


Joined Dec 2007
254 Posts

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Because we're talking oz's not lb's like some of my slopers, there's quite a bit of room for ballast in the FF so you won't lose out by making it light. 





The airframe construction of the FireFly is such that no structural sacrifices were made to get to the weight. The basic construction methods are what Paul has been doing for years. Paul's Mystress and Katana could be built sub 10oz, but the huge wings and larger servos needed for the barndoor flaperons were a serious hold back. The new wing planform and smaller flapperons opened the door, the moving of the joining tennon to be placed under the wing took a lot of weight off the tail; elimination of the wing saddle removed even more weight.
As an airframe gets lighter and leaner, it's structural requirements are also reduced... But Paul has wisely not underbuilt the FireFly. Looking at he layups and materials it's the same airplane he used to make. She will still handle ballasted flying weights up to (and beyond) 10 ounces, but that is well above the performance window. Truth be told, the FireFly could be built even lighter. Sacrifice a layer of pod material, leave the doubler out of the wings, six gram servos for ailerons and a 4 gram for the elevator. Combine those with a 10mm aft shift of the wing mounts and a set of sub 12 gram tails.....she would balance out around 6.5oz! But since the wing seems to like flying loaded around 8 ounces, it's not worth the durability losses to achieve a lighter weight. Of course fiddling will continue and when weight can be reduced without structural losses those changes will no doubt be incorporated...like the change of foam core material that is taking place as we type.... 



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Gerald, I hope you don't mind if I share a couple of my theories regarding your post. These ideas feed each other so bear with me. The plane you chose as an example had a fairly decent sized wing. With the bigger wing it needed more mass to overcome the drag. That is true for launch as well as normal flight. So, the real issue is not total weight but weight in relation to drag and weight in relation to wing area. Every wing will have an optimum wing loading for best performance. The other part of your post has to do with launch height. As an archer I am sure you are familiar with the formula 1/2 * M * V^2. In another thread you commented (yah I read your posts) that a lighter weight radio will allow the thrower to spin faster. It would seem to me that theory would also apply to a plane. The lighter weight plane will bleed off speed faster than the heavier plane but at what rate is highly variable based on the design. I am not saying you are wrong at all. You are right in the case of the T2 but that is a specific plane. As you keep developing better foils, new plan forms are worked out and just great ideas are stumbled into I believe we will find that many of the old school rules no longer apply. On the FireFly I brought many new school ideas together with old school principles. For example I did your foils with a boom mounted wing. 




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that's why you don't see aluminum bats in the show. OK, so what the heck does this have to do with DLG Ken 

Latest blog entry: A new chapter in my RC adventures





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I have a couple of theories why DLG's have gotten heavier over the last couple of years but I will save them for another time. Paul 



United States, CA, Tehachapi
Joined Jun 2011
3,794 Posts

I REALLY like throwing sub8oz planes. I don't get tired after a 2 day contest and my fingers never hurt. And like Paul says, I feel like I can put more energy into it with less effort because of the lighter weight. My Mystic at 8oz launches noticeably higher than my Predator II at 9.8oz.




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The other thing I have really grown to love is dancing a light plane in rough air. It is a lot of fun to thermal in rough air with a plane that just dancing around instead of getting tossed and then it takes a moment to recover because of the inertia. Paul 




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We're good and I agree, I just wanted to make sure you gave yourself some credit for all the design work you put in. 



Joined Dec 2007
254 Posts

I'm sure someone must have posted the mathematical proof for this, but I got intreged as to the puzzle of which launches higher (lighter or heavier DLG) and thought I'd have a stab using my long lost Mech Eng knowlege
Don't flame me as I'm not an expert in equations being dyslexic, and did this kind of stuff a decade ago, but I reckon the answer to "which launches higher a light or heavy DLG" is that in most cases the heavier one will. However this is an 'in general' and there will be an optimal solution for every design and thrower based around the interaction of the following 2 functions: v = f(Sqrt(2/m)) (launch velocity  lighter plane wins and exits faster) a = f(1/2*m) (deacceleration  heavier plane wins and slows down less quickly) However the first equation is a function of a Sqrt so will be a smaller benefit than a function of a 1/2 for a given design....all of course assuming the coefficient of drag for your design is significant  if it's tiny weeny and you have a nice slick DLG, the lighter one will launch higher.....which is kinda what Paul said! Like I said, I'm cr@p at this sort of stuff  so someone please show me the error of my ways (or equations) below.  proof for what it's worth!  What is the exit velocity from any throw? Assuming: The same throwing radius distance (d) is fixed at 2*pi*r where r is approximately your arm length. The force (F) applied through the throw is constant and can be maintained throughout (this does not hold true if you throw faster than your bodies ability to 'follow through') Using the equation of motion: v^2 = u^2 + 2*a*d (where v=velocity, u is initial velocity = 0 and d is displacement = 2pi*r = constant, a = acceleration) This therefore simplifies to: v = Sqrt(2ad) Now using Newton's law of motion: F=m*a (where F = force applied and m = mass, 'a' as before = acceleration) if we solve for 'a' we get: a = F/m Now substituting for a in our original equation we get: v = Sqrt((2Fd)/m) As force (F) applied by the thrower and distance (d) travelled in the throw are both equal for our light plane and heavy plane this simplifies: v = f(Sqrt(2/m)) (where f() is 'some function of') As we're dividing by mass (m) this means the heavy plane is thrown at a slower velocity than the light plane for a given throwing force. How quickly does the plane deaccelerate due to drag? Now the plane has left the hand and is flying but deaccelerating due to drag. Making a few assumptions and assuming the same design for both heavy and light planes: Drag (FD) = (Coeffcient * air density * velocity^2 * area)/2 which simplifies with all the constants to: FD = (v^2)/2 as F=ma, or a = F/m, the deacceleration due to drag for a given velocity is: a = (v^2)/2m Basically this means deacceleration will be faster for 2 different masses in the ratio, where f is some function of and v is assumed equal: a = f(1/2*m) In other words our lighter plane will deaccelerate faster and if launched at the same velocity as the heavy plane will still fail to launch as high.....if the velocities of exiting the throw were equal...but they're not. So which launches higher the lighter or heavier plane? Putting it all together: v = f(Sqrt(2/m)) (launch velocity  lighter plane wins and exits faster) a = f(1/2*m) (deacceleration  heavier plane wins and slows down less quickly) this will give us a set of curves where there is an optimal weight for a given design that gives the highest launch. However, as the throw velocity is a function of a Sqrt(), 'in general' heavier planes will launch higher. 



No flame suit needed as I agree that in general and for most designs the heavier plane will launch higher if the two designs are the same...
We go through this with rocketry competitions. We often have altitude comps with the motor being set and often times the lightest rocket is not the best choice. Aside from the mathematics there is the human factor and for me throwing a light plane is easier on my body and therefore I fly it better as a rule. Check out the boom rolling thread guys !!! Paul 


United States, CA, Tehachapi
Joined Jun 2011
3,794 Posts

Don't forget that the stall speed of a lighter plane is lower, so it can bleed a little more speed before push over. However, in most (if not all) cases a super light DLG won't launch as high as a slightly heavier one. It's been shown experimentally (which, unless your experiments are done improperly, trumps any mathematical model) with several DLGs. For example, Kristoff (maker of XXLite) can build under 200g (7oz), but says he gets the best dead air times at about 250g because of the boost he gets in launch height. The heaviest I've thrown my Mystic is about 230g and it does seem to go a little higher than at 215, but I can't say for sure without an altimeter.




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You lost me at "I'm"..... Ya, the first word of the post: Ken H 

Latest blog entry: A new chapter in my RC adventures





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Me too but I figured I would just keep my mouth shut and hopefully nobody would notice. I seem to really like my FF's between 8.25 and 8.5. The one I am flying right now is 7.8 but it has the CG a little more forward so that makes her a bit easier to fly. I know the general idea is to build as light as possible but why do that if we loose flight performance. I was not overly careful with this build and it still came in below 8oz. the wing seems good s maybe a little more meat in the pod. With a 1pc I would add to the saddle so the weight is right at the wing. Admittedly having to add weight is much nicer than trying to remove it. Paul 



Joined Dec 2007
254 Posts

Lost myself TBH But glad it's born out in practice!
The 2 interesting things that come from it, if the equations are correct are (in general):  Lighter planes will be less forgiving: they should be launched smoothly, without trim changes after release, and taken as near vertically as possible without a trim change (minimising drag and time spent before reaching apex of throw)  Heavier planes will be more forgiving: they should be launched hard, with more emphasis placed on the exit speed from the throw, rather than eliminating small trim changes after release (maximising exit velocity, less importance on obtaining a perfect style) 
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