Vdf/Mdf flight testing

[gus_eng]

Hi,

We are facing some issues with this test in particular. Historically we've been calling "Vdf/Mdf determination" a demonstration maneuver similar to that described in the Part 25.335(b)(1).

The point that arised is: Vdf/Mdf are speeds defined under the provisions of 25.251, not facing extreme buffeting up to Vdf/Mdf, and not the design speeds simulation of 25.335(b)(1), which are design speeds.

In executing in flight the structural simulation of 25.335(b)(1), most of the time we reach speeds that are less than Vd/Md.

So, my first question is this: may we just dive the airplane in a more controlled manner up to the maximum structural speed, or extreme buffeting, whichever occurs first?


Second: @Mad (Flt) Scientist brilliantly discussed this issue back in 2006 in this thread: http://www.pprune.org/tech-log/254943-demonstrated-design-dive-speed-vdf.html (and that is how I got here).

Although I agree with him, the "new" AC 25-7C states in page 149 that:
Section 25.1505 states that the speed margin between VMO/MMO, and VD/MD or
VDF/MDF, as applicable, “may not be less than that determined under § 25.335(b) or found
necessary during the flight tests conducted under § 25.253.” Note that one speed margin must be
established that complies with both § 25.335(b) and § 25.253. Therefore, if the applicant
chooses a VDF/MDF that is less than VD/MD, then VMO/MMO must be reduced by the same amount
(i.e., compared to what it could be if VDF/MDF were equal to VD/MD) in order to provide the
required speed margin to VDF/MDF.
* The bold part is bold in the original text.

Well, if my understand is correct, if we don't have flight evidence that VDF/MDF = VD/MD, than we must reduce the maximum operational speeds VMO/MMO to the difference found between VD/MD - VDF/MDF. Is this right?

i.e. Suppose that my Vc = 360kt and my Vd = 400kt, which were calculated by means of what is described in 25.335. For marketing reasons, I want my Vmo to be 360kt.

Now, my flight test pilot made a dive and, for whichever reason, only accelerated to 390kt. He could have reached 400, but didn't.

According to the provisions of 25.1505, 25.253, 25.335, and what is described in the AC, the difference between Vd and Vdf is 10kt, and so, I must reduce my Vmo to 350kt to respect the speed margins.

What are your thoughts about it?

Thank you for your time!

[Genghis the Engineer]

My experience is that yes - if you reach Vd / Md without anything untoward happening, you declare that as Vdf / Mdf and stop. After all, if Vdf=Vd, then your airworthiness analysts did a good job.

If an aircraft is effectively speed limited and can't accelerate further - which I've seen on some small draggy aeroplanes - then we have set Vdf there. However, I can see that you might not want to do that with a part 25, and I think at that point I'd be having a robust conversation with the authority, but it is only really an issue if Vne or Mne become low enough to create a problem of margin above Vh. However, most Vh margins are relative to Vd, not Vdf.

If a TP simply didn't hit a test point but in theory could have done - then he needs to go and do it again. Irritating within a busy programme, but you may as well just bite the bullet before forced to by the authority.

G

[Mad (Flt) Scientist]

I would disagree that 25.1505 requires one single margin be demonstrated, and i would argue that the interpretation in AC25-7C is not entirely correct. the precise wording of 25.1505 today, which is amendment 25-23 and hasn't changed since 1970, is

The speed margin between VMO/MMO and VD/MD or VDF/MDF may not be less than that determined under Sec. 25.335(b) or found necessary during the flight tests conducted under Sec. 25.253.

i would link the two phrases either side of the two "or" statements to each other only, and read that as:

The speed margin between VMO/MMO and VD/MD may not be less than that determined under Sec. 25.335(b) and VMO/MMO and VDF/MDF may not be less than that found necessary during the flight tests conducted under Sec. 25.253.

That's been the historic interpretation and with the regulation being unchanged, i would argue quite strongly as Ghengis says, that the old interpretation is valid.

The rewrite of AC25-7 was not without flaws, and it has in fact further added confusion to the VDF/MDf "story" by further mixing it up with VD/MD.

[gus_eng]

I do agree with both of you and it doesn't make any sense that, if I pass the speed margin from the upsets of 25.253, then it is not necessary to have a larger speed margin.

But that change in the AC really annoyed me. And I don't see any reason to just add that paragraph out of the blue.

I'll contact FAA regarding this post.

And what about the VDF/MDF dive procedure? Do you understand that it is necessary to fly the procedures of the design speeds simulation? After reading the requirements and the AC I understand that I may dive as shallow and controllable as possible, deploying high drag devices at the speed that I want to reach.

[Mad (Flt) Scientist]

There's no reason whatsoever to fly the VD manoeuvrem ever. It's a theoretical design case and can (and should) be done by simulation/analysis/prediction. (With the data used then validated by various other means)

For the 253 manoeuvres (upsets) you obviously have to fly the upsets as specified. (Though if you know which one or ones are critical, just do those of course)

For the purpose of expanding the flutter envelope you can and should fly the manouvre in a suitably cautious manner. I would say you fly it definitely no more aggresively than the 335 case, because that's the one you designed to. But since all that really matters is achieving the speed, a more shallow dive to the same speed achieves that goal and is safer with regard to pulling out of the dive if a problem is seen.

If you already have the aircraft certified and you're doing a delta, you have two options - if there's no threat to the upset cert, then you can do the "flutter-type" dive to a target speed to re-establish a test point at Vdf/Mdf with your new config. If the upsets need to be re-examined, what has been found acceptable is to repeat the upsets with the new config and take credit for the speed achieved in terms of showing freedom from excessive vibration (251). While you won't hit the exact same speed, you'll be close enough to demonstrate the requirement effectively. (If the original type design max upset speed was 0.88, and you get 0.875 in a "repeat", that's the same speed for all practical purposs, given variability in the test)

These are not low risk manoeuvres, it would be (IMHO) irresponsible for the applicant to conduct these repeatedly in an attempt to perfectly reproduce a previous result, and equally irresponsible for the authorities to insist on the applicant doing so.

[safetypee]

Learned engineers, leaving the unraveling of part 25 to you may I add a word of caution from the pilots union.
Having flown one of those test points once, ...and only once, listen to the test pilot and aircraft; they can talk to you and add to the picture formed by regulations, design documents, and test plans.

As I recall, flying the manoeuvre point was very difficult, particularly to hit the test point by diving and to manoeuvre within height and speed boundaries; there were additional issues with very high manual control forces.
The test point was exceeded by a tiny weeny bit, M.005.
At the debrief the engineers bounced around; aerodynamics and structures with expressions of surprise and relief that everything held together!

[Genghis the Engineer]

At the debrief the engineers bounced around; aerodynamics and structures with expressions of surprise and relief that everything held together!

That's rather concerning given that they should have the best understanding of the safety margins built into Vd, which are considerably greater than .005M. Whilst we should always aim not to eat into safety margins, that is what they're there for.

My experience - admittedly mostly part 23 and smaller, is that we do fly towards Vd in prototype testing. Typically if we get to Vd, we do not pass, and we declare Vdf as co-incident with Vd. Not uncommonly, we hit something of concern (I've seen flutter, canopy distortion, aileron reversal, longstab reversal...) below Vd - at which point we stop and analyse. If the value of Vdf is acceptable for the desired in-service envelope, we have usually stopped at that point, whilst if it isn't, we go looking for solutions.

Of course then, in defining Vne as a margin (usually 10%) below Vdf, we then create a further safety margin. Most regimes will then only go beyond Vne ever again under formal flight test conditions and all in-service flying, including routine air tests don't exceed Vne - that said it's understood that the Vne to Vdf margin is a safety margin, so small Vne exceedences aren't (usually) a cause for concern.

Swap "V" for "M" where required.

G

[gus_eng]

I have the same opinion as G, we should always aim at Vd (and swap V for M where required :P) and should only stop if anything happens in between, i.e. deterrent buffeting, control reversal, etc...

As soon as the authority replies the question regarding the controversial advisory circular understanding, I'll let you know.

Thank you for sharing your knowledge.