I have an Estes Ascender 2" I have flown several times using a Altimeter Two to record data. The OpenRocket predictions of altitude are high by 10% or so, and the ground hit speed is higher than the Altimeter Two data by about 33%.
Is this typical of OpenRocket?
Dan
First, 10% altitude error isn't bad. Your launch-to-launch variability is likely higher than that.
Second, did you weigh the rocket before you launched it? Predicted launch weights are not always accurate.
(If your rocket was a tad overweight you would be short on altitude and high on landing speed. Not saying this happened but it was worth remarking.)
BTW, predicted CG's should be always verified before launch. CG placement can make the difference between a stable and unstable rocket. Everyone worries about CP; they should worry more about CG.
As others have noted, Cd is hard to predict accurately. In particular, the Cd/speed curve before subsonic Cd plateaus is hard to predict. Therefore, flights that spend a large percentage of time in this region are hard to predict.
(Oddly, supersonic Cd's tend to be better predicted.)
Stability corrections, which are largely random, affect Cd as well.
In general (hate to tell you), altitude predictions within 20% are par for the course.
-LarryC