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  #11  
Old 09-08-2020, 08:36 AM
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Quote:
Originally Posted by Ez2cDave
Guppy Youngren, in the April, 1980 edition of the M.I.T. Rocketry Society Journal, addresses the Neutral Point.

Images below . . .

Dave F.

Yup, that's what I've been using to try to arrive at a Neutral Point (and CG) for my glider.
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  #12  
Old 09-08-2020, 10:49 AM
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Quote:
Originally Posted by Rktman
Yup, that's what I've been using to try to arrive at a Neutral Point (and CG) for my glider.


Eric,

I just posted that info up on YORF, so we could get more "eyes" on the situation.

Dave F.
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  #13  
Old 09-08-2020, 11:13 AM
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Quote:
Originally Posted by Ez2cDave
Eric,

I just posted that info up on YORF, so we could get more "eyes" on the situation.

Dave F.

Thanks!
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  #14  
Old 09-08-2020, 08:36 PM
olDave olDave is offline
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Default How much aero theory are you familiar with?

Quote:
Originally Posted by Rktman
.....[snip]..... I may just abandon my shape and use a truly elliptical planform, or give in and use a plain rectangular planform with square tip chords, which all the online calculators appear to support.


For model rocket boost-gliders/rocket-gliders with fixed wings, AR (wing aspect ratio) can usually be chosen at values upwards of 6 to 8 depending on the strength of the wing materials. AR values of 10 or 12 or higher begin to push the bounds of where aerodynamic flutter will damage the wings.

The value of AR is found through dividing the square of span by total wing planform area:
AR = (b2) / S

Model rocket boost-glide or rocket-glide designs with pivoting wings, folding wings, etc., are a whole different animal.

Taper ratio for subsonic wings (usually represented by the Greek letter lambda, but go ahead and use TR) can range across a variety of values and still get you a wing design that is 95+ percent of the efficiency of a true elliptical spanwise lift distribution, without all that PITA goofy calculation for a complex shape like an elliptical wing planform. TR values of .20 to .30 get your wing efficiency into the 99 percent neighborhood.

The value of TR is found by dividing the wingtip chord by the wing root (projected to centerline) chord:
TR = cT / cR

Using a trapezoidal wing planform (tapered or constant-chord, your choice) also has the huge advantage of making it much much easier to sand a consistent leading edge shape, trailing edge thickness taper, and even the spanwise wing thickness taper.

If you are lazy like me, you will go straight to a trap-wing design.

BTW, I do indeed admire the lines of the Spit, but like I said before, that is a clumsy way to seek to achieve high values of wing efficiency.
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  #15  
Old 09-09-2020, 02:06 PM
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Quote:
Originally Posted by olDave
For model rocket boost-gliders/rocket-gliders with fixed wings, AR (wing aspect ratio) can usually be chosen at values upwards of 6 to 8 depending on the strength of the wing materials. AR values of 10 or 12 or higher begin to push the bounds of where aerodynamic flutter will damage the wings.

The value of AR is found through dividing the square of span by total wing planform area:
AR = (b2) / S

Model rocket boost-glide or rocket-glide designs with pivoting wings, folding wings, etc., are a whole different animal.

Taper ratio for subsonic wings (usually represented by the Greek letter lambda, but go ahead and use TR) can range across a variety of values and still get you a wing design that is 95+ percent of the efficiency of a true elliptical spanwise lift distribution, without all that PITA goofy calculation for a complex shape like an elliptical wing planform. TR values of .20 to .30 get your wing efficiency into the 99 percent neighborhood.

The value of TR is found by dividing the wingtip chord by the wing root (projected to centerline) chord:
TR = cT / cR

Using a trapezoidal wing planform (tapered or constant-chord, your choice) also has the huge advantage of making it much much easier to sand a consistent leading edge shape, trailing edge thickness taper, and even the spanwise wing thickness taper.

If you are lazy like me, you will go straight to a trap-wing design.

BTW, I do indeed admire the lines of the Spit, but like I said before, that is a clumsy way to seek to achieve high values of wing efficiency.

Thanks for the feedback, sounds like you've acquired a lot of knowledge from background and/or experience, something I'm still working on (I've been building RGs and BGs for about 3 years, most from kits, and have gleaned most of my theory from forums, existing modroc publications, and sources like the Frank Zaic books). It probably would've been more advantageous if I had started out building/scratch-building model aircraft like the hand launched stuff or even RC. (Been meaning to check out those model aircraft forums to see what I could learn).

Anyway, I've scratch-built several BGs and RGs successfully using accepted design "rules", but realize it wasn't in the most efficient way. Far better to design in where your NP and CG will fall vs adding (probably an otherwise unnecessary amount of) nose or tail weight after the fact to balance it to glide right.
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  #16  
Old 09-15-2020, 01:12 PM
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From "Model Aircraft Aerodynamics", 4th Edition, by Martin Simons.

Dave F.
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  #17  
Old 11-03-2020, 03:24 PM
shockwaveriderz shockwaveriderz is offline
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this may help:
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  #18  
Old 11-03-2020, 04:51 PM
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Quote:
Originally Posted by shockwaveriderz
this may help:


Terry,

The problem is the Wing Planform of Eric's glider.


https://www.oldrocketforum.com/atta...achmentid=57000

Dave F.
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  #19  
Old 11-04-2020, 02:05 PM
shockwaveriderz shockwaveriderz is offline
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dave, this was from Larry Renger circa 1963:

Terry
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  #20  
Old 11-05-2020, 02:36 PM
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Last time I calculated the neutral point on a glider? VERY rarely, for some R/C Rocket Boosted Gliders long ago.

Even so, I'e also flown some high performance R/C R/G's designed by others where the tail was too freaking small and/or the tailboom length was not long enough for good pitch trim. At least not the kind of pitch trim I liked for R/C flying. Yet those was computer-calculated to death. My only option to fix that for other models of the same kind was to lengthen the tailboom as the composite tail surfaces were pre-made. If I'd had a viable option to make bigger tails (pitch axis, more span or effective area), I would have. My fix was to lengthen the tailboom to compensate. Added more weight in theory, but the stock tailboom was overkill so I went with a lighter boom (talking tapered carbon booms) and was actually lighter as-built.

But for simple gliders like these....

I use "TLAR", based on similar models known to fly well, and experiment with noseweight.

Any non-R/C glider I've made the last 30 years....I have no idea what the neutral point is for any of them.

If I DID want to know, and had this planform, I would simply approximate it as a trapezoid, and get a close-enough ballpark figure.

My long ago experiences with calculated neutral points and TVC's and so on to TRY to determine where the CG should be...... is that nothing beats a calm day, a grassy field, and plenty of spare noseweight to toss and trim and toss and trim to get the trim right.

And oh yeah the incidence in the stab matters so much to the CG and proper glide trim.

When I have done some really complex model where I needed to know where the CG oughta be BEFORE building it (at least before adding critical components affecting CG).....I made a simple small sized model to find out.

When I did the "Orbital Skydart Project", and needed to know the CG for glide, and CG for a stable boost, I made simple 1/3 scale models one night, using BT-50 for the mother ship and BT-20 to represent the 2X sized R/C SkyDart. If I'd tried to calculate it all out instead, it could have taken longer to do that than to build the test model, and I'd have had little faith in the calculation results. Also though I will admit I needed to build and test fly a 1/3 scale prototype to do a test to see if it could glide together as a combo, if the 2X SkyDart's engine failed to ignite as that was the only means of separation. The combo did glide, and I also did a full size test of the completed model once to prove that too before I did a public demo. The glide trim for the test model mother ship was scaled up for the full sized model, and worked out well. I didn't want to add any extra noseweight, or tailweight, and had very little wiggle room to shift R/C components to tweak the CG. What's the neutral point? Dunno... but I did know exactly where the CG needed to be for glide.


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Last edited by georgegassaway : 11-07-2020 at 12:55 PM.
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