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Author Topic: Favorite Stunt model design numbers?  (Read 3838 times)

Offline Dennis Toth

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Favorite Stunt model design numbers?
« on: November 01, 2021, 11:06:10 AM »
As Stunt has progressed different sets of "the numbers" have evolved as dominate in designs. The basic PA stunt numbers seem to start with GMA' s Nobler. Question is how have these number changed and what are the factors that caused them to move in one direction or another?

Best,   DennisT

Offline Brett Buck

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Re: Favorite Stunt model design numbers?
« Reply #1 on: November 01, 2021, 11:21:45 AM »
As Stunt has progressed different sets of "the numbers" have evolved as dominate in designs. The basic PA stunt numbers seem to start with GMA' s Nobler. Question is how have these number changed and what are the factors that caused them to move in one direction or another?

Best,   DennisT


Start here:

https://library.modelaviation.com/ma/1979/9/designing-imitation

   Brett

Offline Walter Hicks

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Re: Favorite Stunt model design numbers?
« Reply #2 on: November 01, 2021, 01:59:21 PM »
What Brett said for sure, easy to fly, everything well thought out well proven over and ever design.

Offline Mark wood

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Re: Favorite Stunt model design numbers?
« Reply #3 on: November 01, 2021, 03:44:35 PM »

Start here:

https://library.modelaviation.com/ma/1979/9/designing-imitation

   Brett

Good read, thanks for sharing Brett. It's nice to look back in history and see previous work.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #4 on: November 01, 2021, 07:19:50 PM »
Let's not forget how the change in power we have available has influenced design and we are still not done figuring out if the vibration free electrics will bring on more design changes, especially with twin motors, logarithmic flaps and active timers.  Brave new world.

Ken
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Offline Brett Buck

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Re: Favorite Stunt model design numbers?
« Reply #5 on: November 01, 2021, 08:43:08 PM »
Good read, thanks for sharing Brett. It's nice to look back in history and see previous work.

   Numbers-wise, there is not much "previous" about it, this is the prototype. Compare/contrast to the Thundergazer.

    Brett

Offline Brett Buck

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Re: Favorite Stunt model design numbers?
« Reply #6 on: November 01, 2021, 08:44:19 PM »
Step 2 is the Impact article. I couldn't find it in a quick search on-line, although I do have it in the original magazine somewhere.

    Brett

Offline Mark wood

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Re: Favorite Stunt model design numbers?
« Reply #7 on: November 01, 2021, 09:15:55 PM »
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #8 on: November 06, 2021, 07:10:49 AM »
I was hoping we could get into a discussion of how the numbers impact the design and what changing say tail moment, tail volume, nose moment, airfoil thickness or control ratios changes how a ship reacts. Why have the numbers changed from the Nobler to where we are today? As a particular design evolved and the designer made changes what was the performance improvement they expected from the changes made? Just why are one set of numbers good for IC but not electric?

Best,   DennisT

Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #9 on: November 06, 2021, 07:27:10 AM »
Just why are one set of numbers good for IC but not electric?
Best,   DennisT
y1

Ken
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Offline Brett Buck

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Re: Favorite Stunt model design numbers?
« Reply #10 on: November 06, 2021, 09:44:17 AM »
I was hoping we could get into a discussion of how the numbers impact the design and what changing say tail moment, tail volume, nose moment, airfoil thickness or control ratios changes how a ship reacts. Why have the numbers changed from the Nobler to where we are today? As a particular design evolved and the designer made changes what was the performance improvement they expected from the changes made?

   The linked article is exactly that.

    Brett

Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #11 on: November 06, 2021, 10:44:07 AM »
   The linked article is exactly that.

    Brett
That is true if it is 1980.  It is for all intent and purposes 2022.  Has nothing has changed in 42 years?  The awesome IC motors in use today were still 10 years off and electric was what powered the space heater in your shop when Ted wrote that article.  Nothing in it is outdated it simply doesn't consider the impact of the technology we have today.  It is the starting point for this discussion that I hope unfolds before we are too deep into the building season to make changes.  Personally, I think the new line size rules, logarithmic controls,  active timers,  twins, etc. are going to make our ships of tomorrow as different as the ships of today are of the ones wining in the 70'.  I just don't know how yet.

Consider the tufting work Mark Wood is doing.  Could that impact airfoils, especially electric airfoils which appear to respond better to thinner sections and forward CG's than IC but I have no clue why.  I am designing my PA for next year.  It will be electric and I really would like to know.

Ken
« Last Edit: November 06, 2021, 07:23:50 PM by Ken Culbertson »
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Offline Brett Buck

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Re: Favorite Stunt model design numbers?
« Reply #12 on: November 06, 2021, 05:18:27 PM »
That is true if it is 1980.  It is for all intent and purposes 2022. 

     Compare a Thundergazer to an Imitation, then get back to me.

     Brett

Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #13 on: November 06, 2021, 07:46:57 PM »
Mark,
I think you should look up some of the airfoil work done by Al Rabe. Al wanted to build close to scale stunt ships, he pushed the thinking on stunt ships design things like wing load, airfoil shape, thickness and flap size. On his makeshift test rig that he installed on the hood of his car he started with the Nobler airfoil and tested several arrangements including what he thought would be the best arrangement with the rear part of the airfoil forming the flap. This he later had to admit was not the super airfoil he had hoped for. One thing he found was that airfoils thicker than 15% didn't make much more lift except when the flaps were included. He did a lot of reshaping of the main airfoil and flap airfoil to come up with a shape that flowed into the deflected flap. His ships even with the higher wing loads of the close to scale numbers of his ships, which he found using larger than normal flaps gave the performance he wanted. His biggest issue was always being right on the edge of having enough power. He had extra material welded on to the engine blocks to be able to machine larger cooling fins. When the line rules and engine restrictions were revised he finally was able to get the kind of power he needed and did it with large 2 cycles and 4 stroke. Interesting developments. At the same time frame Ted Fancher was doing his development work as outline the Intimidation design articles as well as his other numerous columns in various magazines.

Best,   DennisT 

Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #14 on: November 06, 2021, 08:33:01 PM »
Mark,
I think you should look up some of the airfoil work done by Al Rabe. Al wanted to build close to scale stunt ships, he pushed the thinking on stunt ships design things like wing load, airfoil shape, thickness and flap size. On his makeshift test rig that he installed on the hood of his car he started with the Nobler airfoil and tested several arrangements including what he thought would be the best arrangement with the rear part of the airfoil forming the flap. This he later had to admit was not the super airfoil he had hoped for. One thing he found was that airfoils thicker than 15% didn't make much more lift except when the flaps were included. He did a lot of reshaping of the main airfoil and flap airfoil to come up with a shape that flowed into the deflected flap. His ships even with the higher wing loads of the close to scale numbers of his ships, which he found using larger than normal flaps gave the performance he wanted. His biggest issue was always being right on the edge of having enough power. He had extra material welded on to the engine blocks to be able to machine larger cooling fins. When the line rules and engine restrictions were revised he finally was able to get the kind of power he needed and did it with large 2 cycles and 4 stroke. Interesting developments. At the same time frame Ted Fancher was doing his development work as outline the Intimidation design articles as well as his other numerous columns in various magazines.

Best,   DennisT
Just a follow up.  Al's work has contributed a lot to our hobby.  As the illustrations point out, one of the things he tried to do, with some success, was to smooth out the top surface of the wing as the flaps extended which ties directly into what Mark is experimenting with.  Also note that the highpoint of his airfoils are well back from what is today's norm and the aft curvature is more European.  One of the highlights of my flying "career" was watching Al do square 8's with one of his Sea Furys.  Probably the worst flying plane of his fleet but one of the most impressive planes in the air and I have seen him fly all but the Critical Mass.  Everyone stopped what they were doing to watch his landings on the shock gear he perfected.  Interesting on this list of his airfoils, the Mustang is missing.  It was thin compared to the others and had a sharper LE.  I think one of them was laminar like the real p-51 but I don't remember which one.

Ken
« Last Edit: November 06, 2021, 09:30:17 PM by Ken Culbertson »
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Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #15 on: November 07, 2021, 07:43:47 AM »
Mark,
This is the Super airfoil that Al Rabe was at first very hot on. As he did his test he was not impressed with it's performance and in a rare moment admitted he was wrong. Keith Trostle used this type of airfoil and had success with it, it is a pain to build and I don't know how it can be sealed. Have you done test with sealed and unsealed flap hinge lines? Sealing the hinge line seems to be one thing that was an improvement over the early flap installations. It benefits the lift prospective but also from insuring that the pressure is consistent along the hinge line to minimize inboard/outboard roll as the gap can be less then perfect as the flap moves.

Best,  DennisT

Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #16 on: November 07, 2021, 10:10:02 AM »
It's somewhat apparent to me the interest is low and the pressure is to simply do what has been previously done which is not what I am about.
Well at least you have one person interested in perusing this.  It may or may not produce anything usable but isn't the process  ​enjoyable regardless of the outcome? 

You are barking up the same tree Al fell out of but you are starting off knowing why he didn't fully achieve his goals.  The "Bump" at the hinge line caused by the flap rotating was the problem (i was typing this when you made your last post!).  I have been trying to figure out how to make a self sealing thick imbedded flap for years.  I don't have the resources to do the kind of testing you can do so I am going to bait my breath and watch. 

I have succeeded in making imbedded elevators that seal quickly by extending the 1/32" top and bottom sheeting slightly past the hinge line (Robarts hinges).  As soon as the elevator rotates, it seals on the bottom (lower) side and is thin enough to flex without binding. My stab is 1/2" thick airfoiled and the elevators 7/16".  I have not been able to duplicate this with the thinner flaps.

I have mixed feelings about sealing.  Probably since we did without it for so long and nobody seemed to notice and also because I don't fully understand how it works.  Is the "bleed" coming down from the top or up from the bottom and does the shape of the curve with the flaps engaged affect it?  How does having the actual gap imbedded in the TE change it?  Does the air stick to the tape in the gap( LL~)?

As technology grows, a lot of old ideas that were correct but impractical,  because the implementation was either too heavy or too difficult to achieve given the tools of the day, suddenly make sense.  I wonder how many of the top fliers in 1979 told Ted he was wasting his time, just go with what works?

Ken

PS: Let me add an afterthought instead of a new post.  I understand having the flap hinge line further back but I am not sure you can without creating the "bubble" even if you make it elliptical.  What does moving the hinge line back do other than taking some pressure off of the flaps?  The problem with recessing the flap into the wing close enough to be effective is the binding when the wing flexes.  I have tried "piano" hinges using aluminum tube with no success.

« Last Edit: November 07, 2021, 11:53:42 AM by Ken Culbertson »
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Offline Brett Buck

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Re: Favorite Stunt model design numbers?
« Reply #17 on: November 07, 2021, 10:38:47 AM »
   You can tell everyone exactly what you do, and why, point to the results - and they will go off and do something else.

     Brett

Offline Mark wood

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Re: Favorite Stunt model design numbers?
« Reply #18 on: November 07, 2021, 10:42:16 AM »
   You can tell everyone exactly what you do, and why, point to the results - and they will go off and do something else.

     Brett

You're right Brett. Point taken.
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Offline Serge_Krauss

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Re: Favorite Stunt model design numbers?
« Reply #19 on: November 08, 2021, 11:24:53 PM »
I am interested and following along. Maybe (!) I will try to find my one NACA report that shows planar flaps as producing superior lift but being rejected, because in the military and in aircraft where cruise is important, they have too much drag. Because of NACA's revisiting their data to improve allowances for mounting interference and the effect on RN, how well their tests (at our RN's but 'adjusted') relate to our sizes was not entirely clear. I don't think that in models the variance must always make a difference that will stand out from accuracy of building, trimming, and proportioning. Al Rabe's articles tend toward the same solution. When I was working in Profili and X-Foil (at our RN's), a planar flap out-performed an integrated flap by having a slightly higher Cl peak and less confusing stall. BUT *I* would probably not notice this difference in my own flying. I also don't know how valid X-Foil was/is for our RN's. That's one reason I'm interested in your flow fields.

So I do look forward to your further posts!

SK

Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #20 on: November 09, 2021, 11:33:14 AM »
OK, so one thing can be established is that flaps on a CLPA model allow a smoother exit from a square corner. Now, what is the optimum airfoil numbers around that? What shape leading edge, what thickness, what CG location, what cord, what wing aspect ratio, what width should the flaps be? How does the basic Nobler airfoil differ from the optimum? Is it useful for CLPA? Then we need to add in the tail moment and tail volume. This is a good discussion but we need to get into how these things make a ship better or worst then ships in the past.

Best,    DennisT
« Last Edit: November 09, 2021, 02:43:02 PM by Dennis Toth »

Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #21 on: November 09, 2021, 06:16:53 PM »
OK, so one thing can be established is that flaps on a CLPA model allow a smoother exit from a square corner. Now, what is the optimum airfoil numbers around that? What shape leading edge, what thickness, what CG location, what cord, what wing aspect ratio, what width should the flaps be? How does the basic Nobler airfoil differ from the optimum? Is it useful for CLPA? Then we need to add in the tail moment and tail volume. This is a good discussion but we need to get into how these things make a ship better or worst then ships in the past.

Best,    DennisT
It will be impossible to define the perfect airfoil.  There are just too many variables and the most important ones are style.  Do you fly slow or do you fly fast?  Different airfoils will be optimal.   Are rounds your problem or corners?  Different flap ratio's will maximize each and different moments will change things.  And that is just the start.  The math and genesis for all of these things in out there in other threads, some multiple times.  Little things about your style matter in this discussion.  For example do you trim your plane to fly a given lap time or do you fly the lap time that the plane is most comfortable?  Do you time your corners or do you follow the yellow brick road ala Ted?  Do you fly nose heavy or tail heavy.  Point is that we are all different and the optimal plane for each of us is going to be different.  So what is your style and we can focus on what is right for you.

Ken
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Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #22 on: November 09, 2021, 07:09:08 PM »
Ken,
I agree that there is no perfect one size fits all stunt ship. But we have gotten to a general set of numbers that seem to work reasonably well for most fliers. Question is over time how have these number changed and what improves a ship and what not so much? For example one thing that is very obvious is the increase in tail volume on current day ships vs. most Classics. With this also is some increase in tail moment length. From what I see this has allowed more rearward CG's while keeping the ship stable and allowing crisp corners. But is it tighter than a light, well trimmed Nobler with good power? Or is it that the larger airplanes just making it look like things are tighter?

Best,    DennisT

Offline Dave_Trible

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Re: Favorite Stunt model design numbers?
« Reply #23 on: November 10, 2021, 09:09:38 AM »
Ken,
I agree that there is no perfect one size fits all stunt ship. But we have gotten to a general set of numbers that seem to work reasonably well for most fliers. Question is over time how have these number changed and what improves a ship and what not so much? For example one thing that is very obvious is the increase in tail volume on current day ships vs. most Classics. With this also is some increase in tail moment length. From what I see this has allowed more rearward CG's while keeping the ship stable and allowing crisp corners. But is it tighter than a light, well trimmed Nobler with good power? Or is it that the larger airplanes just making it look like things are tighter?

Best,    DennisT
I believe airplane design changes have mostly followed the weight and power of engine development.  Better engines got larger and heavier.  The airplanes grew and shuffled stab areas and moments to offset a doubling of weight since the Fox .35.  The flying agility of some of the old classics built light with light engines (and no muffler weight) might surprise some.  However airplane design had to keep up with engine/ power development just to fly as well.   

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Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #24 on: November 10, 2021, 10:49:44 AM »
The flying agility of some of the old classics built light with light engines (and no muffler weight) might surprise some. 
You would have had to have flown in that era to fully comprehend what you just said.  Mufflers killed it.  Less power, more weight.  Enter the .46.  More power but more weight.  Pipes and on to today - unlimited power but heavy.  My 1963 "PA", if they called it that back then, I can't remember,  probably just "Stunt",  was 37oz and 620sq".  A rebuild I did of that plane with modern power weighed 60oz.  We all thought we were doing 5' corners but in reality they were probably like 8-10'. 

Ken
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Offline Howard Rush

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Re: Favorite Stunt model design numbers?
« Reply #25 on: November 12, 2021, 08:11:19 PM »
I encourage you to try spades and tell us what you find.  They look like a good solution because they have no friction.  I haven't messed with them because they look like they'd do weird nonlinear stuff.  Trimming a stunt plane is hard enough.

I used balance tabs on three airplanes.  They sorta worked.  I found an NACA  paper that showed that they were most effective around neutral, which is probably not what we want.  A fancier mechanism might have compensated for this. 
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Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #26 on: November 14, 2021, 09:22:52 AM »
A couple more as they are bigger than allowed in masse. I'm getting ready to send these to the printer and plan on installing them today. We have a KS unicorn day today with light winds and cool temperatures.
This is exciting, my breath is baited.

Ken
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Offline Ken Culbertson

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Re: Favorite Stunt model design numbers?
« Reply #27 on: November 14, 2021, 01:59:48 PM »
Ken

While interesting, I'm not convinced these are a practical solution as they are too vulnerable to damage.
Agreed but they could be made removable.  I really just want to know if they work.

Ken
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Offline Tim Just

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Re: Favorite Stunt model design numbers?
« Reply #28 on: November 14, 2021, 02:31:48 PM »
As Mark mentioned stick snatch is an issue with spades.  The force required to return the control surface to neutral can be very high.  In a full size airplane the pilot has the luxury of reducing speed.  If this occurred on a model it is very likely unrecoverable.   

Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #29 on: November 14, 2021, 03:17:27 PM »
Not sure what advantage these spades would give a CLPA ship other than something interesting to talk about. If you need more force to move the flaps use a longer control horn and bellcrank.

Best,   DennisT

Offline Howard Rush

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Re: Favorite Stunt model design numbers?
« Reply #30 on: November 14, 2021, 04:02:33 PM »
I like a little snatch from time to time.
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Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #31 on: November 14, 2021, 06:25:43 PM »
If finding neutral is the aim, in CLPA we address this with feel for neutral on the handle setup, more overhang at the handle or the angled inboard bellcrank arms all give a little more feel around neutral. For the handle the less overhang the more like power steering it feels but the trade-off is quicker response with less feel. For me I set up my handle with no bias, just straight vertical and when I've flow other guys ships that are set for a bias handle can really feel and have a hard time flying level and low with them.

I like the idea that adjusting the flap width can reduce the amount of needed deflection. With the larger flaps and less deflection where is the equivalent point 0 20%, 30%? If the flap is 50% increase in width it's full length how much would that reduce the deflection? Do you include the flap area as part of the wing or not (Al Rabe said no, Ted said yes), Is it just that the surface area is increased?

Best,    DennisT   

Offline Dennis Toth

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Re: Favorite Stunt model design numbers?
« Reply #32 on: November 15, 2021, 08:11:21 AM »
I guess the idea I had for this thread was to look at how a design evolved and what the thinking was behind making those changes. In Dave Fitzgerald's article on the development of the Thunder Gazer he gave a very good description of the changes to the stab airfoil and why he made the chooses he made. What was not explained was what the increase in tail moment length was suppose to do. These are a the things in the "numbers" that would be interesting to hear about from designers that have evolved a particular design - what worked and what fell short of the intended results.

Best,    DennisT

Offline Howard Rush

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Re: Favorite Stunt model design numbers?
« Reply #33 on: November 15, 2021, 10:41:56 AM »
Stability is the inverse of maneuverability.

That’s what combat fliers used to think.
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Offline Brett Buck

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Re: Favorite Stunt model design numbers?
« Reply #34 on: November 15, 2021, 11:46:48 AM »
That’s what combat fliers used to think.

   Indeed, This is why you can't have a sensible discussion on the topic, you need to rehash the last 30+ years from scratch every time. I tried above, nobody wants to do the homework.

    Brett

Offline Mark wood

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Re: Favorite Stunt model design numbers?
« Reply #35 on: November 15, 2021, 03:23:27 PM »
One thing I find particularly interesting is that fighter jets and helicopters are both statically unstable in order to improve their maneuverability.
Life is good AMA 1488
Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman


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