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Author Topic: Finally satisfying a long standing desire to build a Mini Twin Fin  (Read 344 times)
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PaulBrad
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« on: May 22, 2021, 12:00:03 AM »

Back in the late 70's the National Free Flight Society published a full size plan for Bob White's Mini Twin Fin P-30 in the Digest. I decided way back then that I would build that model. It is hard to believe that so much time has elapsed since that plan first arrived in my mail box tucked inside the Digest and when I finally made good on the build. I have just completed my Mini Twin Fin built to the original plan. My only exceptions are a few 3D printed parts, the use of a Gizmo Geezer prop/thrust bearing, and an electronic DT timer rather than a fuse.

My rendition of the model came in at 43 grams. The DT timer, actuator, and battery weigh 3.65 grams so had I used a fuse I would have been very slightly under weight. I am using a pop off wing DT set up. The wing to fuselage lanyard is not present in the photos of the completed model.

One nice thing about waiting so long to build this model is the availability of 3D printing. I was able to create a number of fixtures that made building this model so much easier to get things aligned and assembled.

Paul Bradley
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Finally satisfying a long standing desire to build a Mini Twin Fin
Finally satisfying a long standing desire to build a Mini Twin Fin
Finally satisfying a long standing desire to build a Mini Twin Fin
Finally satisfying a long standing desire to build a Mini Twin Fin
« Last Edit: May 22, 2021, 12:18:05 AM by PaulBrad » Logged
kkphantom
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« Reply #1 on: May 22, 2021, 04:14:30 AM »

Still the best looking P30 design ever, good job.

Gary
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PantherM100
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« Reply #2 on: May 24, 2021, 05:08:24 PM »

Hi Paul Brad:
I am also taking the scenic root on building
A Mini-Twin-Fin.  Since you have one finished,
I have a few questions.  I have heard many good
Things regarding the MTF.  I have the NFFS down
Load of the plans.  The first thing that caught
My eye is the 21.0” wing span and the 3.5” chord
Of the main panels.  The P30 rules in the USA
Call for not more than 30.0” in any dimension.
The wing area falls near the low limit sweet spot
For a P30 model.  The all up flying minimum weight
of 40 grams For the model plus 10 grams max of
Rubber.
 I’m going by the premise that
Wing area is your friend, especially since the
Model will be expected to thermal on the
Light varieties we get here in on the East coast.
I was thinking of increasing the chord to about
4.0” and the w/s to about 29.0”, keeping roughly
The same aspect ratio.  I already have a wing
Drawn up in DevCad Wing Cam v 3.03d, and I’m
Not certain that is the route I want to follow.
Any comments?  Go with the Original or Increased
Span and chord.
Thanks in advance
Jon B. Shereshaw
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Tmat
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« Reply #3 on: May 24, 2021, 05:49:53 PM »

Jon,
The NFFS pan is a bit confusing. It shows only one center panel with a tip on either side. Presumably to fit onto the sheet?
You are supposed to build two center panels, thus 30" wingspan.
The chord is narrow that is true. But while additional wing area does reduce wing loading, it also reduces aspect ratio. Bob wanted a high aspect ratio P30 to reduce the drag in the climb to get high. The Mini Twin fin gets high and thermals well. I'd leave the wing chord alone. If you want a bigger chord there are plenty of other designs that go that way.
Just my 2 cents!

Tony
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dosco
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« Reply #4 on: May 25, 2021, 12:04:49 PM »

Paul:
She's a beauty, well done!

-Dave
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Tapio Linkosalo
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« Reply #5 on: May 25, 2021, 12:55:48 PM »

The chord is narrow that is true. But while additional wing area does reduce wing loading, it also reduces aspect ratio. Bob wanted a high aspect ratio P30 to reduce the drag in the climb to get high. The Mini Twin fin gets high and thermals well. I'd leave the wing chord alone. If you want a bigger chord there are plenty of other designs that go that way.

I still remember one P-30 fly-off at sunny, calm lake Mjösa, in Smalliday on Ice compeition. I was flying my "Kultasiipi"-design (4in wide wing, satrong climb on 6 strand motor) against one of the Norsk (cannot remember who) that was flying their style of design - wide wing and tail, slow climb on 4-strand motor. We launched at the same time, in the same air. Kultasiipi climbed much. much higher. But then, it also started to sink faster, and the closer they came to ice, the smaller the altitude difference. In the end, the models landed almost at the same time, and checking the stpwatches, it turned out that I had lost by 2 seconds. So it was funny how similar the performance was from two quite different designs (or could you even say concepts)!
 
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DerekMc
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« Reply #6 on: May 25, 2021, 03:14:52 PM »

The chord is narrow that is true. But while additional wing area does reduce wing loading, it also reduces aspect ratio. Bob wanted a high aspect ratio P30 to reduce the drag in the climb to get high. The Mini Twin fin gets high and thermals well. I'd leave the wing chord alone. If you want a bigger chord there are plenty of other designs that go that way.

 So it was funny how similar the performance was from two quite different designs (or could you even say concepts)!
  

Tapio, that's exactly why P30 is a great class.  There are several design concepts that work well and are competitive.  
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PaulBrad
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« Reply #7 on: May 25, 2021, 08:35:10 PM »

Jon - As it has been noted, the wing span of the Mini Twin Fin is just a bit over 29" when you include two main wing panels.

Tapio - A very interesting demonstration of how two different design philosophies can provide very similar results.

Gary and Dave - Thanks for the kind words.

Paul Bradley
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