Quote Originally Posted by Marc Zeitlin View Post
I would be very interested in understanding where you got the #'s for climb rate when at Vx. I looked at two different C-172 manuals:

https://static1.squarespace.com/stat...C172N-+POH.pdf

and:
http://www.lanierflightcenter.com/wp...s/172S-POH.pdf

Both of them have tables for Vy rate of climbs, and both have tables for Vx and Vy at different altitudes, but NEITHER have a table of actual climb rates when flying at Vx. If you don't have this #, you cannot calculate the angle of climb.

Maybe some of the other aircraft have these #'s, but you claimed ALL cases to have Vy steeper than Vx (which others have pointed out is the opposite of the definition of Vx and Vy).

When testing my COZY MKIV in Phase I, I did complete climb tests at airspeeds from 60 KIAS to 140 KIAS and at fwd, center and aft CG, as well as lightweight, mid-weight and MGW. In ALL cases, the maximum climb angle occurred at a speed lower (by about the right amount) than the maximum climb rate speed - IOW, Vx is always below Vy, and Vx's climb angle is always higher than the climb angle at Vy.

So I think that you're confusing something in the POH with the climb rate when at Vx.

Can you point me to a specific POH where they give the climb rates at both Vx and Vy, and where, if you calculate the climb angle at both speeds, you come up with a steeper angle at Vy?


Hi Marc, Very insightful questions,

As I think I noted in the SS none of the manuals I looked at gave the climb rate for Vx, nor did they give the ground roll distance for a Vy TO.
Looking further into why all the manuals had pretty much the same format and types of data (or lack thereof) I found that GAMA has a spec for POHs.
It seems to be the source for not specifying the inclusion of those 2 essential bits of information:

Link to GAMA Specification No. 1 for POHs (+ others):
https://gama.aero/facts-and-statisti...pecifications/

Not having those numbers available from the POHs I derived them as follows:

Vx gives the Vx IAS, ground roll distance and total distance over a 50' obstacle, 4 pieces of info.
1) Subtracting ground roll distance from total distance gives the distance from lift-off to the 50' obstacle, taking the ATAN of 50/that delta distance gives the flight path climb angle.
2) Converting the IAS in mph or Kts to fpm for common units then correcting (reducing) the air speed to Ground speed due to the climb angle.
3) Now knowing the GS from lift-off to the obstacle and the distance from lift-off to the obstacle I ratio distance to speed and get the time (fraction of a minute) from lift-off to the obstacle.
4) Now knowing time from liftoff to the obstacle and that the obstacle is at 50', I divide the fraction of a minute into 50' and get the Vx average rate of climb from Lift-off to 50'.

One set of numbers in the spread sheet (left side) assumes that the Vy Lift-off occurs at the same lift-off point as Vx (clearly not true) - but the point here is to determine which speed gives the greater angle of climb so the answer here is valid.

Having some concern about how much longer the unstated Vy ground roll might be than the Vx ground roll a friend of mine assumed a constant acceleration to Vy, thos numbers are the yellow and blue high lights to the right.

The Vy numbers were then derived similarly to the Vx numbers.

I realize it's a PITA to try and figure out what the thinking is behind someone else's spread sheet formulas, hope these alleviates the pain somewhat.