Demonstrated design dive speed (Vdf)

[geminid]

Hi folks,

I read this in the 'limitations' chapter of the Oxford ATPL POF manual.

".(Vd)..this is the maximum speed which has to be considered when assessing the strength of the aircraft. It is based on the principle of an upset occuring when the aircraft is flying at Vc, resulting in a shallow dive, during which the speed increases, until recovery is effected. If the resulting speed is not suitable due to other high speed effects, a demonstrated speed may be used. This is called Vdf or the flight demonstrated design speed."

Now my questions...

I understand what design dive speed is. Can somebody please enlighten me as to what demonstrated dive speed is, as i find the above rather confusing (particularly the use of the word 'suitable').

Secondly, I've seen the word 'demonstrated' before in relation to (or in lieu of) max crosswind limitations on particular aircraft. Is this something similar to the above?
:confused:

[Mad (Flt) Scientist]

I understand what design dive speed is. Can somebody please enlighten me as to what demonstrated dive speed is, as i find the above rather confusing (particularly the use of the word 'suitable').

It's indeed somewhat confusing, because it's trying to compress a rather complex topic into one paragraph, not entirely successfully.

Firstly, there is no option to substitute another speed for Md/Vd. The relevant structural regulations use Vd/Md, and give no option to elect some other, lower speed. Check the "Flight Maneuver (sic) and Gust Conditions" section of FAR25 - Vd occurs repeatedly as a speed which is to be used in the assessments.

Secondly, Vd is not necessarily defined by the 25.335(b) design manoeuvre - the wording (at least as of Amdt 25-91) is:

Sec. 25.335 Design airspeeds.
The selected design airspeeds are equivalent airspeeds (EAS). Estimated
values of VS0 and VS1 must be conservative.

(b) Design dive speed, VD. VD must be selected so that VC/MC is not greater
than 0.8 VD/MD, or so that the minimum speed margin between VC/MC and VD/MD is the greater of the following values:
(1) From an initial condition of stabilized flight at VC/MC, the airplane
is upset, flown for 20 seconds along a flight path 7.5 deg. below the initial
path, and then pulled up at a load factor of 1.5 g (0.5 g acceleration
increment). The speed increase occurring in this maneuver may be calculated
if reliable or conservative aerodynamic data is used. Power as specified in
Sec. 25.175(b)(1)(iv) is assumed until the pullup is initiated, at which time
power reduction and the use of pilot controlled drag devices may be assumed;
(2) The minimum speed margin must be enough to provide for atmospheric
variations (such as horizontal gusts, and penetration of jet streams and cold
fronts) and for instrument errors and airframe production variations. These
factors may be considered on a probability basis. The margin at altitude
where MC is limited by compressibility effects must not less than 0.07M
unless a lower margin is determined using a rational analysis that includes
the effects of any automatic systems. In any case, the margin may not be
reduced to less than 0.05M.

So 25.335(b)(1) is the case alluded to in the text, but the 0.07M margin is often the determining factor for high Mach aircraft.

The bit about using Vdf in lieu of Vd really shouldn't be there, because Vdf and Vd are, strictly, not related.

Vdf is actually referred to in 25.251, Vibration and Buffetting:
Sec. 25.251 Vibration and buffeting.
(a) The airplane must be demonstrated in flight to be free from any
vibration and buffeting that would prevent continued safe flight in any
likely operating condition.
(b) Each part of the airplane must be demonstrated in flight to be free
from excessive vibration under any appropriate speed and power conditions up to VDF/MDF. The maximum speeds shown must be used in establishing the operating limitations of the airplane in accordance with Sec. 25.1505.

25.1505 is the regulation on setting the maximum operating speed (variously referred to as Vmo/Mmo or Vc/Mc interchangeably within FAR25):
Sec. 25.1505 Maximum operating limit speed.
The maximum operating limit speed (VMO/MMO airspeed or Mach Number,
whichever is critical at a particular altitude) is a speed that may not be
deliberately exceeded in any regime of flight (climb, cruise, or descent),
unless a higher speed is authorized for flight test or pilot training
operations. VMO/MMO must be established so that it is not greater than the
design cruising speed VC and so that it is sufficiently below VD/MD or VDF/
MDF, to make it highly improbable that the latter speeds will be
inadvertently exceeded in operations. 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.

That last sentence is pretty hideously worded, but what it means is that in selecting Vmo/Mmo there are two constraints:

1. The margin between Vmo/Mmo and Vd/Md must be at least that in 25.335(b)
and
2. The margin between Vmo/Mmo and Vdf/Mdf cannot be less than that in 25.253 - which is the High-speed characteristics requirement, and is associated with practical upset manoeuvres - and not, significantly, the theoretical manoeuvre in 25.335(b)(1).

What does that all mean in practice? Well, suppose I'm designing a plane and for marketing reasons I want a nice high cruise speed. So I decide that Mmo needs to be 0.85, and Vmo 350kts.

First step is to find out what other design speeds result. From 25.335(b) I can work out my predicted Vd/Md - I will run some simulations to predict the result of the manoeuvre, and see what i get. Let's say I predict a max speed of 410kts and max Mach of 0.91. Because of the 0.07 Mach margin rule, the latter gets overruled, so now I know that Vd/Md=410kts/0.92M. Now the structures and loads guys can get on with designing the structure.

I still don't know what speed I need to test to, though - there's no absolute requirement to flight test at Vd/Md - its just a design speed. I do know, however, that I need to have enough margin between Vdf/Mdf and Vmo/mmo to allow me to pass the 25.253 tests (otherwise I'll have to reduce my Vmo/Mmo). So I'll take a guess at what margin I need (based on simulation, again) and maybe pick 0.90/400 as a target Vdf/Mdf. So that's the speeds I fly to in my flutter trials.

Later I fly the 25.253 manoeuvres, and hopefully my sim results are validated and I pass.

I end up with an aircraft with different values of Vdf/Mdf and Vd/Md, quite legitimately.

Now, I can simplify things a bit and elect the set Vdf/Mdf at Vd/Md. But I don't gain anything in terms of Vmo/Mmo; I just get a bit of simplicity in not having to consider Vdf/Mdf independently of Vd/Md.

Secondly, I've seen the word 'demonstrated' before in relation to (or in lieu of) max crosswind limitations on particular aircraft. Is this something similar to the above?
:confused:

Nope, it's nothing similar really. Although it does indeed mean it's a flight condition achieved in test, the demonstrated crosswind is intended to give a feeling of comfort that someone has been there already, so as a line pilot you know it should be safe, and I'd expect you to be willing to operate to a demonstrated crosswind. There's certainly nothing wrong with doing so.

The 'demonstrated' in Vdf is emphatically NOT an invitation to try it yourself!

[geminid]

Wow! As I feared it would not be a simple concept. Your explanation was excellent Mad (Flt) Scientist though it took me some time to digest it!

Regarding attempts at achieving the Vdf - I'll definitely refrain from that.

:)

Thanks for taking the time, i appreciate it.

Geminid

[wotdafukwasat]

As far as I am aware `demonstrated` means just that. The a/c may well be capable of much more than than that - but your licencse & insurance are invalid if you exceed it. For example, during an a/c test/evaluation procedure the best x/wind that could be found/used was, say 20kt, therefore it has to be stated that the demostrated x/wind limit is 20kt. In a reality a 40kt xwind may be fine, but is your licesne worth it ?

[Mad (Flt) Scientist]

As far as I am aware `demonstrated` means just that. The a/c may well be capable of much more than than that - but your licencse & insurance are invalid if you exceed it. For example, during an a/c test/evaluation procedure the best x/wind that could be found/used was, say 20kt, therefore it has to be stated that the demostrated x/wind limit is 20kt. In a reality a 40kt xwind may be fine, but is your licesne worth it ?
The bit in bold isn't legally true, AFAIK.

If the crosswind is listed in the "Limitations" section of your flight manual, then it is exactly that - a limitation

If, however, it is listed in the 'Performance' section, such as:
The maximum demonstrated crosswind component for takeoff
and landing [at 33 feet (10 meters) tower height] is 29 knots and is not considered limiting for takeoff and landing.

Then it is NOT a limitation (note the text bolded for emphasis) and unless your SOPs require you to observe the demonstrated value as a limitation then you may legally exceed the demonstrated value. It's an airmanship call at that point - it may not be a sensible decision to elect to be the first man to land in that 40kt xwind on type - but its not a violation of the limitations of the type.