Mach Limit

[donv]

The 840/980 (and I think 690 series) mach limit is 0.52. Are there any insights into how this limit was set?

The 1000 has a higher mach limit-- I think 0.60-- although it is hard to know if that is just the result of flight testing, or if there were aerodynamic tweaks made as well.

I became curious about this after a discussion about flying the 840/980 right on the barber pole-- which represents the mach limit.

[donv]

Incidentally, I know that sometimes there are some weird considerations which apply to mach limits. For example, the Citation X is 0.92, because in flight testing you have to go 0.7 beyond the proposed mach limit, and they didn't want to take the airplane supersonic. So 0.99 was as fast as they were willing to go in flight testing.

I also know they had some issues at high speed during flight testing of the 690 series, although this doesn't look mach-related.

https://aviation-safety.net/wikibase/129350

Incidentally, they also crashed the 690 prototype doing VMCA demonstrations with an aft CG.

[Bruce Byerly]

So the empennage separated and no fatalities.  I want to hear that story!

Maybe Jerry White or someone remembers someone who remembers the flight test engineering and reasons.  Bob Hurley or Getz? I can’t imagine anything aerodynamic that is significantly different between them and it’s the same wing and tail on 840-1000 as far as I know though there are some changes between the serial numbers that make the some of the parts incompatible between them.

It’s a great question Don. 

[JimC]

I've flown one plane (an experimental) where the Vne was set by windshield strength.

[donv]

If I remember correctly from almost 40 years ago, I believe the windshield blowing in was a not uncommon issue on the Darter Commander we used to have!

[Bruce Byerly]

I should revise my earlier statement: there are beef ups on later model tails.  They were installed for a reason.  I have been made aware of a proposed 218 revision and Commander owners need to be prepared to support the service centers trying to get the issue under control and corrected reasonably.

[donv]

Bruce, can you expand on that at all? An inspection, or a mod?

[TXGold]

SB218 EMPENNAGE - VERTICAL STABILIZER........................19-May-94..........ALL S/N, MODEL 685, 690, 690A,
INSPECTION & MODIFICATION 690B, 690C, 690D, 695, 695A REVISION NO. 1 (07/11/94)
REVISION NO. 2 (09/23/94)

[donv]

I just finished my recurrent yesterday, and we were looking at the doubler required by 218. It sounds like there is some concern that the doubler just shifted the stress to a different location.

There is apparently also some concern about cracking on the vertical stabilizer spar as well.

[Bruce Byerly]

It’s an entire re-do of 218 with no cracking allowed plus reinspection intervals.