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Design => Stunt design => Topic started by: Tim Stagg on November 27, 2009, 07:51:17 AM
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All,
I am sure this topic has been covered before but I need help on my new mustang design. What effect does a higher stab position have verses a stab position that is in line with the wing center line? Should I design everything on a center reference line, thrust, wing & stab, or does it not matter?
Thanks
Tim
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I have built so many planes thru the years that had stab higher than the wing. When I got the plans and built Bill Irvin's Cotton Candy, I wondered about the stab, wing engine all on a center line down the fuse. That plane was one of the best I had. So if you are wondering about the high stab on a Mustang there is no worry other than trimming for some blanking out of the stab during some maneuvers. Of course to me I never really worried about whare a stab was mounted as long as it stays on the plane. H^^
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Tim,
It doesn't really matter where you put the stab in relation to the wing, except, lower might not be as good a position as somewhat above.
Jim Pollock
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Lot's of planes have been "In Line" designs, where the thrust line, wing centerline, and stab centerline are all on the same line. Some like this arrangement, but it seems most shy away in favor of the stab being a bit higher than the wing.
There are good reasons most modern designs have the thrust line a bit higher than the wing centerline, with the stab also higher.
I'd go further in explaining the reasoning, but don't want to presume being an "Expert".
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John, Jim and Doc,
Thanks for the input, I am happy to see everyone with pretty much the same view point......does not happen very often, when you ask a question about preference. I will continue down the path of a raised horizontal stab with the wing a little lower then the thrust line.
This will be an electric project, molded fiberglass or carbon fiber fuse, built up wings and stab. I have a friend with access to solid works software and a CNC machine to cut the female molds. If nothing more than a fun learning exercise, we will see what happens. Am shooting to have a prototype flying at Brodak.
something in the 700 sq inch range and less than 64 ounces using 5 cells.
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There are several designs with wing, tail and thrust lines in line, and some seem to fly well. Mike Pratt's P-Force, now ARF'ed by Sig, is one such model that seems to fly quite well. Mine, built from his short kit several years ago, does OK. Some feel that behavior around neutral in straight flight is better with the stab not aligned with the wing chord, since it avoids the turbulence and perhaps some down-wash from the wing.
Whatever the height of the horizontal tail, other dimensions must be attuned to it, including the longitudinal c.g. location. Downwash from the wing can affect the elevator's neutral setting and input, perhaps the desired stab incidence and/or down-thrust, and definitely changes the neutral point of the aircraft. The vertical center of drag is affected, since the drag on a raised stab tends to pitch the plane up (as does gyroscopic precession), to balance drag on the landing gear and a raised thrust line, which tend to pitch it downward. It's pretty complicated, but I think that except at the highest levels, c.g. placement and elevator setting are probably sufficient to trim a plane for any reasonable stab height. You should do a search on SSW Forum's archives, where this question has been discussed in detail more than a couple times by some of the best minds in CL stunt.
One thing thing I believe true though is that you cannot make the plane symmetrical even with everything in line. Symmetrical architecture is no more aerodynamically symmetrical than the best balanced/trimmed "conventional" designs. I like the in-line idea, but have no expectation that aerodynamic adjustments can make up for P-factor, gyroscopic precession and even landing gear drag/polar moment. So I think that regardless of how well anyone thinks "inside" maneuvers match "outside" maneuvers, they really don't. The human mind and senses adjust in a marvelous way for physical asymmetries. For instance, raising the stab may avoid wing/tail interference to some extent upright or level, but somewhere inverted or doing outside maneuvers, those wakes will interfere. Still, I have to admit that my present project started out fully "in-line", but I raised the stab before completing the fuselage work. Oh, well...
SK
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The stab must be out of the wing wash.
In my opinion there is no advantage whatsoever to be gained from inline or nearly inline designs.
Big Jim had it right....
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I believe that if investigations were to be made, you would find that the wing was is primarily directed down from the trailing edge of the wing as a function of lift production.
In all the reading I have done, well that which I can remember?, the wing wash is actually DOWN wash, and as such would travel below the stab. I am sure there is some turbulance behind the wing, but as for the accelerated air mass, it will be moving downard. Howard, or Ted , Brett or someone will correct me if I am wrong I am sure,,
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That's right. Mind you, the tail is on the other side when you're upside down. I haven't figured the downwash angle on a stunt plane tail, nor the round-air effect from loops (one goes up when the other goes down), so I don't know where a wing wake would go. I've flown nice in-line and nice high-tail planes, and I can't feel any wake, but I'm not nearly as sensitive to stunt plane anomalies as good fliers are. Has anybody calculated downwash for a stunter? I don't remember it being hard to do.
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One thing thing I believe true though is that you cannot make the plane symmetrical even with everything in line. Symmetrical architecture is no more aerodynamically symmetrical than the best balanced/trimmed "conventional" designs. I like the in-line idea, but have no expectation that aerodynamic adjustments can make up for P-factor, gyroscopic precession and even landing gear drag/polar moment. So I think that regardless of how well anyone thinks "inside" maneuvers match "outside" maneuvers, they really don't. The human mind and senses adjust in a marvelous way for physical asymmetries. For instance, raising the stab may avoid wing/tail interference to some extent upright or level, but somewhere inverted or doing outside maneuvers, those wakes will interfere. Still, I have to admit that my present project started out fully "in-line", but I raised the stab before completing the fuselage work. Oh, well...
The biggest effect of the in-line arrangement I noticed with the Yatsenko Shark was in takeoff and landing.
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[Howard Rush]The biggest effect of the in-line arrangement I noticed with the Yatsenko Shark was in takeoff and landing.[/quote]
This could be just in time, before I cut and fix the tail in the new model. Can you say what was the effect?
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I'm guessing that having the engine low made the gear taller than it would be otherwise. I didn't notice anything I'd attribute to the different tail location. I hope we hear from Brett or Igor on this topic.
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A formula from the Frank Zaic '51/52 yearbook for wing downwash is CsubL X 5. In level flight we typically fly at about 1 degree and .1 CsubL, thus the level flight downwash is about .5 degree. When pitching up with flaps deflected, who knows?? Where's our wind tunnel photos?? I'm sure there are much more sophisticated formulae for full size stuff but this is probably close enough for our purposes. He also states the downwash effect reaches out above the wing at least 1/2 the chordwidth.
If one studies photos of full size aircraft such as the F-51 Mustang, you will notice the exhaust stains follow an arc similar to the airfoil and this effect starts ahead of the wing leading edge. Creating lift disturbs a lot of air!!!
Lots of good info in the "1951-52 Model Aeronautical year book" and "Circular Airflow and Model Aircraft" books by Frank. I think one can get them from the AMA store.
Don
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I'm guessing that having the engine low made the gear taller than it would be otherwise. I didn't notice anything I'd attribute to the different tail location.
Aha! As it happens I've just revised the new "simple" model from upright to inverted engine for just that reason.
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The biggest effect of the in-line arrangement I noticed with the Yatsenko Shark was in takeoff and landing.
That's a little too succinct, even for you. What sort of takeoff differences? What sort of landing differences? Worse? Better?
Larry Fulwider
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. . .
. . . It's pretty complicated, but I think that except at the highest levels, c.g. placement and elevator setting are probably sufficient to trim a plane for any reasonable stab height. You should do a search on SSW Forum's archives, where this question has been discussed in detail more than a couple times by some of the best minds in CL stunt. . . .
. . .
SK
Serge --
I think in one of the SSW threads (or another somewhere) wasn't it generally agreed that higher stab location "required" more positive stab incidence? I don't recall any recommended numbers, but stab incidence was "Kentucky Windage Linear" with stab height.
Larry Fulwider
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Thats if its in the wing wash . ( air in area of pressure altered by displacement / lift -
OR exhaust stains .) the exhaust stains on hard pressed aircraft tell you a thing or two.
Contrary to one or two .. Rumours ?
Flat plate 1 / 4 sheet (hard ) tailplanes way up on top of vertical stabilizers are positioned to remove themselves from such influances.
and as such should be set at 0 - 0 incidance. Thick ( 5 / 8 ? ) ones , with circular airfoiled leading edges up there,according to reports
do stange things.
Standing on the back of the ute with smoke generators and suchlike , would enable you to vidieo the results.
Keeper rolling if the highway patrol turns up . ! " just investigating aiflow at 110 mph , officer ! " Rightio then ! . . ?
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3 / 4 mm up in 3 1 / 2 in on mine . as baseline . as in not less than .
Mr Rabe uses 3 / 4 Deg. as hes shifting a bit of air, diheadral etc .
Flat wing'd require not as much .say 1 / 2 whichever way .
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Larry-
I think we agreed that the higher it was, the more likely positive incidence was to be successful, but there are other ways besides lift to counter a drag induced pitch-up. I think that as Matthew says, down wash from a wing is a counter torque. But that motion is variable and perhaps not entirely straight. I never computed anything here, but I think Igor may have modeled this. Flaps and incidence make things awfully complicated, and I don't even know whether the relative downwash angle varies linearly, since the plane is pitched and the elevator is deflected. Induced incidence angles are pretty standard considerations, but I don't see them as very accurate for our models. My original impression was that positive incidence was a way of insuring that negative (decolage) didn't occur, for which a downward deflected elevator might or might not be a cure. I did however put some positive stab incidence into my current slow project.
IOW, I haven't re-thought this enough. What I have seen recently is that Martin Simons shows the aircraft neutral point moving rearward for higher stab locations. He gives the following for a stabilizer efficiency factor:
1) .9 for a T-tail
2) .6 for a "normal tail
3) .3 or .4 for a tail near the wing wake.
4) .95 - 1.0 for a canard or foreplane
Incidentally, the last should not get us thinking of stunt canards, since the wing is greatly compromised by canard down-wash and stability requirements, relating among other things to a far forward N.P.
SK
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What I have seen recently is that Martin Simons shows the aircraft neutral point moving rearward for higher stab locations. He gives the following for a stabilizer efficiency factor:
1) .9 for a T-tail
2) .6 for a "normal tail
3) .3 or .4 for a tail near the wing wake.
4) .95 - 1.0 for a canard or foreplane
Incredible. That suggests that the air slows down 20% behind a wing. Does this square with the Cd of a stunt wing?
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That's a little too succinct, even for you. What sort of takeoff differences? What sort of landing differences? Worse? Better?
I regret that I forgot.
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Howard-
Not having studied this yet, I just had the impression that he was referring to the direction of the relative wind. I thought he meant that the closer the stab to the wing's wake, the more negative its aoa (not angle of incidence) and thus more upward it would pitch the nose with increased wing aoa. In other words, he seemed to be implying that the tail was less stabilizing when placed nearest to the wing chord level. I'll get around to finding out what I'm talking about sometime, but unfortunately not tonight during club newsletter editing week.
SK
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That sounds more like it. I don't remember a "stabilizer efficiency factor", and I'm too lazy to look it up. I just figured it was the usual factor folks throw in for air slowing down.
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Howard-
He just termed his symbol the "stabilizer efficiency." I used the word "factor", because that's how he used it. Here's the page...
SK
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All,
I am sure this topic has been covered before but I need help on my new mustang design. What effect does a higher stab position have verses a stab position that is in line with the wing center line? Should I design everything on a center reference line, thrust, wing & stab, or does it not matter?
Thanks
Tim
One place to look is RC pattern. The biggest effect of the stab height is on how the plane behaves in a sideslip(which stunt models do all the time!). RC'ers see big effects when doing knife edge maneuvers, like the our wingover. As I recall, a high stab, such as on a CAP 20, required a lot of elevator mix to keep the plane flying straight in knife edge. It wants to pull towards the canopy, making it harder to do a clean wingover.
The common stunt set up with the thrustline slightly higher than the wing(~1/2 in) and the stab a bit higher yet(~1in) generally gives the least amount of trouble. Really high-mounted stabs, more than the wing thickness above the top of the wing, require some negative incidence or asymetrical elevator movement to equalize the inside and outside turns.
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Serge,
Yep, that's the one. It doesn't have downwash in it. d(epsilon)/d(alpha) is the downwash effect, which he's simplified to a constant. http://naca.central.cranfield.ac.uk/reports/1939/naca-report-648.pdf gives the lowdown on wake location and strength and downwash angle. I'm kinda curious about whether having the tail above the centerline makes a difference between inside and outside loops, but wading through those charts is too much work for me.
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I regret that I forgot.
Damn. Thought I had come across something useful I could plagiarise! :(
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Howard-
Thanks for the link. When I have time, I'll look through this one. I may even have it , but last look would have been over ten years ago. Despite what has sometimes been said here, there has been some interesting and probably relevant work done by the researchers of long ago. I'm really curious about the vertical range of downwash effects. CRS gets in the way!
SK
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A couple more tidbits about this:
Brett calculated that the gyroscopic moment due to turning left with the prop rotating requires some down elevator. He mentioned this again on SSW recently in the bit about tail incidence.
I posted some flowfield pictures on p. 4 of the wing thickness thread here, but I'm not sure they are useful for anything.
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Gyro moment is very small and needs only very small fraction of tail AoA to compensate. But if you will look to flow field analyze of 18% airfoil in 0 AoA, on place where we have tail, you will see also small fraction of down wash (java foil exports coordinates, so it is easy to calculate). But we have also landing gears with some amount of drag and prop with thrust line over the wing, what easily eliminates gyro moment and even more.
I mean - the bigest reason for down elevator and down offset of motor is P-factor. It is possible to trim also by prop down offset to some extent. So I will foloow what David says - give a little bit negative incidence to stab. The rest is "trimmable" either by elevator, or I use also motor offset. It is extremaly visible on my indoors ... I recomment to do indoors in winter, it gives very good trimming practice and sometimes open eyes VD~
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Tim,
It doesn't really matter where you put the stab in relation to the wing, except, lower might not be as good a position as somewhat above.
Jim Pollock
Stab position might be one of the Holy Grail's of stunt design... The position has a tendency to create a holy snowball effect.
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It might be interesting to mount one below the wing centerline to see what happens. Would it need negative incidence relative to the wing?
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"It might be interesting to mount one below the wing centerline to see what happens."
Don't you get the same effect when flying inverted?? ;D
Merry Christmas! #^
"Tight Lines!"
Wes
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Yes, for the wing effect. Some of the engine effects would have the opposite sign.
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It might be interesting to mount one below the wing centerline to see what happens. Would it need negative incidence relative to the wing?
Maybe ..You would use that with pusher props??? y1 S?P S?P
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Maybe ..You would use that with pusher props??? y1 S?P S?P
Ha ha... what do you want to bet that planes still need down elevator, even with pusher props? Doooooh!!!!
Down elevator is not down elevator at all... it is down flap. Look at the umpteen million in flight pictures of stunt planes. They all have flat elevators and down flap in level flight.
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Mine probably does, because zero flaps corresponds to about 3.5 degrees down elevator. Inverted, though, mine would have quite a bit of positive elevator (toward the sky) and the same amount of flaps as right-side up, but in the opposite direction (toward the ground in both cases).
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Ha ha... what do you want to bet that planes still need down elevator, even with pusher props? Doooooh!!!!
Maybe not... hmmmmm.
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OW,
My head hurts guys quit that........ VD~
Jim Pollock n1
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A symmetrical airfoil gets lift from angle of attack. You can fly a Ringmaster level. It will even do stunts. Here's the Impact data from Gary James's Xfoil run. Black is zero flaps. "Alpha" is angle of attack.
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When the flaps move up and down on the wing hingline, it does change the wing, It in effect,changes the angle of attack of the wing, just as when the elevator moves it changes the effective AOA on the stab. This is the reason many people tweek in some down elevator to raise the tail of their stuntships when they see them flying with the aft portion of the plane flying low (dragging the tail)
R
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I have thrown out hte *inline* question before, also. I had a Junar (Billy's smaller design) that was inline. It had some good qualities. The Sky Writer is inline as were most of the Chinese planes I saw in '04 were, IIRC. That design (and updated) has won quite a few WC's. From what I understood in '04, the Chinese were still basically flying the same numbers and all as Xaing's original Sky Writer. Fo curse they had replaced the *ST style .60* with four strokes.
Maybe it is all what we are use to doing. ???
Anyhoo.......
Big Bear
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My old mans center line designs flew fine...problem was, trike gear drag made em turn quicker outside and without slop, 3" bellcrank, they were more sensitive around neutral. With limited power in 67...35s, on the repeat of the triangle, you had to lean forward to lessen the load at the handle, and crank it quick and short back to neutral. A hair trigger moment.... or stall and bitch because she is so fat now! LMAO n~ n~