Climb rate vs. speed or weight
Thread Starter
Joined: Apr 2002
Posts: 86
Likes: 0
From: Denham
Climb rate vs. speed or weight
How can I correct (approximately) rate-of-climb numbers for different speeds or weights?
I did such a test on an C182 I'm planning to buy a share in a few days ago but unfortunately both pilots got the test slightly wrong: we used the speed for MTOW with significantly less than that. Since I'm looking only for an approximate number some form of adjustment for either weight or speed will do.
Thanks.
Mak
I did such a test on an C182 I'm planning to buy a share in a few days ago but unfortunately both pilots got the test slightly wrong: we used the speed for MTOW with significantly less than that. Since I'm looking only for an approximate number some form of adjustment for either weight or speed will do.
Thanks.
Mak
Moderator
Joined: Feb 2000
Aviation Qualifications: CPL
Posts: 14,480
Likes: 178
From: UK
Firstly, you've posted this on three different forums - which is firmly against PPrune rules. There are less cuddly moderators than me who would temporarily ban you for that - so I suggest that you either delete two of them, or change them to simple links to the third.
Correcting for weight is technically very straightforward. Standard aerodynamics textbooks will tell you that you can scale inversely with weight, so that
Climb rate at MTOW = (actual climb rate * MTOW / actual weight).
In practice there are too many variables not accounted for by the rather simplistic treatment of most textbooks, for which reason the CAA won't let us use that for more than 5% weight variation in extrapolations. However, for a crude check on "is the climb rate about right", I'd say it's good enough.
The correction for speed however is much more problematic. The speed for best climb will increase with weight - so at the lower weight, the best climb speed will be slightly slower. However, what the actual correction is will depend upon the excess power versus airspeed curve looks like. This is complex, and will depend upon the particular engine / prop / airframe combination - what it is for a 182, I wouldn't dare even guess.
So, if you are looking for a crude check, use the formula shown above. If you want any kind of accuracy, I'd re-do the test at the right weight and speed.
G
Correcting for weight is technically very straightforward. Standard aerodynamics textbooks will tell you that you can scale inversely with weight, so that
Climb rate at MTOW = (actual climb rate * MTOW / actual weight).
In practice there are too many variables not accounted for by the rather simplistic treatment of most textbooks, for which reason the CAA won't let us use that for more than 5% weight variation in extrapolations. However, for a crude check on "is the climb rate about right", I'd say it's good enough.
The correction for speed however is much more problematic. The speed for best climb will increase with weight - so at the lower weight, the best climb speed will be slightly slower. However, what the actual correction is will depend upon the excess power versus airspeed curve looks like. This is complex, and will depend upon the particular engine / prop / airframe combination - what it is for a 182, I wouldn't dare even guess.
So, if you are looking for a crude check, use the formula shown above. If you want any kind of accuracy, I'd re-do the test at the right weight and speed.
G
Joined: May 2002
Posts: 1,210
Likes: 56
From: Melbourne, Australia
Mak,
I'd support Genghis' view that you don't have adequate data to do the analysis properly. You say that you only want an approx number so I wonder about the purpose of the test. If it is a 182 and the engine is working properly then it will perform like a 182. I'd concentrate on whether the airframe has had any major repairs, rigging and the condition of the engine.
FAA AC23-8 Appendix 2, in part 2 has the method for you. If you really want to try your hand at number-crunching, have a look at this:
http://www.avweb.com/news/airman/182418-1.html
I'd support Genghis' view that you don't have adequate data to do the analysis properly. You say that you only want an approx number so I wonder about the purpose of the test. If it is a 182 and the engine is working properly then it will perform like a 182. I'd concentrate on whether the airframe has had any major repairs, rigging and the condition of the engine.
FAA AC23-8 Appendix 2, in part 2 has the method for you. If you really want to try your hand at number-crunching, have a look at this:
http://www.avweb.com/news/airman/182418-1.html
