Philipp,
I thought about a lot of what has been brought up in the posts below way too much when I was flying gliders, so I can reiterate a couple of things. First, a more rearward CG will make your plane faster. Most competition gliders have small water tanks in the tail for exactly this reason. We always try to keep the CG on the aft limit for maximum glide performance. A rearward CG will also make your plane more difficult to recover from a spin. Gliding performance was improved for the reason Gordon reported below, less drag from the elevator having to provide less downforce to counteract the heavier nose.
Second, bugs, dirt, dust or rain drops are all enemies of laminar flow airfoils. Your entire plane must be absolutely clean, not just the leading edges of the wings. To give you an idea of how much this matters, it was well documented by the use of drag probes on gliders that glide performance can drop by as much as 15% with significant bug accumalation. Some competition gliders actually have small devices the pilot can deploy from the wing root area that scrape the bugs off the leading edges while in flight.
All that being said, 8-10 knots does seem like a lot to be coming up short and I don’t think you are going to find it moving the CG around or wiping the wings.
You might take a look at both ailerons and flaps when nuetral to be sure they are dead even, check the gear fairings for security, etc.
If you don’t see anything suspicious, I would start recording some accurate numbers in flight and talk to CD. They will probably be able to home in on the problem.
FWIW,
Greg
I would be intertested if other SR20 customers are happy with the cruise speed of their aircraft, because our SR20 (C version, 3-blade-prop) really cannot keep up with the book specs.
At standard temperature and 2500 lbs weight, it normally lacks about 5-6 knots and if temps are high and weight are high it even lacks 8-12 knots!
Therefore, we sometimes don’t even reach the 150-knot mark.
Because engine gauges might be inaccurate, I think a good benchmark is to climb to 8000 feet pressure altitude, feed in full power and lean to 75° rich of peak (normally about 1375° EGT on our aircraft). This gives us 155 KTAS in winter and 153 KTAS in summer.
Would like to know your exact observations. Thanks.