Yes, the flaps are mandatory when flying with the 24...it will do about 160-165 in cruise, which is just slow enough to require flaps...especially for the jet.
C to D...I'm no expert on D's but I can give first impressions. I've flown Crazy Horse and Lady Jo from the back seat but until now never flew a D from the front. Its different, alot different.
Cockpit:
C, In the cockpit you are much farther aft of the panel and windscreen and much more reclined. It feels like you are sitting lower and the forward sight picture is confined to the peripheral vision. The trim and throttle quadrant is much more user friendly in the C as the hands fall more naturally on the trim wheels and flap handle. The early style "ball" throttle is much more forward and aligned with the left arm than the D's handle bar throttle which juts out from the longeron.
D, In the cockpit you are closer to the panel and wind screen, it feels higher but that is because of the canopy rail structure sitting higher on the top longerons. The big one piece canopy is a blessing and almost makes every other issue moot. The seat feels more "upright" and the visibility is outstanding. The trim wheels and flap handles do not fall naturally under your hand and require a bit of getting used to, but NOT bad. The throttle grip really sticks out and I found myself using the elbow of the throttle rather than the handle to manipulate the manifold pressure.
Taxi:
C, Its blind, but I am so used to it I don't even notice, "S" turning is mandatory as is a closed canopy, too easy to damage with all that prop wash.
D, With the canopy open you can pretty much drive straight, BUT "S" turning is still mandatory, 1000% easier to see what is ahead of you.
Takeoff:
C, The C is pretty blind until the tail comes up, and even then its not that great and you never stop using your peripheral vision.
D, The D is beautiful, as soon as the tail is up its like sitting in a 172. The D got off the ground ALOT faster than the C but this could be because of the weight, I think the D is lighter than the C, but I am sure that is airplane specific. They both handle the same on the takeoff run and directional control feels identical.
Flying:
C, Of course the visibility is restricted with the canopy framing but the handling is identical. I really think the visibility the D affords you really gives you a sense of invincibility. I think the saying "head on a swivel" originated in the C because there is no way to move side to side...just left and right and straight up.
D, Both aircraft feel exactly the same in roll, pitch and yaw. Aerobatics are identical. The forward sight picture in level flight is distinctly different as the D model affords a much greater field of fire with less cowling visible. The B trims level with about half the cowling still visible.
Landing:
C, Forward visibility goes away once you are lined up and starting to flare...back to peripheral, the C also favors a tail low touchdown and is easy to 3 point, although we don't to save stress on the tail structure.
D, Forward visibility isn't an issue and since my first landing was on a 4,000ft strip I crossed the threshold at 110mph tail low...still had plenty of forward visibility. The second landing was on a much longer runway and I took my time, wheeled it on and flew the tail down...really really comfortable...the best part was cranking the canopy open while exiting the runway.


