob flying the Imitation, having crashed my Excitation due to a control system failure warming up that AM prior to the Top five flyoff....that's my excuse and I'm sticking with it!
After the flyoff was over Bob asked if I would like to fly his Avanti II. Because, historically, the Bob Baron/Wild Bill Netzeband team had been in the forefront of then current "innovative" stunt design I was delighted to take him up on his offer. Again, the Avanti was exactly the design concept under discussion in this thread...thrust line, wing and stab/elevator on exactly the same vertical plane.
Long story short (I know, I know you don't believe it) following an uneventful but not necessarily assisted by the pilot take off I was struck by the fact that I was unable to fly the ship in steady state level flight and, in fact, was concerned about two things...it didn't want to fly at level flight altitude and I was thus unwilling to do any maneuvers beyond a handful of inside/outside test loops and was scared to death about getting the ship on the ground in one piece. It simply felt unresponsive to inputs and I felt I had no ability to even maintain "level" flight so simply held on as the ship went up and down more or less at will.
Even more strangely, when the engine quit and I braced myself for impact the ship simply gently lost altitude and without assistance on my part made pretty much a 40 point touchdown! Weird! The plane with which I found myself unable/unwilling to attempt even a “safe altitude pattern” had just finished a very competitive fourth below Les, Bubba and Billy; the reigning Championship US FAI team.
I was totally embarrassed and unable to account for what had just happened. At the time I more or less in my mind thought it possible that the all in-line configuration was likely the culprit, but...Bob had kicked my butt with it! Looking at the construction article in my "library" just now I noticed a couple three other things that were "different from the norm at the time that could be extrapolated by a non-aerodynamicist like myself to have contributed to the apparent instability.
The elevators on the plans were significantly thinner than the stab; perhaps as little as 50% of the stab's thickness. Could there have been a "null" or something in pitch response to initial pitch inputs due to the elevators needing to first transit that "gap"??? The flaps were a larger percentage of the chord than I would prefer now but hadn't given it any thought back then. The stab/elevator was ~a solid 23% of the wing area which seemed just fine but the empty CG (on the mag plans) seemed a bit forward of where I would plan it nowadays at 18% (wagged) MAC...and of course, due to the fuel load, forward of that following takeoff when I first noticed the "instability" (for me, I must add...Bob flew it like it was on rails)
I find my assessment now…some 40 years later…not much different than the above thoughts. I suspect Bob was simply perfectly comfortable with much greater inputs at the handle and was thus able to overcome the “gap” in response that I felt unwilling to apply. How he flew it straight and level, however, escapes me to this day.
Combined with the top crop's current “love affair” with “in line set-ups” my suspicion is that Bob’s ship was anomalous with respect to the general concept likely, in part, for the reasons. I’ve suggested.
At any rate, for the balance of my competitive years I blatantly shunned any such configuration on my own ships.
Ted
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version