Don answers some questions about launching the Spectre 2 Meter.

The only problem I am having is with the launch. Let me say that I've been flying competition level sailplanes for about 7 years now and am considered to be quite good at flight trimming. I've probably tuned 20 sailplanes of my own and maybe 50 more for friends. The Spectra 2M is the first one that I cannot get to go vertical on launch. Balanced per the plans and with the towhook as indicated, it stalls terrible on climbout. Moving the hook forward or rearward still has it stalling. I then moved the CG forward about 1/2 inch and played with hook location until the launch was stable, but it was at a rather low angle. Bit by bit, I reduced the nose weight until it began to stall again...this was still at a relatively low angle.

I tried a bit of flap...stalling. I tried a bit of elevator trim...stalling. Period, I just can't get that thing to launch vertical (really can't exceed maybe 30 degrees?).

From : Don Stackhouse

Well, if you mean 30 degrees form horizontal, that does sound shallower than what I'm used to seeing. However, the Spectre 2-m on long lines does launch a bit shallower at first than what you might be used to. OTOH, you get it all back (plus maybe with some interest) in the zoom if you do it right. Watching Joe fly it in competition, it is competitive with the other ships on launch height, and superior on both range and thermalling ability once it's off the line.

Yes, you will need to play with control, C/G and towhook setups to get the model to rotate properly at the beginning of launch. However...

Let's go back to fundamentals.

If an airplane stalls, it's because it doesn't have enough airspeed and/or area and/or max lift coefficient to make the amount of lift that's being demanded of it.

We have to either reduce the amount of lift being demanded, and/or increase one/some/all of those other three parameters.

At the beginning of launch, when you're trying to go as vertical as possible, the lift of the wing is pulling nearly horizontally against the towline tension, while the combination of airplane weight, airplane drag, towline weight and towline drag are all pulling downwards. Because the turn-around is at ground level while the airplane is up around shoulder level, there's also a small component of the towline tension that's pulling downwards.

All of these downward forces have to be counteracted by the relatively tiny vertical component of the wing's lift. If the flight path is truly vertical, then that upward component of lift is zero. Obviously then, no airplane can launch completely vertically without some help from wind. How close to vertical the airplane can launch depends on how much upward component of lift you need, and how much total lift your wing is making. The easiest of these to improve is the amount of total lift.

First I'd recommend using more flap, maybe about 20 to 30 degrees of deflection. Also, look closely at the upper surface of the wing above the aileron servo. How recent is your wing? Can you make out an oblong carbon doubler over the top of the aileron servo cutout (you can see a faint step around the edge of this doubler in the wing's upper surface) and running about 1" spanwise on both ends of it? We added that shortly after the 2006 Nats. If your wing has that doubler, then it's safe to droop the ailerons as well, probably about 1/2 to 3/4 of the amount you droop the flaps for launch. If you don't have that extra reinforcement, then it's risky to droop the ailerons more than about 5-10 degrees. The extra twisting moment concentrates the loads forward, the extra lift of the drooped ailerons adds to the bending moment, and the combined effect causes a huge increase in the compressive load in the upper spar cap at the front of the servo cutout.

The net effect of these extra flap and aileron deflections is an increase in the max lift coefficient of the wing. By using extra camber, we're increasing the total lift (and therefore the upward component of lift), so we can then support more line drag, airframe drag and airframe weight. A stronger throw to give the model more initial airspeed will also increase lift (remember, lift is proportional to the square of the airspeed, so even a small increase in speed can be very significant) and make a more vertical launch possible.

Yes, this much flap hurts the L/D of the model in normal flight, but while on the towline during launch about 3/4 of the total drag is from the towline. Even if you double the drag of the aircraft, you only increase the total drag of the airplane + towline by about 20%. The increase in lift is proportionately much greater than that, so the overall L/D of the complete system is improved. Of course you must remember to put the flaps and ailerons back to neutral as you begin your acceleration just before the release and the zoom.

Turbulators at about 12-15% chord on the upper surface may help as well, particularly on the outer panels. This could also help your performance and handling in thermal turns. At low Reynolds numbers we've found that it's often possible to get a better Clmax from even a good airfoil than can be done without them, even in comparison to airfoils optimized for no turbulators. The down side is that it increases high speed drag a little, although usually not by a lot. You would be most likely to notice it in the height gain from the zoom on launch. Try it and see if they help. I'd recommend about three layers of 1/16" trim tape about 15% aft of the leading edge on the outer panels only.

Don Stackhouse
DJ Aerotech