new glider altitude record over 50,000'
 

Two-pilot glider team sets altitude record - BBC News

A big well done to Morgan and Jim for running this project for so many years and now enjoying well deserved results. Their aim is to reach 90,000'. I hear this glider handles and flies better once its above 40,000', this is where most normal gliders exceed VNE and stall simultaneously .


bb

They don't "exceed VNE and stall simultaneously" - the 'stall speed' in straight & level unaccelerated flight and VNE are both indicated speeds. At those sorts of altitudes the glider is in 'coffin corner' - the region of flight where a fixed-wing aircraft's stall speed is also near the critical Mach number.

Current UK Absolute Height Record for a glider was set in 1995 and is 37,959' (11570 metres) in a DG 500 glider.

The world record (ratified so pre-dates the current claim) from 2006 is 50,721' (15,460 m) in a DG505 glider.

My glider's VNE (IAS) decreases from 151 kts at surface to 97 kts at 32,808' (10,000 m). I G stall occurs at around 37 kts IAS. Figures for greater altitudes not provided.

Thud 105 glider VNE is often limited by wing or control surface flutter, which is a true airspeed limit if i remember my basic bearodynamics. Some gliders can increase their VNE by mass balancing some control surfaces and is done on gliders that are likely to cruise at high speed in wave. I'm not sure when a critical mach number might become relevant, but these guys have taken all those aerodynamic challenges and designed a glider capable of 90, 000' a truly amazing achievement compounded by finding the suitable meteorology and the pilot skill to bring it all together. I understand the really high flight will involve soaring making use of the polar vortex, way beyond this bears limited understanding

bb

For most light aircraft the limiting speeds are all indicated, because the limiting factor is dynamic pressure (Q to aerodynamacists). However, flutter isn’t. It’s a factor of TAS, and as TAS varies widely with altitude then if you do go high flutter needs to be factored in. And flutter is not to be trifiled with, for as the great aerodynamacist Theodore von Karman once observed “some fear flutter because they do not understand it – and some fear it because they do.”
As Jim59 explains, his glider's VNE reduces at altitude to compensate for flutter, as will Perlan's. However, to suggest some gliders will simultaneously stall and exceed VNE at altitude is clearly nonsense. The reason why an indicated stall speed at altitude for Jim's glider is not given is simply because irrespective of whether it is at 500ft, 5,000ft or 50,000ft it will still stall at 1g at 37kt IAS.

Based on a couple of calculators to convert TAS to CAS I think my glider reaches 'coffin corner' somewhere between 55,000' and 60,000'. I don't plan to verify that since I don't have a pressure suit.

Don't blame ya Jim! I wonder what the TAS of 37kt IAS is at 90,000ft? I would think Mach 1 is probably less than 100kt TAS at 90,000ft too, so you can see how the stall speed and critical mach number do converge. I guess a wing like Perlan's would have a pretty low limiting mach number.

A great many gliders have mass-balanced control surfaces, possibly all GRP designs and quite a few earlier ones. I know my glider which is a 35yo design does.

Perlan 2 has exciters to induce flutter in flight! That’s brave. :eek:

Details here:
Flutter Excitation | Perlan Project

My electronic E6B says 90 KIAS is 512 KTAS and M.945. 37 KIAS is 245 KTAS and M.42. Amazed that it worked at F900

GF

Something done on all aircraft, the 747 had issues prior to remedial action. A F-117 was lost after a loose elevon led to flutter strong enough to cause a structural failure. Not unusual.

https://www.wired.com/images_blogs/a...1996104074.pdf

The following is a NASA test flight made by astronaut Fred Haise. Of the flight Fred commented, “I’m fearless, but that scares me.” Test pilots earn their money, as you say IFT "thats brave"..

Here's a video of a flutter test-flight that is fairly well known in the gliding community.


The aircraft is a DG-300/17, a specially-built research glider. The wingspan was increased from the 15m of a production DG-300 to 17m, by adding new wing panels at the the wing root and the water ballast tanks were increased in size.

The flutter tendency with an increased amount of water was predicted, based on flutter calculations and static flutter tests.

The video shows the flutter occurring at airspeeds between 140 and 150 km/hr (75-81 kts)

More information here on the DG website:
https://www.dg-flugzeugbau.de/en/lib...of-the-dg-1000

PS megan, thanks for the link to the NASA paper. Exactly what I was searching for.

Anyway, back on topic. Congratulations to all at 'Perlan.' A truly inspirational project. Looking forward to the record being raised (and ratified) further.:ok:
https://www.facebook.com/bbc5live/vi...5768246446108/