1950-60 era yellow trainer wing stripes.

It was Graham who raised it as a possibility at high speeds, though I think we agree it isn’t directly causable by merely painting.

The oscillation of a control surface is bilateral, caused by a resonating dynamic ‘feed-back’ between varying aerodynamically and mechanically induced moments of inertia. This occurs at a specific harmonic frequency. Change the centre of mass relative to the pivot on one of the moving bodies and you change its fundamental frequency, such that you might get ‘constructive’ feedback causing oscillation, ie you hit a ‘harmonic’.

The vibration of a flap isn’t a bilateral oscillation as such, its a buffet. Moving the centre of mass of the flap might alter the buffet frequency, but as it’s a mechanically truncated, asymmetric and low-amplitude rattle not a smooth and symmetrical wave it is less likely to hit any kind of harmonic with the aerodynamic behaviour of the flying surface to which it is attached, however you paint it. I am struggling for an analogy, but when I get one I’ll write it 🙂

Is there any possibility the aim was to avoid any paint in hinge or actuator mechanisms?

I am not at all certain that flap-flutter is at all common, in the same way as control-surface flutter.

I certainly never mentioned Flap flutter – not sure if anybody else has.
I think it might have been variations of paint schemes/markings which might have made a plain Flap colour desirable/more pragmatic,nothing upsets the military mind more than Lines not Lining up ; )

Any Flap vibration (close to flutter) would usually be caused by play in the linkages,however depending on the design of the Flap and Hinges/Arms etc it still might be desirable to have the C of G as far forward as possible to negate flutter/vibration/buffet effects – especially on a completely manual system.

I’ve a few pictures of Horsa and Hotspur gliders where they are painting in the training colour scheme with replacement rudders/tails in the operational scheme.

Not to mention all the patch work B17’s.

I’m sure you’re right that flap flutter never took place, but suspect that any limitations on painting were more because of a too-generic understanding of the term “control surfaces” rather than easing replacement of parts. It’s not as though Chipmunks or Balliols existed in any range of colour schemes.

I am not at all certain that flap-flutter is at all common, in the same way as control-surface flutter. A flap is held rigid against the airflow – in any given deployment it is meant to remain static relative to the airframe, regardless of any variation in force applied. An aileron, elevator or rudder can move freely on a pivot, it isn’t ‘propped’ in position, for want of a better phrase. It is this potential freedom of movement, combined with torsion when acted on by variable forces that can cause an oscillation of the control surface. So I am not convinced that not painting flaps was about avoiding flutter.

The other element of flutter, aeroelasticity, might cause flap-flutter, possibly as part of a larger wing flutter – but a bit of paint wouldn’t make a solid object held rigid against the airflow (or for that matter held flush to the wing) flutter, in the way it might affect a ‘balanced’ control surface.

Did the Bolton Paul Sea Ballilo have a flap arrangement that was only underneath the wing?
This aircraft has a yellow stripe the full length of the top wing and only half way underneath the wing?

Longer service life and so more likely to need component replacement?

Which raises an interesting challenge: are there any pictures of WWII aircraft with mis-matched markings resulting from changed components?

I’ve a few pictures of Horsa and Hotspur gliders where they are painting in the training colour scheme with replacement rudders/tails in the operational scheme.

A few more examples…http://www.highgallery.com/Foreign-Aircraft/Great_Britain/Training_Aircraft.html

The above Typhoon looks to me as though it has had the starboard wing replaced, with the flaps and various panels from the old wing fitted to the new one. I think this shot is part of a sequence taken of R8831 while it was being used for bombing trials, hence the lack of cannon and the stripes not being repainted on the new wing.

It seems that flaps and ailerons were treated differently. Here is a puzzle pic of a typhoon. Spot the issue.

Never done? To the contrary, it was normal practice during WW2 to avoid flutter, this being a resonant interaction between the aerodynamics and the structural design that rapidly shakes an aircraft apart. This was something that only became understood after analysis of the in-flight disintegration of the Parnall Plover. Typhoon elevators are perhaps the most noted wartime example. Examples of normal practice include the white markings on SEAC aircraft. D-Day markings being a notable exception, perhaps because of the pressure of the time and the thought that they would be temporary, but even then you will see that they are generally located away from the ailerons, and may be omitted when they would encroach. It is something that seems to have been more relevant to fabric-covered surfaces, but I suspect that the main difference with 1950’s aircraft is that they have powered flying controls which are more resistant to flutter.

Flaps presumably would flutter if left down to high enough speeds, but this doesn’t occur, and are more likely to rip off at lower speeds if left down.

Longer service life and so more likely to need component replacement?

Which raises an interesting challenge: are there any pictures of WWII aircraft with mis-matched markings resulting from changed components?

Which raises the question as to why this was never done before with invasions stripes and other identity markings happily trailing across control surfaces. What was different about post 1950 aircraft?

Moggy

As Graham alluded to in post #2 – one has to be careful when applying paint/dayglo to Mass Balanced manual control surfaces as the majority of the paint is aft of the control surface centre of gravity and therefore would move the C of G aft.This would mean a re balance would be required and the weight of manual control surfaces are usually kept as low as possible.
Also from a purely pragmatic point of view and especially in the case of flaps which would not normally be mass balanced – there are usually variations in aircraft paint/marking schemes and as Mike alluded to in post #3 any spare/replacement control surface would not necessarily match the aircraft markings/scheme – so much easier just to have plain basic surface finish on the control surface.

Just wondered why …. Chipmunk, Vampire etc have a yellow stripe that does not go the full length of the wing?

Simply, according to Bruce Robertson in his markings book, that this was the scheme specified. The yellow stripe was not to be carried on control surfaces.

Of course this doesn’t actually answer the question as to why this was specified.

Moggy

Just a guess… but, perhaps it was because the item (flap. aileron, etc), if damaged, could be changed for a stock/ex-stores item, pre-finished in silver, without the aircraft having to go into the paint shops afterwards to have the continuation of the stripe applied?

Anon.

You often find that coloured identity bands are not painted over the control surfaces, to avoid possible problem with weight and balance. However this is not the case with flaps, so either someone was confused by the term “control surfaces” or just thought it looked better that way.