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Author Topic: Stiletto tail size  (Read 1631 times)

Online Dietmar Morbitzer

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Stiletto tail size
« on: January 20, 2018, 11:36:48 AM »
Hello stunt pilots,
I've started to build a Stiletto 660. The tail size is under 20%
of the wing. I've learned that the tail shout be in the range of 25+%
of the wing. Does somebody have experience with a bigger tail
on a Stiletto and in which direction should it grow?
Dietmar


Online Brett Buck

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Re: Stiletto tail size
« Reply #1 on: January 20, 2018, 02:12:10 PM »
Hello stunt pilots,
I've started to build a Stiletto 660. The tail size is under 20%
of the wing. I've learned that the tail shout be in the range of 25+%
of the wing. Does somebody have experience with a bigger tail
on a Stiletto and in which direction should it grow?
Dietmar

      At least around here, that will make it not legal for Super 70s/Nostalgia 30, if you care about that.

     I will preface this with  - I probably wouldn't do this for a variety of reasons, but if I was, and the only option was to add to the existing tail, I would add it entirely in chord and only to the stabilizer. That will reduce the aspect ratio and reduce the stabilizer/elevator split. If I was going to replace it entirely, I would design a tail with about a 4.25:1 aspect ratio, a 65/35 split, and about 25-26%

   Brett

Offline Matt Spencer

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Re: Stiletto tail size
« Reply #2 on: January 20, 2018, 09:06:56 PM »
Perhaps youd better let Les McDonald Know. He might want to change it before He starts Flying It . :) ;)




Offline Target

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Re: Stiletto tail size
« Reply #3 on: January 20, 2018, 11:25:22 PM »
Les himself told me that in later years the following mods were popular on the Stiletto 660:

Make the cord 1/4" wider on the stab.
Make the cord 1/4" wider on the elevator
Both of the above is for the full span.

Make the outboard flap 1/32" wider.

Hope that helps you.

R,
Target
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Chris
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Offline Target

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Re: Stiletto tail size
« Reply #4 on: January 21, 2018, 09:52:03 AM »
That's what he told me....
Take it or leave it.

Puts the precision in precision stunt.
« Last Edit: January 21, 2018, 03:32:00 PM by Target »
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Chris
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Online Brett Buck

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Re: Stiletto tail size
« Reply #5 on: January 21, 2018, 02:05:14 PM »
1/32" on the flap?  One less stroke of the sanding block & use a good micrometer.

 You had darn well be able to build to much better than this sort of tolerance if you want to have any sort of predictable results.

     Brett

Online Dietmar Morbitzer

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Re: Stiletto tail size
« Reply #6 on: January 22, 2018, 01:04:12 PM »
      At least around here, that will make it not legal for Super 70s/Nostalgia 30, if you care about that.

     I will preface this with  - I probably wouldn't do this for a variety of reasons, but if I was, and the only option was to add to the existing tail, I would add it entirely in chord and only to the stabilizer. That will reduce the aspect ratio and reduce the stabilizer/elevator split. If I was going to replace it entirely, I would design a tail with about a 4.25:1 aspect ratio, a 65/35 split, and about 25-26%

   Brett

Thank you all for answering.
Because we only fly F2B and a little OTS around here I don't have to care for being legal in Nostalgia.
If I add the chord on the stabilizer should I also lenghten the hinge line flap to stab-elevator, if so how much?
What advantages can I get from split stabilizer/elevator size instead adding stabilizer and elevator the same?
Dietmar

Online Brett Buck

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Re: Stiletto tail size
« Reply #7 on: January 22, 2018, 01:46:34 PM »
Thank you all for answering.
Because we only fly F2B and a little OTS around here I don't have to care for being legal in Nostalgia.
If I add the chord on the stabilizer should I also lenghten the hinge line flap to stab-elevator, if so how much?
What advantages can I get from split stabilizer/elevator size instead adding stabilizer and elevator the same?
Dietmar

   If you are not concerned with replicating the original, with all due respect to those involved, why not just copy the Impact or Trivial Pursuit and put a Stiletto fuselage on it? 

   Adding to the stabilizer is intended to get your desired area while reducing the aspect ratio of the tail, and moving the ratio of stab/elevator to be more linear. The very high aspect ratio tails are *very* sensitive and also non-linear - a little elevator deflection does a lot, a little more does a lot less.

      Overall, if you are willing to change this much, definitely put an Impact or Infinity stabilizer on it, make the tail moment 18-18.5" and move on. Also, make the flaps removable so you can change them later if necessary (probably to have less taper and possibly smaller, depending on the weight).

     Brett

Offline Howard Rush

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Re: Stiletto tail size
« Reply #8 on: January 22, 2018, 09:50:56 PM »
The very high aspect ratio tails are *very* sensitive and also non-linear - a little elevator deflection does a lot, a little more does a lot less.

Why’s that?
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Offline Target

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Re: Stiletto tail size
« Reply #9 on: January 29, 2018, 08:32:38 PM »
Yes, why IS that, or why ISN'T that??
Brett and Howard-
Whatever the answer is or isn't about the high aspect tails, please pass that along, I would like to know your thinking.

I was always told that in sailplanes, the same tail area at the same moment arm was more effective with a higher aspect tail plane.
But there was always the asterisks next to that statement to warn of the lower Reynolds Numbers when the chord was too small.
Personally, with the sailplanes, I have seen them BOTH work pretty well; but to me, the designs with the greater tail volume trumped everything for control, but not for speed.

Thanks gents!

R,
Target
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Chris
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Online Ted Fancher

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Re: Stiletto tail size
« Reply #10 on: February 16, 2018, 05:46:59 PM »
Tails are lifting surfaces just like wings and in most respects act just like wings with respect to their "jobs"...which in both cases is to produce lift.  Higher aspect ratio lifting surfaces develop less drag per unit of lift than lower aspect ratios and as a partial result result in less drag while producing those "units" of lift.  They also produce a given unit of lift at lower angles of attack.  Thus sailplanes have long wings (high aspect ratios) commensurate with achieving prolonged flight with no visible means of thrust! They also provide the variations in lift necessary for various stages of flight through extremely modest angle of attack changes.

High speed aircraft are pretty much the opposite of that using low aspect ratio surfaces to minimize form drag and great mechanical thrust to attain and maintain high speeds while maneuvering despite much greater angles of attack...and, therefore, drag...developed to support the much greater wing loading under such aerodynamic distress.

The profound configuration differences between sailplanes and fighter planes are the result of their grossly different missions.  Most "normal" airplanes...like stunt ships...fall somewhere in the great middle ground between those two extremes.  The effects of aspect ratio of the lifting surfaces for such airplanes is somewhat broader than for the previous two extremes and useful variations can and have exhibited comparatively modest variable with respect to lifting surfaces for their missions.

The fundamentals of lift/drag development still, however, impact the performance of aircraft utilizing varied aspect ratios and those impacts are obvious to the lay person (your "average" stunt flyer") and verifiable and quantifiable by the cognoscenti amongst us (the less average Howards, Igors, Bretts etc.)

Among those observable and verifiable differences due to aspect ratio is the fact that higher aspect ratio wings and tails on stunt ships will produce greater lift per unit of angle of attack increase than will lower aspect ratios and will produce less drag while doing so.  (Note that the wing's angle of attack is driven by the lift generated at the tail which drives the angle of attack at which the wing strikes the ambient air mass.  The tail, on the other hand, in order to drive that wing A of A must reshape itself so as to produce--for instance--negative lift to drive the wing to a positive angle of attack and, as a result any angle of attack achieved by the tail must be "greater" than the angle at which the aircraft is changing its overall angle of attack.  You might have to cogitate on that for a bit.

The effect of aspect ratio on a tail trying to drive the ship in the opposite direction of the lift that tail is producing is significant.  Remember that a high aspect ratio surface of a given "area" produces a given amount of lift at a lower angle of attack than a low aspect ratio surface of the same area.  Inasmuch as "X" amount of lift must be produced to drive the wing to an A of A necessary to support the lift required for the pitch change (think "corner") the high aspect ratio tail will accomplish that at a lower angle of attack and with less resultant drag than will a much lower aspect ratio tail.

If you can find a picture of it, check out the tail of Bob Baron's spectacular flying Humbug design way back in the dark ages of stunt.  A flapless triple boom operation with a tail aspect ratio of 10 to one which turned absolutely incredible corners (and more than a few of other competitors heads) when it had the highest flight scores way back at the 1966/7 (or so) Nats.  It's also instructive to read the issues Bob (and designer Wild Bill Netzeband) encountered trying to develop a control system that was insensitive enough to fly competitive round maneuver while still maintaining the ability to fly the Humbugs trademark "where'd it go" corners!  The very high aspect ratio stab/elevator were an important component to the aircraft's well remembered response!

Of course, I could be wrong.  I don't build much anymore but I would be interested in the results of someone building a ship with the ability to mount stab/elevators with a variety of tails of the same area with A/Rs ranging from say 3 to one up to 8 to one or so both with say 60/40% split stab to elevator all of which would be attached at the same location with respect to the location of say 25% of their average chord.  The CGs should be adjusted to be in the same place for all of the tails.  My guess is you could tell the difference.  Both in the feel at the handle (high load with low aspect ratio and low with hight aspect ratio) and in the response of the airplane (brisk with high aspect ratio unresponsive with the low).

Offline Serge_Krauss

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Re: Stiletto tail size
« Reply #11 on: February 18, 2018, 07:38:41 PM »
If you use high aspect ratio for the wing, you can get l-o-o-n-g glides in calm air without whipping and greater lift per handle-input amount, but gusts will upset the plane easier, due to greater sudden increases or decreases in lift with relative-wind changes. The tail, being smaller doesn't have the larger drag of a wing; so it's drag probably doesn't affect the plane's turn performance much - except regarding its sensitivity in and ability to recover smoothly from sharp corners. As Ted says, the increased lift of a high-aspect-ratio tail can increase the angular acceleration into a turn, but the sensitivity can also make a smooth exit more difficult. Brett has often posted the limiting factor of a pilot's own reflexes anyway. So, I'd just add that lower horizontal-tail aspect ratios, having more turn drag (and lower CL-curve slope), should aid in returning the fuselage to equilibrium more smoothly, by always creating a greater restoring force from drag that's present regardless of which direction the handle input goes. I probably said that clumsily, but that's what I can do at the moment.

If I understand my CL history, Bob Baron was known for his great ability to control pretty "skitish" planes.

Offline Target

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Re: Stiletto tail size
« Reply #12 on: February 19, 2018, 12:37:33 AM »
Thanks to both Ted and Serge for the time in explaining the sensitivity of the higher aspect tails.
I also would think it would be COOL to have a plane that could interchange tail planes easily to see what the real world differences are.

I STILL am interested in Brett's comment about high AR tails having the "reverse expo" effect for pitch changes; more sensitivity close to zero lift, and less at increased pitch rates.
And interested in Howards apparent lack of agreement.

At some point, unless you increase the thickness percentage of the tail profile, you might have issues with stiffness with higher AR's. Luckily we AREN'T dealing with sailplane wing and tail profiles, 'cause they are generally thinner than what we are doing, yes, even the tails.

R,
Target
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Chris
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Offline Howard Rush

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Re: Stiletto tail size
« Reply #13 on: February 19, 2018, 12:26:17 PM »
I STILL am interested in Brett's comment about high AR tails having the "reverse expo" effect for pitch changes; more sensitivity close to zero lift, and less at increased pitch rates.
And interested in Howards apparent lack of agreement.

I was hoping Brett would explain the nonlinearity.  I don't think I disagree.
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Offline Target

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Re: Stiletto tail size
« Reply #14 on: February 19, 2018, 12:51:37 PM »
Oh, OK, well, I was still hoping that Brett would explain, and I guess I misinterpreted your "Why's that?" response as an opposing opinion, my mistake.

Brett-
Please explain if you get some time to do so!

Regards,
Target
Regards,
Chris
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Offline Howard Rush

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Re: Stiletto tail size
« Reply #15 on: February 19, 2018, 01:10:30 PM »
The tail, being smaller doesn't have the larger drag of a wing; so it's drag probably doesn't affect the plane's turn performance much - except regarding its sensitivity in and ability to recover smoothly from sharp corners. As Ted says, the increased lift of a high-aspect-ratio tail can increase the angular acceleration into a turn, but the sensitivity can also make a smooth exit more difficult. Brett has often posted the limiting factor of a pilot's own reflexes anyway. So, I'd just add that lower horizontal-tail aspect ratios, having more turn drag (and lower CL-curve slope), should aid in returning the fuselage to equilibrium more smoothly, by always creating a greater restoring force from drag that's present regardless of which direction the handle input goes. I probably said that clumsily, but that's what I can do at the moment.

I don't think your explanation was the clumsy part.  There was a discussion on this awhile back, either here or the Preacher's board.  I can't find it, but I found the calculation.  Yes, the stabilizing moment is greater with a higher aspect ratio tail of the same area because its lift curve slope is greater.  The stabilizer's lift, acting through the moment arm that's the tail length, is what does the stabilizing: not the drag acting through the the tiny moment arm that's the distance of the tail above or below the CG. Intuitively, one would think that drag matters, but calculation shows that it doesn't, and textbooks generally leave it out.  The attached graphs show relative lift and drag and relative pitching moment contribution from lift and drag.  I can't find the particulars, but I suspect the numbers came from an Impact.

You may be attributing damping (the moment opposing pitch rate) to drag.  Drag doesn't matter there, either.  Damping comes mostly from lift from the added angle of attack on the tail from airplane rotation (see Igor's article).
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Offline Serge_Krauss

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Re: Stiletto tail size
« Reply #16 on: February 19, 2018, 02:50:56 PM »
Thanks for the graphs, Howard. I should have thought of L/D ratios to realize that the drag contribution is minimal. I wasn't thinking though of stabilizing moment from lift, but rather of the overall pitching moments in both directions and how to diminish them near neutral to alleviate bobbles on turn exits. I wasn't thinking of contributions from raising the stab either, but the tininess of those quantities is interesting. What I had in mind was  a restoring torque from drag that always works toward neutral.as caused by the "R" forces in my diagram. However, as your graphs may show (I don't know whether they represent the forces I'm thinking of or contributions from raised stabilizers), and typical L/D's indicate, this too is very small compared to the lift forces on the tail that can aggravate recovery around neutral, when the pilot over-corrects or is behind the plane. In either case, I stand corrected; it shouldn't have taken computer data for me to see that differences in my drag force are too small to matter in matters of horizontal-tail aspect ratios.

Offline Howard Rush

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Re: Stiletto tail size
« Reply #17 on: February 19, 2018, 11:16:16 PM »
I guess the reason for low aspect ratio tails is to take advantage of the lower lift curve slope to keep the tail from stalling (or operating in the angle-of-attack range where it's unpredictable) at high pitch rates.  Also, Frank Williams observes that it keeps more of the tail in the prop blast.
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Online Ted Fancher

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Re: Stiletto tail size
« Reply #18 on: February 20, 2018, 01:48:03 PM »
I guess the reason for low aspect ratio tails is to take advantage of the lower lift curve slope to keep the tail from stalling (or operating in the angle-of-attack range where it's unpredictable) at high pitch rates.  Also, Frank Williams observes that it keeps more of the tail in the prop blast.

If I recall correctly I believe Bob and Wild Bill addressed that general issue on the Humbug with a combination of very limited elevator deflection and some sort of unusual geometry of the bellcrank to reduce sensitivity to the point reasonably normal control inputs and responses were achievable.  I'll have to look it up again and reread that part.

It's worth noting that Bob was extremely successful flying control set-ups that some might find objectionable.  I, for instance, flew one of his more conventional ships (forget which one) after he had flown it to a third or fourth place finish at one nats and had to quit flying maneuvers with it as I found it impossible to fly straight and level and actually was concerned about landing it safely.  I've no explanation as to why when the engine quit it settled down and essentially made a 40 point landing on its own.

I say again, he finished a very competitive mid top five at our national championships with the ship.  Clearly, there appears to be more than one way to set up and stunter to do the tricks! 

Ted

Offline Serge_Krauss

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Re: Stiletto tail size
« Reply #19 on: February 20, 2018, 02:51:45 PM »
I was also thinking that the lower lift-curve slope for tails of lower aspect ratio would make control feel less "jumpy" (possible through other geometry changes) while providing all the input needed to corner as sharply as necessary, and still, as Ted implied, reducing deviations from gusts.

Offline Target

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Re: Stiletto tail size
« Reply #20 on: February 20, 2018, 04:48:30 PM »
Maybe what Ted wrote is one of the reasons that Bob said most guys add 1/4" of chord to the stab and also to the elevator?
Reduces AR while at the same time adding tail volume. More normal human friendly, perhaps?
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Chris
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Offline Howard Rush

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Re: Stiletto tail size
« Reply #21 on: February 21, 2018, 12:20:09 AM »
I was also thinking that the lower lift-curve slope for tails of lower aspect ratio would make control feel less "jumpy" (possible through other geometry changes) while providing all the input needed to corner as sharply as necessary, and still, as Ted implied, reducing deviations from gusts.

Hmm. Less jumpy response to control input?  It would also be less stabilizing.

Wouldn’t increasing tail lift curve slope relative to wing lift curve slope reduce airplane gust response?  Time to do a full-zoot dynamic analysis.
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Online RandySmith

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Re: Stiletto tail size
« Reply #22 on: February 21, 2018, 07:32:15 PM »
Quote from: Brett Buck on January 22, 2018, 03:46:34 PM
The very high aspect ratio tails are *very* sensitive and also non-linear - a little elevator deflection does a lot, a little more does a lot less.

Why’s that?
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I have  not found that to be the case at all,  I flew my design  DreadNought VECTRA  at 9 NATs  and countless other contest,  It has a very hard sharp corner, but is  NOT at all  overly sensitive or non linear , It   is very stable, yets turns the plane with great authority in calm and  high winds, it is very consistent  with little or a lot of control movement . The control movement vs turn is very  honest.  IF I could find a way to make one as stiff as the  Low  A/R    tails, I would go back,  ITs  just so much easier to make Low A/R tails  stiffer,  and  stiffer  is  hard  to argue with

Randy

Offline goozgog

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Re: Stiletto tail size
« Reply #23 on: February 25, 2018, 09:18:10 AM »
  The tail of the 660 Stiletto has a very curious trait.
I have always wanted to ask Les McDonald if he is a
musician or Lutier.

   The spacing of frets on a guitar fingerboard are
positioned according to the "Golden Ratio" of 1.61804.
The spacing increasing by that amount per fret.

  If you build a 660 Stileto tail surfaces exactly to the plans,
and then run your finger over the edges of the ribs,
you will hear a perfect musical scale .
Do - Re - Mi - Fa -So - La - Te - Do.

Only Les MacDonald knows for sure if this was intentional..

BTW My 660 Stiletto is a wonderful thing.. Excellent in the wind.

Cheers!
« Last Edit: February 25, 2018, 10:09:22 AM by goozgog »
Keith Morgan

Online Dietmar Morbitzer

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Re: Stiletto tail size
« Reply #24 on: February 26, 2018, 09:37:47 AM »
   If you are not concerned with replicating the original, with all due respect to those involved, why not just copy the Impact or Trivial Pursuit and put a Stiletto fuselage on it? 

I build a Stiletto profile kit, so everything is in the box. My idea was to take it for an engine test bed also, finding out the difference between 4-2-4 run
vs. high-low vs. electric engine. Building the profile seemed to me reasonable, also, for faster and easier engine change, and easier operating in wintertime for practice, because a side mount engine is much easier to prime and start in cold weather conditions when you are alone with your stooge. Now I'm not shure if it makes more sense building a full fuselage instead a profile fuse because the stiffness? The tail is grown to 26%, only on the stabiliser.
Any suggestions on this?
Dietmar

Offline Dave_Trible

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Re: Stiletto tail size
« Reply #25 on: February 26, 2018, 11:24:35 AM »
Arriving late to this discussion,  On your final point of using the airplane as a test bed for engine runs etc., A side mounted will run differently than one mounted upright or inverted in our application.  Whatever you learn from a profile-mounted motor may not apply to otherwise.  I'd mounted it as you intend to use it in the final analysis.  Also for your express purpose you might just leave the airplane as designed.  Yes we do things a little different now for top end competition and you can move on to that but you might to more harm than good to a proven design and gain nothing.  One thing ALWAYS affects something else.  The larger stab/elevator allows us to move the CG back for cornering in wind etc., but the airplane, moments and the rest weren't designed with that concept.  It is/was pretty dang capable to begin with.  Worm cans don't usually have screw on lids.

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Offline Ron Varnas

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Re: Stiletto tail size
« Reply #26 on: February 28, 2018, 10:54:03 PM »
I've had a Stilletto 660 build on the backburner for a longtime now, I also received the same information from Les's recommendations
on increasing the stab & elevator chords.

One thing on these 70's designs that rarely comes up in conversation is that ubiquitous 60's/70's 'diamond' section stab airfoil,
which today I'd change to a more conventional airfoil section.
RJV Melb. Australia


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