Flight Tests

[LowNSlow]

As part of the Star Annual flight test the aircraft has to be dived to Vne (Never Exceed Speed).

WHY :eek: What does it prove :confused: In a Cub one has to be nearly vertical to get the required 106 mph on the ASI. Loads of fun, not with the aileron cables vibrating and pretty much everything else shaking.

What is the reasoning behind this seemingly bizarre rule?

Has anybody damaged / overstressed their aeroplane complying with this rule?

[2Donkeys]

Amongst other things, the Vne dive is there to ensure that the aircraft is capable of being controlled through its entire certificated speed range. This means verifying that control surface flutter and other factors that might affect controllability do not manifest themselves to unacceptable levels.

I have no idea if anybody has ever overstressed an aircraft performing this manoeuvre in a flight test, but I don't recall reading any accident reports to that effect.

[Hilico]

Surely if damage occurred at any airspeed up to and including Vne, it would mean that Vne had been set too high.

Did get up to Vne in a C150 once, recovering too enthusiastically from a spin. Drove the prop round at redline rpm (throttle closed of course) but nothing fell off and I'm still here.

[Genghis the Engineer]

It proves that...

- Nothing in the control system or aerodynamic surfaces has stretched, contracted, tightened, loosened, etc. enough to cause the start of flutter. (I've seen that)

- The pitch control circuit hasn't done something similar so as to cause a reversal in pitch stability, which if it occurs, is likely to be seen first near Vne (the phrase CH601 springs to mind).

- Control authority remains at high speed, this is why Paul, Bob or Darrol, depending upon who checked you out from CAA, will have taught you to make small control inputs at each speed increment. Control authority can be degraded by cable stretch, something gone in the surfaces or wear in a joint. I've seen an aircraft lose aileron authority at Vne.

- The canopy is still stiff enough not to implode on you at high speed (I've seen that as well, quite exciting, fortunately in my case a bendy one so it just changed shape).

- Propeller control is still correctly set-up to allow you to avoid red-lining the engine (although I'm sure you are aware of the need to retard throttle towards the late part of the dive).


Regarding overstress, Vne is no more than 90% of Vdf which is the limit the prototype (and depending upon company procedures the individual aircraft) was taken before approval. Given wind energy ("Q") goes with the square of airspeed, you are at Vne only taking it to a state of 81% of the energy the aircraft's designed to cope with.

I've certainly not heard of an overstress, and hope that if the airframe was that degraded, it would have been picked up in maintenance and sorted before the air test.

G

[M14P]

I've conducted lots and lots of airtests and have never had a problem at Vne. The provisos are, however, it should be approached with caution (you don't need to accelerate straight through it, just a short visit will do) and this should be conducted in smooth conditions.

More likely that the aircraft dropping to bits at Vne is that the pilot will have forgotten to retract the flaps after the stalling sequence! This has been done before (not by me though) leaving some very unwell flaps.

The simple fact is that the airtest is designed to examine the certied range of operations of the aircraft - something which could occur in normal operations. Sounds like your Cub might need some attention with the aileron rig?

Why is it 'bizzarre' to operate an aircraft throughout it's normal operating range once every three years?

[LowNSlow]

Thanks for the info chaps. The purpose of it makes sense to me now.

Genghis It's especially reassuring to know that it equates to 81% of the max energy. I realised that there was a safety margin and the wings shouldn't fall off at Vne but it nice to be able to quantify it. I've replaced virtually all of the tail surface control cables on the Auster and had the elevators and rudder painted. If I end up doing the airtest I will be approaching Vne very carefully, feeling for the onset of flutter.

M14P now that I understand the purpose to the Vne check I agree that bizarre was the wrong word and yes, the Cub did need adjustment to the aileron cable tensions.

[Kingy]

L&S,

C'mon its not that difficult to get 106mph out of a Cub - intrestingly, in military service L4s had a max dive speed of 120mph - that must be approaching 100% max 'Q' - mind you, with a BF109 on you're tail that would be the least of your worries!

My Isaacs Fury has a VNE of 160mph. Being a draggy biplane, it's a case of.. 4000ft ...stick the nose down, wait... and wait... and wait, whilst watching the altimeter unwind at an alarming rate!

Good fun though :D

Kingy

[Thrifty van Rental]

No problem getting a Cub to go to Vne. Its just that the nose down angle take a little getting used to if you normally trundle around straight and level.

So are you an authorised CAA Test Pilot LowNSlow, or were you there to take the notes?

[LowNSlow]

Thrifty as owner you are allowed to do your own flight tests to the supplied schedule. It's a matter of sitting down beforehand and working out what you have to do from the schedule followed by figuring out how you are going to do it.
As with everything else the 6P method works (Prior Planning Prevents P1ss Poor Performance).
Doing the Cub flight test was the first time I had done anything like that so I dug around all the bods I knew who had done flight tests and tried to learn from their experiences. I subsequently enjoyed it, the Vne dive wasn't that bad really. I would have been quite happy to do the Auster FT but I'm out of the UK until just before the Pprune fly-in on 5th April so somebody else will be doing it.

Going back to the Vne dive, I was unused to pointing the Cub at the ground so steeply. I was used to it when I had an Aerobat and did (poor) loops and stall-turns. My Cub flying was more steep turns, chandelles, side slips and rabbit chasing ;)

[Thrifty van Rental]

Nothing stays the same LowNSlow. Those rules are changing.:(

Glad to see that you are none the worse for the experience. Cubs are lovely aircraft and you've had the privilege of seeing part of the flight envelope that many have not experienced.

I enjoy flying a Cub occasionally at high-alpha close to the motorway near my home and watching the cars overtake me.:D Beats the day job!

[LowNSlow]

Thrifty as I understand it, I could do the flight test on my CofA Auster. Is this changing? Another privilege removed by a Nanny Institution?

In normal cruise in my 65hp L4 I was regularly overtaken by motorway traffic ;)

At high alpha I could be overtaken by a vicar cycling to church. :) (do they still do that Keef?)

[M14P]

As far as I know you can still do your own airtests - sole owner etc.

That said I think that it is of benefit to have an experienced airtester along as the observer simply because we see lots of different types andmight be able to provide valuable info.

I am always concious of the fact that many owners do not want some unknown flying their pride and joy so I will always offer to observe instead.

In that case it is also a perfect opportunity to sign off your Dual Flight with Instructor too (at no extra charge)!

[LowNSlow]

M14P your suggestion is good, if I was doing the flight test I'd take you up on it. An extra pair of experienced eyes is worth having in most situations.