Performance glider scratch-build: delta wing or traditional/swept wing planform?

[Rktman]

It was excerpted from that report and appeared in Tim Van Milligan's (Apogee Components) book Model Rocket Design and Construction.

 

[Ez2cDave]

This PDF file, by Geoff Landis, is interesting . . .

Dave F.

 

[Ez2cDave]

It was excerpted from that report and appeared in Tim Van Milligan's (Apogee Components) book Model Rocket Design and Construction.

Eric,

I have his book . . . I'll look it up.

Dave F.

 

[Rktman]

This PDF file, by Geoff Landis, is interesting . . .

Dave F.

Thanks for that Landis article. It seems to support my suspicion that the figures in the last row of that chart are for gliders using a "Lifting Tail". Doesn't say so anywhere on that chart or in the text, but it's the only thing that would make sense.

 

[Rktman]

CG location has been revised; it'll fall somewhere between 25% to 40% of the wing chord back from the LE. Will finalize after I get an estimate of where the neutral point falls using one of the online calculators.

Not sure where Tim Van Milligan (Apogee Components) got the CG estimates from in the chart that appears in his book "Model Rocket Design and Construction". Neither does he (I emailed him regarding the chart and he said he couldn't recollect the source). Seems part of it was excerpted from Dr. Gregorek's "Glider Design Rules", but not the last part of Tim's chart that I got the CG location from. Obviously I won't be using it as it appears to be inaccurate, at least for "standard" BGs and RGs as they're designed today (unless, as I speculated earlier, it might work with gliders designed to use a "lifting stabilizer"). Not sure, I'm still working my way up that learning curve.

 

[Ez2cDave]

Eric,

It sounds to me like he "cited it from memory", without re-verifying the data.

Dave F.

 

[Rktman]

Eric,

It sounds to me like he "cited it from memory", without re-verifying the data.

Dave F.

Not sure how those CG figures were calculated, but given the benefit of doubt, it must've been derived from some formula on glider design and stability, since they're so exacting...except maybe the formula(s) were misunderstood and/or the math was off?

 

[TheAviator]

Here's what my calculations say for your neutral point and my recommendation for a suitable CG point. The exact trim location will depend on your stabilizer incidence, don't be afraid to bend the trailing edge of the elevator a little to get the glide you want.

 

[Rktman]

Here's what my calculations say for your neutral point and my recommendation for a suitable CG point. The exact trim location will depend on your stabilizer incidence, don't be afraid to bend the trailing edge of the elevator a little to get the glide you want.

View attachment 428093

Cool! Fairly close to 40% of the chord aft of the leading edge.

 

[Rktman]

Here's what my calculations say for your neutral point and my recommendation for a suitable CG point. The exact trim location will depend on your stabilizer incidence, don't be afraid to bend the trailing edge of the elevator a little to get the glide you want.

View attachment 428093

Hoping the answer is a simple one, but how were you able to so accurately determine the NP and CG? The online calculators drive me crazy because they always assume your wingtips are square. Because mine are basically ellipsoid and come to a point, I had to guesstimate a measurement (for the tip chord) using a point halfway along the curve and the very tip. Not very accurate I admit. Did you use Rocksim or some other method?

 

[TheAviator]

Hoping the answer is a simple one, but how were you able to so accurately determine the NP and CG? The online calculators drive me crazy because they always assume your wingtips are square. Because mine are basically ellipsoid and come to a point, I had to guesstimate a measurement (for the tip chord) using a point halfway along the curve and the very tip. Not very accurate I admit. Did you use Rocksim or some other method?

Pretty accurately. I traced your planform in Inkscape and ran the integrations by hand to get the MAC numbers for the wing and tail. I have a spreadsheet I wrote I've been using for years to calculate the NP from that point. CG is just rule of thumb, as long as it is at least 5% ahead of the NP, you'll have a stable airplane. You can then adjust the CG by feel from there.

 

[Rktman]

Pretty accurately. I traced your planform in Inkscape and ran the integrations by hand to get the MAC numbers for the wing and tail. I have a spreadsheet I wrote I've been using for years to calculate the NP from that point. CG is just rule of thumb, as long as it is at least 5% ahead of the NP, you'll have a stable airplane. You can then adjust the CG by feel from there.

What I use isn't accurate (online calculators). Is your spreadsheet in Excel format (manual calculus/math is my Kryptonite) and something you'd be willing to share? Designing around stability first would sure make life a whole lot easier.

 

[TheAviator]

I'm not going to share my spreadsheet, sorry. If you're confused by the online calculators, this will make it worse, not better. However, I will give you some rules of thumb to go by:

Horizontal tail volume should be between 0.4 and 0.7. V_H = (tail area)/(wing area) * (distance from wing to tail average quarter chord)/(wing average chord)
On top of that, a longer moment arm with a smaller tail area (same tail volume) gives you an airplane that is slower to react in pitch. This is part of the reason that real sailplanes have long tails.

Vertical tail area is a bit more varied depending on what the application is. For our free flight gliders, about 1/4 to 1/3 the area of the horizontal is a good starting point.

As for balancing a small FF glider, I tune the trim by feel anyways. I get the CG about where I think it needs to be and then add or remove nose weight and tail incidence as needed.

 

[Rktman]

I'm not going to share my spreadsheet, sorry. If you're confused by the online calculators, this will make it worse, not better. However, I will give you some rules of thumb to go by:

Horizontal tail volume should be between 0.4 and 0.7. V_H = (tail area)/(wing area) * (distance from wing to tail average quarter chord)/(wing average chord)
On top of that, a longer moment arm with a smaller tail area (same tail volume) gives you an airplane that is slower to react in pitch. This is part of the reason that real sailplanes have long tails.

Vertical tail area is a bit more varied depending on what the application is. For our free flight gliders, about 1/4 to 1/3 the area of the horizontal is a good starting point.

As for balancing a small FF glider, I tune the trim by feel anyways. I get the CG about where I think it needs to be and then add or remove nose weight and tail incidence as needed.

Thanks for the pointers, appreciate them.
The online calculators don't confuse me, they just frustrate me because they all assume squared off wingtips (same for the stab tips). How do you adjust for a curved tip chord like mine? (I'd hate to have to resort to squared off tip chords just to pinpoint NP and CG locations, as my understanding is that an ellipsoid shape, especially one terminating in a sharp point, reduces tip vortex and it's associated drag). Thanks for any clarification you can provide.

 

[TheAviator]

Thanks for the pointers, appreciate them.
The online calculators don't confuse me, they just frustrate me because they all assume squared off wingtips (same for the stab tips). How do you adjust for a curved tip chord like mine? (I'd hate to have to resort to squared off tip chords just to pinpoint NP and CG locations, as my understanding is that an ellipsoid shape, especially one terminating in a sharp point, reduces tip vortex and it's associated drag). Thanks for any clarification you can provide.

Gotcha. Let me scratch something together in a spreadsheet and send it your way. There are calculations to "convert" a generic planform to a unswept, untapered wing for static pitch stability. I ran that by hand for your case and ran it through my regular straight-taper (just like the online calcs) with the converted values.

 

[Rktman]

Gotcha. Let me scratch something together in a spreadsheet and send it your way. There are calculations to "convert" a generic planform to a unswept, untapered wing for static pitch stability. I ran that by hand for your case and ran it through my regular straight-taper (just like the online calcs) with the converted values.

Thanks Brian, really appreciate this!

 

[Rktman]

Been awhile but I’m finally able to start the glider build after some unavoidable delays. Should go fairly fast since it’s a fairly simple and straightforward design that’s already been finalized and there’s not that much to assemble. The only change was substituting a tapered carbon rod in place of balsa for the aft part of the fuselage and mounting the stab with enough overhang to allow tweaking its trailing edge to fine tune trim.

What will probably take the longest is trying to adjust the parts so that the CG falls where it should to avoid having to add much clay to either the fore or aft ends.

"Icarus" Final Glider specs:
13mm

Wingspan: 12.0”
Fuselage: 10.70”
Stab span: 5.0”
Rudder 1.0” x 1.20”
Wings: 3/32” contest balsa, airfoiled
Stab and rudder: 1/32” contest balsa










The piece of carbon rod I sliced off feels like it weighs almost nothing. At 0.57 gram, it’s the lightest piece of anything I’ve ever used in a glider.

 

[Rktman]

Sliced out all the parts and started airfoiling the wing. I masked off the 25% and 50% wing sections and started tapering the trailing edge area first.



To prevent over-thinning the trailing edge I butted it up against a piece of 1/32” ply so that my sanding block can’t cut the edge any thinner.



Once the TE was done I stripped off the forward masking tape strip to expose the leading edge for sanding. Used the same method to prevent the LE from over-thinning.



Wings get a flat bottom Clark Y type airfoil. Best I could manage with the thin 3/32” soft contest balsa. 320 grit cut quickly and made short work of the job, followed by 400 grit for final shaping and smoothing. Stab and rudder received a symmetrical airfoil.

 

[Rktman]

Wing, stab, and rudder edges all get stroked through a line of CA to strengthen and reinforce them. Good way to ward off dings or tears.



Flight surfaces ready for assembly.

 

[Ez2cDave]

Eric,

One thing concerns me . . .

I think that the "slab-sided" forward fuselage may reduce the effectiveness of your rudder.

Dave F.

 

[Rktman]

Attached doublers to the forward fuse to form a channel for the carbon rod boom.



Decided to go with a Xerclod type pylon attachment to save weight. The piece X weighs in at only 0.06g (0.00212 oz.) whereas an Apogee universal hook is 1.2g (0.04oz.).



Aft fuselage rod installed.

 

[Rktman]

Eric,

One thing concerns me . . .

I think that the "slab-sided" forward fuselage may reduce the effectiveness of your rudder.

Dave F.

Thanks, noted.
Build is too far along to change anything at this point. My original design used a more traditional all balsa fuselage with a much taller profile anyway, and by that definition would've had more of an impact on the rudder. But most gliders go that route and are ok so hopefully I will be too.

 

[Ez2cDave]

Thanks, noted.

Build is too far along to change anything at this point. My original design used a more traditional all balsa fuselage with a much taller profile anyway, and by that definition would've had more of an impact on the rudder. But most gliders go that route and are ok so hopefully I will be too.

Eric.

Trimming and Test Flights will "tell all" . . . Hope it goes well !

Dave F.

 

[Rktman]

Eric.

Trimming and Test Flights will "tell all" . . . Hope it goes well !

Dave F.

Thanks! It's all for fun and discovery, to experience what it's like to build for performance instead of durability. Whether stuff works or not, I see it as a valuable learning experience so it's all good.

 

[Rktman]

To dihedral the wings I used a method I saw used on a catapult glider build. Wings are carefully cut partially through with a razor saw first.


The cut is then widened into a narrow “V” channel with fine sandpaper (or with an emery board in this case)…



…then one wing is weighted down while the other side is bent up to the correct total dihedral angle, splitting neatly along the weakened cut area.


Thin CA is then flowed into the cut and everything locked in place with accelerator.

 

[shockie]

https://www.rocketryforum.com/threads/amrocs-wombat-k-5-gallery.35204/
https://www.spacemodeling.org/jimz/amrk-5.htm

 

[Rktman]

This is my first experience working with contest balsa and it’s extremely soft and somewhat wobbly. The 1/32” balsa stab and rudder were especially brittle and fragile and probably susceptible to flutter. Found out the hard way when I snapped the tip of the rudder just trying to pick it up off the workbench. Fortunately it didn’t take long to rebuild it.

So all flight surfaces were tissued today.

 

[Ez2cDave]

Eric,

Great color for high visibility !

Looking forward to flight test results . . .

Dave F.

 

[Rktman]

Finished assembling. Now for some trim tosses as soon as the weather decides to cooperate.



This is the smallest, lightest bird I’ve ever built. It tips the scales at only 10.69 grams (0.38 oz.)

 

[Ez2cDave]

Eric,

Looks great !

QUESTION : What is the Grain Direction on your Xerclod ?

In the pic, it looks parallel to the Fuselage . . . Might be prone to breaking during Boost ?

Dave F.