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Author Topic: Coefficient of Lift help for a Mini Twin Fin  (Read 441 times)
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PantherM100
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« on: June 26, 2021, 05:11:55 PM »

Hi Guys:
I’m planning on building (for a long time now),
the Mini Twin Fin P30 by Bob White.  I have a friend
Who might have some suggestions for improvement.
I need the Coefficient of Lift for the Mini Twin Fin.
To obtain this number I need a couple of pieces
Of information that I am not sure about:
*Wing Area: 108.29 Sq. In.
*Lift Force in Lbs (All up flying weight should be 53 grams
with rubber) I’m not sure how to obtain this number.
*Flow Speed in Feet/sec (30 ft/sec??) Not sure if 30ft/sec is
Accurate for a P30.
*Air Density: 1.225 Lbs/cu. ft.
Any help would be appreciated
Sincerely,
Jon B. Shereshaw




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lincoln
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« Reply #1 on: June 26, 2021, 06:03:22 PM »

12 feet per second would be more like it, unless you have your model trimmed for a steep dive.

The coefficient of lift will depend on the trim of the model. I suspect, for most P-30s, that would be between 0.7 and 1.0 in the glide, but don't hold me to it. Unless you are working from measured flight data, you should probably calculate the speed based on the Cl you want, not the other way around. Particularly considering how far off you were. Keep in mind that the Cl for the entire wing will be less than the hypothetical max for the wing section in 2d flow. Also, in real life I suspect it's best to trim a little faster than the absolute minimum speed so that the mode won't stall every time someone sneezes.

You might find it handy to look up the Wikipedia article on coefficient of lift.

I tend to remember flight speed, for a Cl of 1, in feet per second, as 7.3 times the square root of the wing loading. This will vary a little depending on conditions, but it's close. The speed will be greater on a hot humid day at high altitude, and less at the Dead Sea in the middle of the winter. If the CL is really 0.8, then you'd divide by the square of the new Cl. If the flight speed was 10 feet per second, at a Cl of 0.8 it would be 11.2 feet per second.

For any design work, you probably need the Reynolds number. My rule of thumb is 532 times the speed in feet per second times the chord in inches. Again, this depends on the conditions.

I'm sorry about the perverse English units. They're in my head in a way that the more reasonable metric units mostly aren't. Keep in mind that the English unit of mass is slugs. Pounds are just force. The weight (not the mass) of a slug is 32 lbs. At least where the gravitational field is nominal. It may vary a little in different places on the Earth's surface and a whole lot elsewhere. But it will still be 1 slug. The density of air in a standard atmosphere at sea level is 0.0023769 slug/ft3. The figure you give is in kg per cubic meter. Just for kicks, remember that the metric unit of force is Newtons. That's the amount of force to accelerate one Kg at 1 meter per sec2. If you aren't careful with units, you are guaranteed to get ridiculous results.

What exactly are you trying to do here?
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OZPAF
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« Reply #2 on: June 27, 2021, 02:02:00 AM »

To add a little to Lincoln's post - i would advise that you stay in metric units since you already have the mass in gms.

I agree with Lincoln's advice re starting with a estimated CL and calculating the speed from that. At the low Reynold's number you will probably have - and also to keep away from the stall - i would advise a first guess CL of 0.7-0.8. Calculate the speed for that and check it withe actual models' flying speed which will then give you a better value for the actual CL.

John
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cvasecuk
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« Reply #3 on: June 27, 2021, 06:13:27 AM »

In metric units Cl=M*9.81/(0.5*p*V2*S)
If we put in your measurements we get Cl=12/V2 where V is m/s.
The only way to get a good feel for speed is to take it to a large indoor hall and hand launch it. Get somebody else to measure the time over a set distance starting a short way from you. Do this about 15 to 20 times. Discard the obvious anomalies and average the rest. This will give you a fairly good approximation to what you want.
Ron
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flydean1
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« Reply #4 on: June 27, 2021, 09:59:16 PM »

Just an idle question Panther; how many Wakefield (F!B) models have you built?
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PantherM100
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« Reply #5 on: June 28, 2021, 09:39:55 PM »

Well Flydean:
I have been building and flying
Model Aircraft since I was 6 years
Old, I am now 73 years young.  I started
With the old Top Flite Jig Time rubber powered
Free flight models that were designed by Carl Goldberg.
I then graduated to many free flight glow powered
Free flights powered with the great little Cox Pee Wee
.020, like the Ranger 32 airplane.  I managed to loose
A number via OOS flights.  My modeling then evolved
To R/C. First single channel escarpments and pulse
Systems - Then onto multi channel (reed).  At that
Time in my life, I entered college with the intent on
Becoming an airline pilot.  But that was not to
Be as color blindness issue was uncovered by
The Doctor performing my Commercial physical
Exam.  My desires then led me to motorcycles
and girls. Anyway, I established a small precision
machine shop, and proceeded to build something from
Nothing.  It provided a comfortable living for 20 years.
While attending to my precion shop. We, my father and I
Embarked on the design and manufacture of a model
Aircraft motor intended for the then very popular
quarter scale size models.  The motor was very well
designed and performed well enough to place 2nd at the
Nationals.  The motor was called the Bantam 2.6.  The motor
Project would have required large capital outlays and banks were
Not interested in furthering American manufacturing.
After the closing of my business, I embarked on a second
Career in Quality Assurance positions for Aerospace component
Manufactures.  Through out my working life,
I tried to make time for building and flying model airplanes.  About 12
Years ago I got the bug for building and flying indoor duration models.
Mostly Ez b’s and mini stick class. 
My best time was about 18 minutes at Lakehurst hangar No. 1.
After that time I had various health issues, and to this
Day I’m working through the after effects of COVID and
A 4th heart attack. 
Coming to a conclusion and answer to your question: No, I have not yet built
An “F1B Wakefield” I thought the subject matter(s) discussed here cover mainly P30’s,
however I do have a “Tilka” vintage Wakefield kit that I want
To start later this Summer.  At the moment I am engrossed in P30 class free flight
Rubber aircraft.  I’m not sure what inspired you to ask the question that you did.  I have a
philosophy of measure twice (or maybe thrice) and cut once.  I have a Mini Twin Fin that I have
Started recently.  The plans are barely readible, so I decided to CAD draw a set.  From that point
An Aeronautical Engineer tipped me off to some changes that could improve the performance
Of the Mini Twin Fin.  I’m hoping this post answers your question.
Happy 4th and God bless America! 
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flydean1
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« Reply #6 on: June 28, 2021, 10:17:54 PM »

Just posed the question as what you were delving into is way beyond what someone who is building a Mind Twin-Fin would do.  Me, when people start using equations, my eyes roll back in my head.  The beauty of classes like P30 is it leaves room for everyone.

Look forward to your progress.  You are clearly in your element.

Regards 
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piecost
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« Reply #7 on: June 29, 2021, 04:42:15 AM »

Panther,

I enjoyed reading your CV. One of the good things about the hobby is that one can go the full scientific or artistic approach.

Best wishes with your recovery to full health.
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OZPAF
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« Reply #8 on: June 29, 2021, 05:27:09 AM »

It's always inspirational to see some one follow their interests in life despite the set backs. I also enjoyed reading your CV - continue to enjoy yourself and the best of luck with your health.

I also enjoy applying a theoretical approach to my modelling as I originally wished to be a Aeronautical Engineer and I believe it just adds another dimension to the enjoyment. Always each to his own.

John
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BG
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« Reply #9 on: June 29, 2021, 02:24:35 PM »

Hi All,
While I am all for improving, tweaking and developing FF models in any way you can imagine I think it is worth noting that some classes have rules and restrictions that make spending hours worrying about the details of airfoils pretty much a waste of time (all-be-it an enjoyable and amusing waste of time). P30 is such a class. Because ALL p30s have a 9.5 inch drag plate up front, and an aspect ratio restriction there is little point in sweating the details of the airfoil. Further and potentially more important, the way p30 contests are flown ensures that P30 is a true air picking event. You fly mostly during the day when thermals abound and this includes the fly off flights. So the few percentage points of improvement you might be able to eek out with an exhaustive airfoil analysis will be lost in the noise of thermals and the related turbulence. So given limited time for playing, and an obsession with the p30 class, where should one spend their time?

Answer: Focus, on consistently finding thermals, a trim that stays in thermals, and on rubber that will get you higher. This means 1. working with a thermistor or just getting really good a picking air using a streamer and fluffies, 2. getting a pair of sturdy, to weight, reliable models trimmed for the best climb and glide that can be achieved with a 9.5" plastic prop, and 3. spending time getting your rubber tested, learning to wind to max torque, doing motor break-in,  and using one motor per flight to get the absolute max energy from each motor. 4. Thinking and practicing strategy for different flying conditions at p30 contests. 

Fiddling around with airfoils is fun and feels very sophisticated and significant but even in F1B all of our work with sophisticated wind tunnel tests and computational analysis to find the best possible airfoils has yielded only marginal improvements in a class with unlimited aspect ratio and folding props where the glide is minimally compromised, and where final flyoffs happen at 7 am in truly dead air. So, in the most advanced and sophisticated FF rubber class where the airfoil actually matters and where we have put countless hours of effort with world class aerodynamicists into identifying the best possible airfoil geometry we have only improved the sink rate from about 0.3 m/sec to about 0.28 m/sec.

I think you can probably see that your efforts to "improve" on Bob's airfoil are unlikely to yield measurable improvements given what we have seen in F1B. This is not to say your shouldn't try, if you have the time and you are keen, the i wish you many enjoyable hours of fiddling
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dosco
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« Reply #10 on: June 30, 2021, 12:36:50 PM »


Coefficient of lift ... ...

... ... is Monty Python about?

Cheesy
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piecost
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« Reply #11 on: June 30, 2021, 12:54:48 PM »

Thanks for the explanation BG; it makes me feel better about not analysing the aerofoils on my competition models. I fly indoor duration and still see variations due to weather, winding technique etc. Durations are a lot less deterministic than I first thought
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calgoddard
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« Reply #12 on: June 30, 2021, 02:48:52 PM »

I agree with BG.  To summarize, concentrate on building sturdy but light, optimizing trim, using good rubber, winding hard to torque, swapping out rubber motors after each official flight, and trying to pick good air. You can win a P-30 contest, even against top notch competitors, with any reasonable design, as recently proven with my Three Nite P-30.  See Reply #80 at https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=25280.75

Bob White was a very formidable competitor in both coupe and P-30.  Any improvement on his P-30 design would be infinitesimal in terms of performance.
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Re: Coefficient of Lift help for a Mini Twin Fin
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USch
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« Reply #13 on: June 30, 2021, 02:54:37 PM »

Just my 2-pence worth about this argument.

It correspond perfectly to my approach to a new class, to build somebody else’s model, especially if it is from a world-renowned modeller like Bob White. That gives you the opportunity to appreciate and learn the tricks from an experts, at least if you reproduce the original in all his details.

As a friend of Bob and Tony White, I know how much time and knowledge Bob put into his models. He spend perhaps more time in trimming than on building and the results where spectacular at the least. So to my believe there is little to improve on his models.

Build any model and perfect your trimming and thermal finding skills. You will see that the results will be much more appreciable than modifying an already near perfect, but not easy to build, model.

Urs
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