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I was at Duxford recently, and spotted a Jet Provost Strikemaster that had a rough texture applied to a portion of the leading edge of the wing.

Jet Provost Strikemaster leading edge

Why was this applied?

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  • $\begingroup$ Looks like part of the leading edge is missing and someone put in a rough patch awaiting the proper part, this is definitely not normal for any Strikemasters, and not for the Mk.80/80A in particular that were used by the Saudi air force. $\endgroup$
    – jwenting
    Aug 15 '19 at 11:27
  • $\begingroup$ My guess: cheap attempt at a vortex generator. See answers to this question for the purpose. $\endgroup$
    – DeltaLima
    Aug 15 '19 at 12:14
  • $\begingroup$ @DeltaLima: Almost - it's artificial roughness to trip a laminar boundary layer. $\endgroup$ Aug 15 '19 at 16:51
  • $\begingroup$ Have you considered emailing Duxford to ask them? They'll likely know a lot about their planes. $\endgroup$ Aug 16 '19 at 8:19
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The leading edges are roughened to improve the stall characteristics. Typically vortex generators are used for this purpose nowadays.

The 5 and the 5a were used by the RAF for pilot training and the nose strakes were fitted to improve the spinning characteristics - the leading edges of the wings had a roughened paint finish to improve the stall characteristics. The 5s with the tip tanks (I'm not sure if 5b is an official or 'unofficial' designation) were used by the RAF for nav training and didn't have the nose strakes (not sure about the roughened wing leading edges) I don't know if it was because the tip tanks themselves improved spin handling or if it was because they weren't flown by studes for spin training.

(source: User "Fritag" at www.britmodeller.com)

The rough grey coating on the wing of the aircraft was applied in order to break up the smooth airflow and give an early indication of the onset of a stall as the T5's original clean wing design gave the pilot little prior warning.

source: all-aero.com / RAF Museum

A great deal of work has been put into stalling, spinning and stall-warning over the years of Jet Provost and Strikemaster development. This has led to all sorts of devices, like inverted-aerofoil tailplane, fences under intakes, strakes round the nose, roughened outboard leading-edges and little "eyebrow slats" at the wing roots. Of all these devices, the Strikemaster requires only the "eyebrow slats" to give it that precious smooth spin, a tidy g break in the stall and a distinct stall warning.

source: Flight International 12 October 1972 500-501

At first it was forecast that the T.5 would take the place of the T.4 on certain roles such as high altitude training. Unfortunately the majority of T.4s had to be taken out of RAF service much earlier than anticipated so the T.5 had to take on more responsibilities. As a result, the RAF decided to update its fleet of Jet Provosts with new avionics suites, spin strakes, and roughened leading wing edges in 1973.

source: Jet provost heaven

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    $\begingroup$ Not sure you should use dudes posting on an aeromodeler forum as source data. Just sayin'... $\endgroup$
    – John K
    Aug 15 '19 at 15:50
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    $\begingroup$ Still just anonymous people (like here) who may or may not know what they are talking about. You don't know they really are or what they know, or who wrote the All Aero bit. I've searched numerous pics of Mark 5a's and can't find any with that crazy leading edge treatment and can't find any OFFICIAL accounts that describe it, so I will default to skepticism, and my own knowledge of how dangerous that kind of LE contamination is. If that kind of surface treatment, equivalent to a heavy coating or rime ice, improves aileron effectiveness, well, there's a whole new paradigm. $\endgroup$
    – John K
    Aug 15 '19 at 16:58
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    $\begingroup$ @JohnK I don't think it is increasing the aileron effectiveness, instead it increases the turbulence over the aileron close to stall. This gives an early indication of impeding stall, which is more friendly than the laminar flow suddenly stalling, especially in a training aircraft. $\endgroup$
    – DeltaLima
    Aug 15 '19 at 17:05
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    $\begingroup$ No that kind of roughness will simply cause a full separation and complete loss of aileron control at an angle prior to the normal stall AOA. I could be wrong in all this, but I want to see an authoritative source because this is pretty bizarre, and like I said, all the pics of Provosts I can find all show normal clean LEs. $\endgroup$
    – John K
    Aug 15 '19 at 17:13
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    $\begingroup$ Good find! I'm satisfied that the treatment was intended to modify stall behaviour and will delete my post. I still think tho that it was just done experimentally and may have not worked so well because I still can't find a picture of one in service with that treatment. $\endgroup$
    – John K
    Sep 5 '19 at 15:19
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I am a helicopter pilot (ex RN) but my original training was on aeroplanes (Bulldog at RAF Leeming 1979) prior to helicopters. The FTS at Leeming was operationg JP 3's & 5's.

My understanding of the rough leading edge on the outer part of the wing on the JP.5, I cannot remember if it was also on the JP.3, is that it was there to help keep airflow attached and give/maintain aileron control at lower speeds and therefpre high angles of attack (AoA) i.e. close to the stall. Ideally you want the inner part of a wing to stall first, as this will reduce the chance of wing drop and therefore the likelihood of a spin.

If the outer part of the wing stalls first, and a wing drops, the AoA on that wing increases as it drops increasing the stall, increasing the wing drop etc. As the aircraft rolls due to the wing drop, the other wing goes up and therefore the AoA reduces and the wing may be unstalled. This may assist the roll towards the down going wing increasing AoA on the wing and exacerbating the conditions of a spin.

Many aircraft have other ways of trying to achieve the effect of aileron control at slow speed/reduce wing drop:

  1. Fixed (leading edge) slot - e.g. Douglas Dauntless and Grumman Avenger. Postioned on the outer leading edge, ahead of the ailerons. At higher AoA air passes through the slot re-energising and therefore re-attaching the airflow helping maintain aileron effectiveness. Helpful to have aileron control at low speed (particularly when deck landing).
  2. (moveable/automatic) Slats - e.g. Messerschmitt 109, Fairey Swordfish. Positioned on the leading edge ahead of the ailerons. Some are spring loaded to open (pop out) and create a slot as the air pressure on the leading edge reduces as speed reduces. Reduction in speed means increase in AoA. The slats open to form a slot between slat & wing which helps re-energise and therefore re-attach the airflow ahead of the ailerons. Others open due to the relative change in pressure above and below the wing. Apparently in fighters, like the 109, not having the slats lockable can mean that they can "pop" when the AoA increases uneavenly during hard manouvering causing the wings to rock spoiling aim. Some types Westland Lysander, and some modern light aircraft types, have moveable slats along the whole leading edge. This reduces take off and landing speeds and therefore stall speed. These may be selectable and opened and closed as required, like raising/lowering flaps. The Fiesler Storch had a fixed slat, creating a slot, along the whole leading edge. The Storch has a very short take off/landing performance.
  3. Washout - various types. Effectively the whole wing is twisted (leading edge downwards towards the tip) so that the angle of incidence and therefore AoA is less at the tip than at the root. Thus the wing should stall near the root first.

If any body reads this with proper aerodynamic knowledge and finds I have made some errors, created misconceptions, please correct, explain my errors.

IRW

7 Feb 2021

I recently found AP.3456, now called the "CFS Manual of Flying", online (https://www.gov.uk/government/publications/foi-responses-released-by-mod-week-commencing-13-january-2020) and downloaded it.

So far I can find no direct reference to the "rough leading edge" on the Jet Provost (Strikemaster). From what I have so far read in what I have downloaded, I am still of the belief that the "rough leading edge" is effectively a vortex generator (perhaps it could be called a boundary layer exciter/re-energiser.

I would recommend that anyone wanting more information download and look particularly at; Vol.1 Principles of Flight, Chpt.6 Stalling, Para.8-10 (Tip Stalling) and Para.30-34 (Autorotation); Vol.1, Chpt.10 Lift Augmentation, Para.3-12 (Slats) & Para.44 (Vortex Generators).

Since writing my original comments, I have spoken with a former RAF QFI who instructed on Jet Provost 3's & 5's and he is in broad agreement with what I wrote. He cannot remember however for certain if the 5's had the "rough" leading edges when he was flying them.

I hope that the link above and the references may be of interest and help answer the "rough leading edge" question.

IRW

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