Still more glider talk

[graylensman]

In the <i>Handbook</i> Stine says that one does not need to airfoil wings on a BG, and that dihedral is more important. Yet nearly every plan I've looked at for traditional gliders specifies airfoiling the wings.

Who's right?

 

[Micromeister]

Like everything in modeling, there is no "right" it depends on what your willing to live with.
If your looking for the Absolute best flight profile, you'll want to airfoil, but lots of current designs will not fly well if you attempt to airfoil the wings. Edmonds Aerospace Deltie is a prime example. it has square cut leading and trailing edges. Try to airfoil this model it will not fly at all.

 

[SecretSquirrel]

Dihedral is important for stability, although some designs have none.

Airfoiling the wings is not always needed, but will often result in longer glide times.

 

[shockwaveriderz]

perhaps what Mr. Stine was implying was that at the small Reynolds numbers that typical 1/4A-B size BG/RG fly at, a thin flat plate is just as efficient as any potential airfoiled cross section.... which is true to a large degree....

recent research in MAV Micro Air Vehicles show that at low Rn, airfoil effciiency deteriorates rapidly ..

"For chord Reynolds numbers between 10,000 and
30,000, the boundary layer is completely laminar
and artificial tripping has not been successful. Experience
with hand-launched glider models indicates
that when the boundary layer separates it does not
reattach."

"The range 30,000 to 70,000 is of great interest
to MAV designers as well as model aircraft
builders. The choice of an airfoil section is very important
in this regime since relatively thick airfoils
(i.e., 6% and above) can have significant hysteresis
effects caused by laminar separation with transition
to turbulent flow. Also below chord Reynolds numbers
of about 50,000, the free shear layer after laminar
separation normally does not transition to turbulent
flow in time to reattach. Near the upper end of
this range, the critical Reynolds number can be decreased
by using boundary layer trips. Thin airfoil
sections (i.e., less than 6% thick) at the upper end
of this regime can exhibit reasonable performance."

I would guess that 1/4A-B size BR/G fly at 10,000-70,000 easily....

 

[illini]

Originally posted by shockwaveriderz

I would guess that 1/4A-B size BR/G fly at 10,000-70,000 easily....

My quick calculations show that you are right about the Re for such models. Airfoils with camber will definitely change incidence angles, hence the reason Edmonds gliders don't do well airfoiled...he's carefully built all the incidence angles into his kits. Airfoiling will also change the pressure distribution...there is an adverse pressure gradient from the max thickness to the trailing edge that will promote separation. At these Reynolds numbers, you could very well be heading for laminar separation...no turbulence transition to hold the boundary layer attached. My guess is that weight matters far more for performance of these models than aerodynamics.

Regarding the question of airfoil vs. dihedral...'tis apples and oranges. Dihedral is for roll stability. Airfoiling affects lift/drag characteristics (and, less directly, stability). I suspect Stine's point was simply that airfoiling offers minimal payoff here, but an unstable glider has no payoff.

 

[Zack Lau]

A technique for improving the lift without adding weight is to warp a thin wood sheet into an airfoil--this does add drag, but this isn't necessarily a disadvantage. A slow moving glider is easier to track and recover than one that darts around quickly.

It seems to me that airfoiling doubles the duration time, assuming that both gliders are trimmed properly.

 

[illini]

Originally posted by Zack Lau
It seems to me that airfoiling doubles the duration time, assuming that both gliders are trimmed properly.

You may very well be right. Ultimately, glide times all come down to L/D, and airfoiling *should* reduce Cd and increase Cl as long as it didn't result in separation.

sink rate = sqrt(2*weight/air_density/wing_area)*Cd/Cl^3/2

Note that increasing Cl has a greater effect (exponent of 1.5) than reducing Cd (exponent of 1). And reducing weight has even less effect (exponent of 0.5). In practice, Cd and Cl can be all over the map for various airfoils due to the difficulty of sanding these in with any precision. However, we can reasonably expect a rounded body to have less drag than a blunt body.

In the end, I think Stine's comment (Gray's original question) still has more to do with stability than performance.

 

[shockwaveriderz]

heres a couple of model rocketry related R&D reports on Low Rn Airfols for gliders and Hd

https://www.members.aol.com/rickhyman /smallairfoils/

 

[illini]

Originally posted by shockwaveriderz
heres a couple of model rocketry related R&D reports on Low Rn Airfols for gliders and Hd

https://www.members.aol.com/rickhyman /smallairfoils/

Nice link! That first article is really interesting...really begs for further research.

One good turn deserves another. Take a look at this: https://www.aae.uiuc.edu/m-selig/uiuc_lsat.html

I think the airfoil data on this site is measured down to Re ~ 40,000. Still may be a bit high for some of our gliders.

 

[graylensman]

Wow- great link on the NARAM papers!

Another aspect of gliders occurred to me... BGs (typically) place the vertical stab in a ventral position, whereas traditional gliders place the vertical stab in a dorsal location. Would this have any effect on glide times?

 

[illini]

Originally posted by graylensman
Another aspect of gliders occurred to me... BGs (typically) place the vertical stab in a ventral position, whereas traditional gliders place the vertical stab in a dorsal location. Would this have any effect on glide times?

No. At least not directly. Given the sink-rate formula, the only difference between ventral or dorsal would come in through impact on Cl or Cd.

 

[shockwaveriderz]

by "vertical stab" are we talking rudder or fin?

If so, the only reason that I know of for having the rudder/fin on the bottom as opposed to the top is that if its in the top position the engine exhaust will burn it .......

 

[illini]

The vertical stab is the thingie that on a controlled aircraft would have a rudder. The horizontal stab is the thingie that would have an elevator.

And, yes, you are correct about the reason for ventral placement of the vertical stab.

 

[powderburner]

Something else to keep in mind, when considering whether to use an airfoil or flat-plate wing shape, is the operation of the combined model rocket in boost phase.

That is, a model that is trimmed to be stable with flat wings may not be able to absorb the new lifting force generated by an airfoiled wing if that lift is not acting through the c.g. of the boosting modroc. If the wing is aft, and an added airfoil shape generates lift, the modroc will pitch over during boost.

One of the advantages of flat-plate wings is that during boost, when the wing is mounted at (or near) zero degrees angle from the thrust line, the wing acts basically as a fin and does not try to 'lift.' After transition to gliding mode, the aerodynamic balance of the tail down-force will cause the wing to hold a small positive angle of attack and generate lift. A flat-plate wing might achieve higher transition altitudes than even a well-trimmed airfoiled wing, and in this way often out-performs in glide times.

Trimming an airfoiled wing is a bit trickier than it looks, and almost qualifies as an art (instead of a science).