The recent posts regarding increased speed with increased aft loading, (I’ve seen this done in an old laminar flow comanche 250) prompts me to propose the following question to the aeronautical engineers of the board.
If the wing tip extensions of the SR22 were added to the SR20, would the added lift shift the CG aft, resulting in an increase in cruise speed?
(Anticipating a collective reduction of parasitic drag) RE: A decreased angle of attack and elevator drag plus any increase in parasitic drag resulting from the wingtip extensions?
If the wing tip extensions of the SR22 were added to the SR20, would the added lift shift the CG aft, resulting in an increase in cruise speed?
The CG is based solely on weight distribution; the wing tips would add a little bit of weight to the rear of the CG envelope (but close to it) so the CG isn’t going to shift significantly.
The center of lift would change, though I couldn’t tell you in which direction. The added drag would make the plane fly slower. (Keep in mind that part of the reason the Cirri are so fast is that the wing area is much smaller, i.e. higher wing loading, than most small airplanes. With much less wing area the stall speed would end up too high to pass part 23 certification–the forward strakes had to be added to the SR22 to get the stall speed back down to the required 59 knots.)
Try this one.Some of the -2x’s have more paint and filler in the rear than others.That can make a difference on some of them. -j
Thanks Dave,
You’re absolutely right, my error, the center of lift not the CG would change.
However, would you expect the collective increase in lift, (From the added wing tips), to change the angle of attack of the wing in cruise or the amount of downforce required by the elevator?
Richard
If the wing tip extensions of the SR22 were added to the SR20, would the added lift shift the CG aft, resulting in an increase in cruise speed?
The CG is based solely on weight distribution; the wing tips would add a little bit of weight to the rear of the CG envelope (but close to it) so the CG isn’t going to shift significantly.
The center of lift would change, though I couldn’t tell you in which direction. The added drag would make the plane fly slower. (Keep in mind that part of the reason the Cirri are so fast is that the wing area is much smaller, i.e. higher wing loading, than most small airplanes. With much less wing area the stall speed would end up too high to pass part 23 certification–the forward strakes had to be added to the SR22 to get the stall speed back down to the required 59 knots.)
However, would you expect the collective increase in lift, (From the added wing tips), to change the angle of attack of the wing in cruise or the amount of downforce required by the elevator?
I’m way out of my league here, though I’ll go out on a limb and say that adding three feet of wing will change just about everything. I won’t guess at a magnitude or sign though. 
The short version is that the size and shape and position of the wing and stab is an exercise in tradeoffs, and while the SR20 configuration is unlikely to be perfectly optimal, it is at least their second stab (so to speak) at it (probably the third, given two prototypes prior to the certified configuration) and it’s unlikely that it would be easy to improve the performance (at least not without screwing up something else.)
If they wanted to reduce the downforce from the stab, they could simply stick 100 lbs of lead shot in the empennage, but that would have other tradeoffs…(in the SR22, the lead in the empennage is in the form of plates in a bath of sulfuric acid!)
Dave,
IMHO the final design was a comprimise to get the cruise speed to a “Magical” 160 Kts. Though advertised as such, reading the posts on the board seems to indicate they didn"t get there.
Now that that mile marker (no pun intended) is out of the way, would improved slow speed, short field, load carrying operation become a viable option with (optional) wing tips?
PS. My previous plane was one of the last Beech Sierras. It’s reputatation for nasty landing habits and wheel-barrowing was legendary. My particular aircraft removed three separate nose gears (none at my hands) during it’s short life time (Another story some other time).
The addition of Madras wing tips changed the landings into a pleasure.
The short version is that the size and shape and position of the wing and stab is an exercise in tradeoffs, and while the SR20 configuration is unlikely to be perfectly optimal, it is at least their second stab (so to speak) at it (probably the third, given two prototypes prior to the certified configuration) and it’s unlikely that it would be easy to improve the performance (at least not without screwing up something else.)