VNE vs altitude: glider specs vs rules of thumb

Since some gliders (like mine) don't have tables of VNE vs altitude, I made a plot of relative IAS-VNE vs altitude from seven random gliders that do have tables in their manuals. I got most of it from BGA datasheets.

See the plots at: https://docs.google.com/document/d/1...it?usp=sharing

Some interesting points
1. Most of the seven gliders follow the same curve, but two (Ventus A,B and Discus BT) have constant IAS-VNE to much higher altitudes before they decrease. Is this because they were simply tested to a higher altitude? or were they designed differently?

2. The common rule of thumb I've heard is that IAS-VNE drops by “2 percent per kft after 10 kft�. I plotted this with the data in the first plat, and it's a pretty bad description.

In kft, the correct description is “Constant IAS-VNE until 6.6kft; then drops 1.4% per kft�.

Or an easier-to-calculate rule of thumb is, “Constant IAS-VNE until 7kft; then drops 1.5% per kft�.

This is good to 40,000 ft.

3. In a discussion on RAS in 2002 (https://groups.google.com/forum/#!to...g/iRgr7pc44xg), Ian Strachan wrote about "The normal German (LBA) protocol used for the glider Vne schedule" which I'll rephrase as “Constant IAS-VNE until 2km; then IAS-VNE drops as TAS-VNE is held constant.� Indeed this is the curve that most gliders follow as shown in the 2nd plot. In km the description is “Constant IAS-VNE until 2 km; then drops 4.4%/km�. It's good through 12 km.

Does anyone know the rationale for the details in the protocol? Is 2000 ft just a convenient flutter testing altitude, so most gliders are tested there and no higher?