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Author Topic: Question on Imitation Flap Chord for Ted  (Read 535 times)

Offline jfv

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Question on Imitation Flap Chord for Ted
« on: August 11, 2020, 09:40:31 AM »
Ted:

I love your article on the Imitation, and have a question on the flap chord.  In the article you mention that the flap chord on the Imitation was 17% at all stations.  The plans for the Imitation that I have show a flap chord at the root of 19.5% and at the tip of 13.8%.  My question is whether a tapered chord flap, as shown on the plans I have, has any significant difference, plus or minus, over a constant chord flap?

Thanks,

Jim V     
Jim Vigani


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Online Brett Buck

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Re: Question on Imitation Flap Chord for Ted
« Reply #1 on: August 11, 2020, 04:21:59 PM »
Ted:

I love your article on the Imitation, and have a question on the flap chord.  In the article you mention that the flap chord on the Imitation was 17% at all stations.  The plans for the Imitation that I have show a flap chord at the root of 19.5% and at the tip of 13.8%.  My question is whether a tapered chord flap, as shown on the plans I have, has any significant difference, plus or minus, over a constant chord flap?

    Did you use the entire chord of the wing in the denominator, or just the fixed part?

   Brett

Offline jfv

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Re: Question on Imitation Flap Chord for Ted
« Reply #2 on: August 11, 2020, 11:32:34 PM »
Entire chord.
Jim Vigani

Offline Ted Fancher

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Re: Question on Imitation Flap Chord for Ted
« Reply #3 on: August 14, 2020, 06:40:01 PM »
Ted:

I love your article on the Imitation, and have a question on the flap chord.  In the article you mention that the flap chord on the Imitation was 17% at all stations.  The plans for the Imitation that I have show a flap chord at the root of 19.5% and at the tip of 13.8%.  My question is whether a tapered chord flap, as shown on the plans I have, has any significant difference, plus or minus, over a constant chord flap?

Thanks,

Jim V   

Well, Jim V...this has proven quite embarrassing. 

After digging out my article with all that data in it I can't find any way to dispute what you've brought up...even though measuring with calipers made our numbers a bit different the difference was infinitesimal in the "real world".  I've tried to think up an excuse  for the arithmetical goofs but failed miserably.  Perhaps due to the fact that I wrote that stuff (especially the tables from which we both gleaned the data) 40+ years ago and my memory ain't remotely what it used to be! (maybe that "memory" disappeared early and that's my excuse???? y1 y1 y1...naw, didn't think you'd buy that.

FWIW, I still feel the resulting air frame as presented performed in the exemplary manner declared and the "concepts" of modest area/chord  percentage of flaps, large % area tails and the aft CG those factors permit----while retaining exemplary stability and excellent maneuverability--are valid.  Although I've had several pretty good airlplanes/designs during my competitive career the Imitation to this day brings back the fondest memories of a just plain perfect ship both for performance and ease of achieving that performance.  If you're planning to build one I encourage you to do so; build it straight and at a reasonable weight  and don't go nuts in search of the zero ounce stunter.  The most flights I flew with the original were with a "heavier than most" four stroke in it that required I also add tail weight that brought its dry weight up to ~61-2 ounces dry and it never once failed to perform beautifully despite its modest wing area.

If I still had it I'd probably be flying more than once or twice a year.

Oi vey, you caused me a little embarrassment but I'll get over it.  Thought it would be chicken S$#t to PM this response to you.  Drop me another note when you get yours in the air and let me know how you like it.

Ted Fancher
« Last Edit: August 15, 2020, 04:34:12 PM by Ted Fancher »

Offline jfv

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Re: Question on Imitation Flap Chord for Ted
« Reply #4 on: August 14, 2020, 09:56:50 PM »
Thanks Ted.  Actually, I'm putting my own design together with basically an Imitation wing layout, only slightly smaller.  I like the looks of the Imitation flaps better than the constant chord flaps and that's why I asked.  The Imitation is on my build list though.
Jim Vigani

Offline Ted Fancher

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Re: Question on Imitation Flap Chord for Ted
« Reply #5 on: August 15, 2020, 12:28:21 PM »
Jim,

FYI, the whole purpose of the development of the Imitation and my competition ships of that era was to investigate and confirm in real life my feelings about the desireability of using no more flap than necessary to minimize the negative pitching moment that must be overcome when they are deflected and to configure the rest of the airplane so that the CG could be moved aft so that the moment between the CG and the Center of Lift is minimized or eliminated.  The primary goal in doing so was to minimize to the greatest degree possible the change in response when flying in high winds/gusty conditions.

It was very common in my early days of flying stunt that ships (especially flapped ones) that flew just fine in "good" air had tendencies to open up and/or completely lose the ability to not only fly the corners which were normal in calm/light wind conditions but even to fly loops.  We would frequently see pilots abandon consecutive loops and/or simply crash because even full up/down control was insufficient to keep the maneuver from opening up and even running out of airspace and crashing.

The two things I wanted t mitigate to resolve such problems were to design the airplane so as to allow a perfectly flyable ship with the CG much further aft than was "normal" and...a bit less important but a contributor to the problem nonetheless...reduce flap chord to no more than necessary (a lot smaller than many thought practical) to provide the lift necessary to support the ship with the improved CG location and simultaneously reduce the adverse pitching moment caused by deflecting larger than necessary flaps to say nothing of the reduced loading on the control system to deflect the excessively large flaps.

More or less the structural definition proof of my belief that "more" lift (from big flaps) doesn't make stunt ships turn tighter but can do the opposite in a variety of ways.  We only need "enough" lift to support the ship in corners the size we desire plus a tiny amount to prevent stalls should we get too aggressive. 

Just a quick final note; a ship designed, constructed and trimmed per these factors can be flown in perfect to rotten weather without anywhere near the fanatical "yank and bank" panic corners frequently required to keep from crashing ships with forward CGs, small tails and biiiig flaps!

Gotta run.  Momma says I'm wasting time!

Ted

Offline Howard Rush

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Re: Question on Imitation Flap Chord for Ted
« Reply #6 on: August 15, 2020, 03:42:11 PM »
...to say nothing of the reduced loading on the control system to deflect the excessively large flaps.

I'd say something of that.

Gotta run.  Momma says I'm wasting time!

Considering the fraction of her life that said Momma has spent in the service of Stunt, she has earned the right to so advise. 


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Offline Ted Fancher

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Re: Question on Imitation Flap Chord for Ted
« Reply #7 on: August 15, 2020, 04:32:00 PM »
I'd say something of that.

Considering the fraction of her life that said Momma has spent in the service of Stunt, she has earned the right to so advise.

Howie!

Thanks for the shout out to "Momma". 

Oddly enough, just this  morning as I wandered by the upstairs bookshelves wondering where to "power walk" my Covid-19 mask today I stopped to read the plaques (Hall of Fame and Thanks for her 30 years of service as Nat's/WC Head Tabulator and PAMPA Secretary Treasurer) and the beautiful glass trophy given Shareen by PAMPA on another occasion and was once again struck by how fortunate we were to have found...first one another...and then to have found a mutually satisfying way to share so much of our lives together and with the hundreds of extraordinary people with whom we became friends all because of toy airplanes and caring for one another.

Thanks for the perfect opportunity to say so publically.

Ted

Offline jfv

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Re: Question on Imitation Flap Chord for Ted
« Reply #8 on: August 15, 2020, 08:47:55 PM »
Here's another question regarding flap chord.  By reducing the flap chord toward the tips, doesn't that effectively produce some washout at the tips when the flaps are deflected?  (Less angle of attack the tips)  Any thoughts on how that might affect flight performance?
Jim Vigani

Offline Ted Fancher

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Re: Question on Imitation Flap Chord for Ted
« Reply #9 on: August 15, 2020, 10:54:49 PM »
Here's another question regarding flap chord.  By reducing the flap chord toward the tips, doesn't that effectively produce some washout at the tips when the flaps are deflected?  (Less angle of attack the tips)  Any thoughts on how that might affect flight performance?

Interesting question, Jim.  Speaking strictly theoretically I would think there's validity to the suggestion and that, again, theoretically, the effect would be to minimize any unwanted lift induced roll.  From a practical standpoint, however, I think any such affect would be in the "noise" and is one of those things I think we sometimes decide to make important when it probably really isn't.

As you've learned, however, I've been wrong before!

Ted


Offline jfv

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Re: Question on Imitation Flap Chord for Ted
« Reply #10 on: August 16, 2020, 06:48:52 AM »
Ted:

I think within the norms we typically use, and all the variables we introduce during the building process, you are probably correct.  However, if we can eliminate the "noise" as much, and from as many sources as we can, things might get a bit quieter.  Ever the search!
Jim Vigani

Offline Ted Fancher

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Re: Question on Imitation Flap Chord for Ted
« Reply #11 on: August 16, 2020, 11:43:30 AM »
Ted:

I think within the norms we typically use, and all the variables we introduce during the building process, you are probably correct.  However, if we can eliminate the "noise" as much, and from as many sources as we can, things might get a bit quieter.  Ever the search!

Oh, I certainly can't argue the validity of the "search", Jim.  Spent a bit of my time on the prowl as well!  Now if we could just find a way to stop flying in a yaw all the time... ??? >:D ;D

Good luck, Jim.  Keep us informed as your safari continues!

Ted

Offline Howard Rush

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Re: Question on Imitation Flap Chord for Ted
« Reply #12 on: August 16, 2020, 12:14:18 PM »
By reducing the flap chord toward the tips, doesn't that effectively produce some washout at the tips when the flaps are deflected?  (Less angle of attack the tips)

Your choice of reference from which to measure angle of attack has no effect on the physics.
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Offline jfv

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Re: Question on Imitation Flap Chord for Ted
« Reply #13 on: August 16, 2020, 05:23:04 PM »
Howard, I was referring to the relative angle of attack between the root and the tips.
Jim Vigani

Offline Ted Fancher

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Re: Question on Imitation Flap Chord for Ted
« Reply #14 on: August 21, 2020, 11:14:30 PM »
 (snip) Here's another question regarding flap chord.  By reducing the flap chord toward the tips, doesn't that effectively produce some washout at the tips when the flaps are deflected?  (Less angle of attack the tips) (snip)

Jim,

The above copied from one of your earlier posts!

I believe I've always advocated consistent percentage of flap chord for the flapped portion of the wing.  I have, by the way, built planes my preferred way with "fixed trailing edges" for the span encompassed by the wing tips.  This generally amounted to three inches or so because my ships have pretty much all had "swept" wingtips with curvature "raked" from leading edge to trailing edge. 

I did, however modify my last Trivial Pursuit to increase the flaps to full span when I repainted it from the original red,white,blue of it's "Great Expection" years (which included a win at the 1986 Nats to the Pond Scum Purple refinish (per Bubba Hunt) when it won its last Nats in 2000.  I did so solely because so many of my flying buddies... Dave Fitz, in particular, on very "similar" ships...utilized full span ones and were kicking my butt with unpleasant regularity.

For what it's worth I didn't like the altered version all that much...although it didn't stink.  I felt the same corners required more effort on the part of the pilot which, as a finger and wrist pilot, I didn't appreciate.  I postulated that the longer "flapped" segment increased the negative pitching moment of the now full span flapped wing, thus requiring the greater pilot effort.  Although I'm less confident, I also postulated that the wing tip vortices, all emanating from the deflected flap itself may have also played a part in the increased "load" I was experiencing.

I don't feel that "lift" is the secret to sharper corners.  If you ain't stalling in a corner you've got the lift necessary to accomplish it.  If you don't...you won't!  In addition, if you're developing more lift than necessary to support the weight of the aircraft the "angle" of the fuselage path around the loops, etc. will be modestly different from "straight" ahead per the radius you're attempting to track.  You only "need" enough lift to support the "G" induced load required to maintain the desired radius of a "turn".  If you "get" the required lift prior to the body reaching "parallel" with the arc of the desired path your flight path's intersection (of a horizontal eight for illustration) will occur prior to the fuselage reaching "vertical" and vice versa.  As a result, disputes could be made as to whether you did or did not perform and accurate intersection!

Finally the wisdom of the search for the zero ounce stunter!  First of all see the preceding paragraph and contemplate flying a figure eight with a 700 square inch full span flapped wing.  I think the problem would be obvious.

Second of all, a real world example of the near opposite state of affairs; starring, yet again, the Trivial Pursuit...a "pig" of a plane at 73 oz, 650 sq. inches of flapped wing area...albeit with flaps ending three inches prior to the full span of the wing with tips.  Also a "pig" of a plane that won one US Team Trials, four second place Nats finishes plus four other top fives.  Not bad for a "Porker"!

Here’s the rest of the story, however.  The part that more or less is the proof of the zero ounce pudding!

At one of the later Nats at which I flew the T.P. it was unusually hot and humid and during a practice flight prior to the start of the comp something happened.  For the first time ever the T.P. stalled—dramatically, saved only at the last possible second--in the last corner of a triangle!  Ooopsy!

Long story short, Brett and I diddled around with the ship and ultimately added a primitive form of vortex generator to the top of the wing…we taped a couple of feet of control line to the top of both wings an inch or so forward of the high point of the airfoil.  Result, no more stalls and the T.P. continued to a top five finish!

Ultimately, to get rid of the ugly taped on wire I simply raised the elevator pushrod a couple of turns so as to get just a wiggle of additional flap deflection for a given elevator deflection and forgot about the issue from then to the time I stuck my finger in the prop setting the needle valve, snapped a blade off the three blade Eather  prop and the poor T.P. shook its front end to death as P.W.  tried to shut it off by pointing the nose down…but the froth in the tank just increased the RPM and…Good Bye T.P.

The reverse of all the above was well illustrated with the “career” of my ultra light (32 oz) Tucker Special which was built thus almost solely to experiment with the “zero ounce stunter” concept.  The tale has been told several times here on S. H. but, long story short, the airplane was very difficult to fly well at that weight with corners seeming to take place at random places and radii.  Making simple loops the same size--and round! --was very difficult.  IOW, it pretty much sucked!

Brett and I took the ship to the field one day with the tool box refilled with stick-on lead weights cut in ¼ oz segments.  In a series of flights we added these weights a total of one ounce per flight by sticking them, four at a time, to the wing near the fuselage, one each on the top and bottom of each wing at the CG, thus increasing the gross weight without moving the CG or unbalancing the wing inboard to outboard.

We eventually added eight total ounces to the ship flying it after every added one ounce of lead (four 1/4oz chunks).  With each addition the consistency of all the above problem was mitigated and after a total of eight ounces was added the airplane was “dramatically” better and easy to fly competitively.   No question, heavier was better!

The point of telling these stories is to illustrate how the tub of lard T.P. had always flown close to the edge of its lift margin yet had consistently placed near the top of some of the most competitive comps on the planet.  Almost as illuminating was that it did so with powerplants ranging from, at first, a little piped VF .46 to, ultimately, a Ro Jett .61.  The lesson, more or less, was that—when coupled to the earlier experiences with flap spans, Tucker Special tests, etc. was that building a stunt ship too heavy was almost as hard as building one too light but way less damaging to the score sheets.

What we need is access to “enough” lift but lift much beyond “enough” is likely to be harder on the score sheet.  I’m simply not a big fan of maximizing lift for lift’s sake  Given “enough” lift, centered in the right place with respect to the CG ; with the CG properly located fore and aft (near the center of lift) and a tailplane of the appropriate size and configuration and with the hinge lines sealed to insure uniform distribution of the lift they produce…plus several dozen test flights to optimize flight trim…is the appropriate path to success at the field.

Sure, it’s fun to diddle with the minutiae of the multiple facets of the “aerodynamicness” of the whole thing but, for the most part, I believe the minutiae in an aircraft as fundamentally simple as a CL stunt ship pretty much remains just that: minutiae.

Sorry.  Got carried away.  Between the smoke filled skies of the San Francisco Bay Area and the lock down of the COVID “Pandemic” one has to fill the forced indoor hours with something!



« Last Edit: August 22, 2020, 12:14:38 PM by Ted Fancher »

Offline Mike Ferguson

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Re: Question on Imitation Flap Chord for Ted
« Reply #15 on: September 01, 2020, 06:52:00 AM »
Just some peripheral observations I've made this month about the "old school" planes with "Big Flaps" and "Big Tails" ...

I started work on an electrified Thundergazer this month. As I started to work on it, I decided it'd be cool to make it cosmetically look like Jim Casale's rudderless Spectrum - the 1985 Nats winner, which is the reason my lifelong involvement with Stunt really began. (That, and I think it's one of the coolest looking planes ever.) So - armed with Thundergazer plans and a T-Gazer foam core, as well as the relevant Spectrum plans and one of Jimmy's actual 1986 competition planes - I started seeing just how much would need to change.

I'm still stunned by how similar they are.

Root airfoil is identical. So, Thundergazer=Thunderbolt=Spectrum=Avanti, basically. (And, if an old checking of plans is correct in my memory, =Shark 45.) Tip airfoil for the Spectrum is the same in shape, except it's 1/2 inch longer than the Thundergazer's. The T-Gazer has slightly more wingspan, and has unequal panels.

Nose moment (going by conventional measurement of back of spinner to wing leading edge) is the same. Tail moment (trailing edge of wing to trailing edge of stab) is 17 1/2" for the Spectrum, 18 1/2" for the Thundergazer. Engine/motor Thrust line to C/L of wing is the same, as is the stab/elevator C/L to the C/L of the wing.

The main differences are the flaps, which the Spectrum has much more of a taper (which is why this thread caught my attention), and each Spectrum flap is about 10 square inches bigger. The stab/elevators also are about the same area-wise - the Spectrum is just higher aspect (a 29" span to the T-Gazer's 26" span, and about a 50/50 stab/elevator area ratio to the T-Gazer's roughly 60/40 s/e ratio).

Maybe this isn't terribly surprising to most, but it was a little to me. Not that the differences in tail and flaps aren't significant, they're just smaller than I'd expected. I'd also gone for a long time just assuming that the 1980s East Coast Planes (TM) were all 720 inch "giants", when in reality, they (or the Spectrum Mk III, at least) is about 670, in line with a lot of today's designs.

I'm more convinced that what made those planes different from the modern ones in terms of flight trim were 1) the "East Coast" nose heavy trim style in vogue back then (put in lots of nose weight to 'make it groove'), 2) the 3 inch bellcranks they used, with shorter arms on the flaps/elevators, 3) non-adjustable pushrod - they all basically had 1:1 controls, and 4) handles that had long arms and didn't have adjustable spacing. I'm also sort of convinced that building one of those planes as they were back then with *just* with a modern control system, a Fancher/Walker type handle, and trimmed less nose heavy might be extremely competitive. (I wish the old Spectrum I have could be converted to this - but besides needing to replace the controls entirely, it's from the folding foam wing generation, so I don't think it's worth the exercise.) I do think the smaller flaps and different tail make a positive difference, but the controls and handle make a bigger one. (I think. Just some musings on my part. I guess I'll find out when this new one gets in the air!)


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