Adding Strength To Glider Parts

[BigRiJoe]

What is the best covering for boost glider wings, stabilizers and rudders, and what tools and techniques work best in applying it? Does airfoiling the vertical and horizontal stabs improve the boost and/or glide portion of a BG's flight?

[GuyNoir]

What is the best covering for boost glider wings, stabilizers and rudders, and what tools and techniques work best in applying it? Does airfoiling the vertical and horizontal stabs improve the boost and/or glide portion of a BG's flight?

(a) In my opinion, airfoiling the vertical and horizontal stabs helps, though I confess I have nothing substantive to back that up.

(b) IMHO, the best covering for BG wings, etc. is Japanese tissue.

http://www.a2zcorp.us/freedownload/A2ZCorpModelCatalog_PriceList.pdf

or

http://www.darehobby.com/accessories/tissue02.htm

You'll need to put down a couple of coats of thinned dope (50% dope, 50% thinner) onto your balsa, sanding both coats after drying, then put down the tissue. 320 grit sandpaper works well to trim the overhang.

If I can find my notes on how to apply tissue to solid balsa, I'll post them here.

[Pat_B]

With solid balsa you can't beat jap tissue vac-bagged down. It provides some flexibility to keep the balsa from breaking if it gets bent; unlike fiberglass that will provide a great amount of stiffness but will snap when bent too far. Photo shows jap tissue vac-bagged on a rotor.

[BigRiJoe]

Could you describe the vacuum bag technique for applying tissue, Pat? My wife has one of those food saving kitchen gadgets that she uses to "freeze" foods........

[Pat_B]

The process is pretty simple but you do need a way to pull a vacuum. I'd say you need to pull at least 18 inches of mercury. I'd suggest buying a vacuum gage- you can buy a cheap one for cars at Harbor Freight to see how much vacuum you can get out of your food saver device.

One thing that concerns me however is that you need to have the vacuum applied for at least 4 or 5 hours and you wouldn't want to burn your device out. It's rare to build a vac bag setup that doesn't leak somewhere and your unit will be cycling on and off maybe every 5 minutes or so.

I built a unit from an HVAC pump that is rated for continuous operation. You can buy them on Ebay for about $120. ACP also sells a unit that is ready to go but it's quite expensive at about $450.

Once you have your vacuum source then you simply apply your jap tissue over the balsa with a little bit of finishing epoxy applied. Put inside the vac bag and turn on your vac source. Excess epoxy will seep out from under the jap and you can remove your layup from the bag after maybe 20 minutes to wipe off any excess epoxy. There are also dedicated materials such as breather cloths that are designed to soak up extra epoxy, but they aren't really needed with the small amount of epoxy that we work with. The vacuum needs to be applied at least until the epoxy gets to the green stage which varies with temperature but is usually around 4-5 hours. I usually keep my unit on overnight.

If you decide to go this route then I can answer any questions you may have.

[BigRiJoe]

The process is pretty simple but you do need a way to pull a vacuum. I'd say you need to pull at least 18 inches of mercury. I'd suggest buying a vacuum gage- you can buy a cheap one for cars at Harbor Freight to see how much vacuum you can get out of your food saver device.

One thing that concerns me however is that you need to have the vacuum applied for at least 4 or 5 hours and you wouldn't want to burn your device out. It's rare to build a vac bag setup that doesn't leak somewhere and your unit will be cycling on and off maybe every 5 minutes or so.

I built a unit from an HVAC pump that is rated for continuous operation. You can buy them on Ebay for about $120. ACP also sells a unit that is ready to go but it's quite expensive at about $450.

Once you have your vacuum source then you simply apply your jap tissue over the balsa with a little bit of finishing epoxy applied. Put inside the vac bag and turn on your vac source. Excess epoxy will seep out from under the jap and you can remove your layup from the bag after maybe 20 minutes to wipe off any excess epoxy. There are also dedicated materials such as breather cloths that are designed to soak up extra epoxy, but they aren't really needed with the small amount of epoxy that we work with. The vacuum needs to be applied at least until the epoxy gets to the green stage which varies with temperature but is usually around 4-5 hours. I usually keep my unit on overnight.

If you decide to go this route then I can answer any questions you may have.

LOL..I think I'll stick to clear dope!

[Pat_B]

No problem :)

[Clipper]

Are you building from scatch, with foam core wings? Use 1.4oz glass and resin.

[powderburner]

Adding an airfoil to the horiz tail may seem to improve glide performance but it can also greatly complicate the overall trim to keep the glider straight during boost.

There is also the whole Reynolds-number thing, where it is not clear at all that any airfoil anywhere on the glider actually has a better lift coefficient than a flat panel. In case you don't have the background, aerodynamics for "real" aircraft (at high subsonic speeds, or transonic, or supersonic) are quite different because of the high Reynolds numbers. Airfoil characteristics (like the kind of lift, drag, and moment data you see published in reference books) are for this class of Reynolds numbers.

When you slow down (several orders of magnitude) to the speed range of hand-launched toy gliders, and insects, this gets into a whole different field of specialized low-speed aerodynamics. Bumble bees and dragonflies fly just fine with their hairy, lumpy, crisscrossed with blood vessel ridges (not smooth), overall more-or-less flat, definitely not airfoiled wings, pretty much in direct contradiction of "classical" subsonic (human) airfoil theory.

Our boost/rocket glider wings fall into this speed range, at least on the way down. Even during the ascent our rocket's Reynolds numbers do not creep much beyond this speed range. This all suggests that flat plates will work just as well for our purposes as fancy airfoils and delicately shaped aerodynamic surfaces.

Kinda takes the fun out of things....

Definitely makes if way more simple to trim a glider for boost.

[BigRiJoe]

[
There is also the whole Reynolds-number thing, where it is not clear at all that any airfoil anywhere on the glider actually has a better lift coefficient than a flat panel. In case you don't have the background, aerodynamics for "real" aircraft (at high subsonic speeds, or transonic, or supersonic) are quite different because of the high Reynolds numbers. Airfoil characteristics (like the kind of lift, drag, and moment data you see published in reference books) are for this class of Reynolds numbers.

LOL.....now I understand the reasoning behind G. Harry Stine's Flat Cat BG design. If I knew that 40 years ago, I would have saved a lot of frustration trying to sand that perfect airfoil. Surprising this contradicts every set of BG kit instructions I've ever read. FWIW, would this apply to putting an airfoil on regular model rocket fins,too?

[GuyNoir]

[
There is also the whole Reynolds-number thing, where it is not clear at all that any airfoil anywhere on the glider actually has a better lift coefficient than a flat panel.

Before we all quit sanding, you might want to check out:

http://soartech-aero.com/

and

http://www.ae.illinois.edu/m-selig/

There's an awful lot of low speed / low RN airfoil stuff out there that's convinced me to keep putting airfoils on things.

My $0.02; YMMV.

[hardinlw]

Regarding airfoiling surfaces, for many years I flew competetion hand-launch gliders. The surfaces were always airfoiled. At small angle of attack, a flat plate can generate the same lift as an airfoiled surface by flying at a higher angle of attack, but the drag is higher. Whether the surfaces are set at an angle or airfoiled, the impact on boost trim should be the same. One trick is to put an inverted airfoil on the horizontal tail in cases where the tail must create a downward force to achieve trim. The Beech Muskateer does this.