CD II

For lack of a better moniker, let us start with CD II For reasons oft repeated, maybe it's about time to consider a revised aircraft. I'd like to start with some ground rules of what is necessary (Mandatory), and what are nice-to-haves (optional).

Mandatory

155 Lb maximum weight, otherwise, you lose the advantage of having an ultra-light. To me, I'm not really interested in a non-ultra-light, since I can buy one of those. Also, there is a new kit that should be out before too long, an HP-24, which sounds pretty cool for a `regular' sailplane. The point is the CD does some things that not many other sailplanes can do, primarily taking advantage of micro-lift – I wouldn't want to lose that in a redesign.

Pilot weight 220 Lb – probably the biggest beef against the CD is its pilot weight capacity. If it is robustly designed around the 220 Lb 6'2" pilot, then that envelope could be pushed a little and individual accommodations designed into the basic aircraft

Provisions for a ballistic chute; I am not going to fly without one, and I think there is some bonus weight that can be added to the structure. I think you get 25 Lb for the chute and the chute actually is closer to 18 Lb. 7 Lb could buy you an additional inboard rib and some more D-tube (i.e. bigger wing). I like the airbrake/ballistic chute concept just aft of the spar. So, gross weight is up to 180 Lb still meeting the FAR 103 ultra-light category.

Better weight distribution (i.e. not too nose heavy) – the CG should be forward of the lift surface for stability, but too far forward and there is too much loss as well as a reduction in pitch authority range.

Nice-to have

Low wing loading – I don't know whether it needs to be as low as the original CD/pilot weight, but I think that wouldn't hurt. From what Dan says it doesn't need to be, so maybe it could use the same wing? Or, the option would be to bump up the area to compensate for the heavier pilot. Though, as suggested, that might make hitting the target weight more challenging.

Now, AFAIK no one is foot-launching at 155 Lb, and there is sure to be no one launching at 180 Lb, so I'm for eliminating the whole concept from the plans. I just can't see it provides any useful function and it just adds weight. Car, Bungee, tow, OK

Better visibility...

Design and configuration considerations

With a really light-weight airframe, I think there has to be serious consideration for the wing to be swept forward (maybe leading edge is normal to X axis?) or the pilot more upright or possibly a little of both (for weight balance issues). I think there is less performance impact to the forward-swept wing, assuming it allows the pilot to be more reclined. I think the pilot has to be pretty much right up against the spar. If the wing were swept forward, there might be some change of getting the pilot forward just a bit for better visibility. If the fabric is biased on the 45 it should be very torsionally rigid. I've been contemplating some small internal skin stringers as well as a way to stiffen the skins for stability without adding too much weight. I will have to make a FE model of that to verify.

Additionally, the tail boom should be longer to give more authority and help with the aft CG issue; if you do need some ballast you will need less of it to balance the aircraft.

With a 155/180 Lb w chute weight target, it would seem that the current wing concept of a D-tube with a covering would be retained with an emphasis on weight-saving (ribs, spar). Also, I think that a solid laminate fuselage is simply too heavy, as is fully molded wing skins (ala Axel). The Fuselage needs to be some sort of truss structure with a skin, or it will likely bust the 155 Lb limit. Silly really as it's an arbitrary number and for another 15-20 Lbs you could add some safety. It's the Government – what are you going to do?

The canopy would be single curvature for the home-builder – maybe something along the lines of the Robin that I posted. I think Mark Calder did a good job with that. It the singly-curving plastic is combined with a composite canopy cover, I think you could get something pretty aerodynamic without resorting to having the expense of a blown canopy. Also, remember that the CD forte is slow-speed, so there is less of a penalty associated with a less-than-perfect pod. I think as long as it's clean, it should be good to go.

I think there is also not much change of making a constant section wing and meeting the weight targets unless you are willing to sacrifice significant performance (i.e. trading some external bracing etc. for weight, which will increase drag).

Maybe the wing could be the same size, but some winglets added to bump the efficiency up a bit?

Anyway, just some thoughts