VNE vs altitude: glider specs vs rules of thumb

On Monday, June 8, 2020 at 7:17:13 AM UTC-7, Bret Hess wrote:

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?

My understanding on this topic is that for gliders in particular flutter is a significant concern at higher speeds. Flutter is an aeroelastic effect. It is partly a function of IAS (the relationship between lift, Cl and alpha) and partly a TAS effect (the time it takes a parcel of air to traverse the chord of the wing - which affects the frequency response of the aerodynamic parts). It's also a function of the structural rigidity of the wing, hinge moments of control surfaces, Cl and Cm vs alpha, depending on the type of flutter. Which is to say it's complex to calculate and somewhat dangerous to test.

TAS vs IAS does increase at about 2% per 1,000 ft so the totally conservative thing to do would be to tie Vne to TAS, but this leaves you with less room between stall and Vne at higher altitudes, so they sort of split the difference, while remaining conservative (the method is likely more sophisticated that splitting the difference literally). I suspect the reason they hold Vne constant up to a certain altitude is that it's easier for pilots to have a single number for the most common operating altitudes and leave the table lookup to less common operating scenarios. My best guess is that holding IAS-Vne constant with altitude means you have more flutter margin at lower altitudes and that margin falls to whatever limit they calculate/test for at the top of the constant speed band.

The other thing about high speeds at lower altitudes is that the relationship between control inputs and G-loading goes up with IAS, which is why you have a maneuvering speed (and similar logic for the yellow arc) which is calculated off of the aircraft's V-N diagram.

Andy Blackburn
9B

On Monday, June 8, 2020 at 1:27:57 PM UTC-6, Andy Blackburn wrote:

TAS vs IAS does increase at about 2% per 1,000 ft

Although looking at the data of the protocol and the glider specs, it's really closer to 1.4% per 1000 ft. Certainly 2% is easier to use and 1% is too little. The "10Kft, 2% rule" kind of splits the error. V is too high at 10K and too low at 40Kft. I have to say while it's not accurate enough for the table I will make for my glider, it's a good rule of thumb for on-the-fly estimates.

On Monday, June 8, 2020 at 2:41:56 PM UTC-7, Bret Hess wrote:
On Monday, June 8, 2020 at 1:27:57 PM UTC-6, Andy Blackburn wrote:

TAS vs IAS does increase at about 2% per 1,000 ft

Although looking at the data of the protocol and the glider specs, it's really closer to 1.4% per 1000 ft. Certainly 2% is easier to use and 1% is too little. The "10Kft, 2% rule" kind of splits the error. V is too high at 10K and too low at 40Kft. I have to say while it's not accurate enough for the table I will make for my glider, it's a good rule of thumb for on-the-fly estimates.

The difference between 1.4% and 2.0% is a dangerous place to play...

The glider specs use 1.4%, so it's neither play nor dangerous.