Interesting Aerodynamic Fix on the Do-335

[Broncazonk]

I saw something last night in a photo of a Do-335 that I've never seen before.

Look directly below the GI's feet at the leading edge of wing root. Notice how 'pinched' and 'sharp' the leading edge is? Then right next to the handle of the step ladder the leading edge transitions back to a normal profile.

You can see this design feature again (faintly) on the 107 bird.

It took me an hour of searching before I discovered what this is. On Pg. 67 of the Smith, Creek & Hitchcock text is a photo of it with the following caption. "A close-up of the wing leading edge stall strip. This device acts as a stall originator which improves the lateral control thereby allowing the wing tips to stall last." (See photo below.)

The wing of Do-335 apparently didn't utilize 'washout' which is the standard and usual (Ta-152H) way of dealing with wing tip stall. (Washout is difficult and time consuming to build into a wing.)

Now here is the interesting part! Not every Do-335 had the stall strip installed. See the last photo. I've looked at clear, very large copies of this photo and the stall strip isn't on the plane. It's a normal wing root.

Bronc

[Revi16]

Isn't your last photo the V1? If so, that's probably where they learned that they needed a stall strip.
https://www.worldwarphotos.info/wp-c...o_335_v1_2.jpg

F4U Corsairs evolved much the same way, with an additional stall strip being added to the right wing.

[Broncazonk]

Originally Posted by Revi16

Isn't your last photo the V1? If so, that's probably where they learned that they needed a stall strip.

Bingo! Of-course. Yes it is.

Photos of the V2, all later pre-production aircraft, and all production aircraft have the stall strip installed.

The book with the stall strip photo is the Monogram Monarch 2 [1983/1997] (Smith, Creek and Hitchcock.)

The [2006] Smith, Creek (with Roletschek) book notes that the stall strip was first test flown on 19 Dec 43 (Flight 24) with Quenzler at the controls.

Karl-Heinz Regnat's [2003] book states: "Dieterle reported that the aircraft's (V1) stalling characteristics were unpredictable, with no warning of an impending stall." After the stall strips were installed everything got better.

The Ta-152H had 3-degrees of washout to address this problem. (Essentially, the wing was twisted. At the wing root the center of the leading edge was 3-degrees above the airflow, but at the wing tips the center of the leading edge was 0-degrees to the airflow. As a result, at any angle of attack, the wing root would stall before the wing tips.) The impressive wing span and high aspect ratio of the wings demanded this ingenious and elegant solution.

Bronc

[Graham Boak]

The stall strip is not a straightforward alternative to washout, but usually used (as on the Corsair) on one wing to make the stall completely predictable on aircraft where the stall can mean unpredictable wingdrop to either side. It was also used on the Grumman Tiger light plane, presumably for similar reason. It is a cure for low speed bad behaviour, and tells us that this was present on the Dornier.

Washout is to allow for the wing twisting with load, as incidence increases. This because this twist increases the incidence of the tip above that of the inner wing, so that the wing stalls at the tip first giving wingdrop. In combat turns, it may be unpredictable which wing drops first. A few degrees of washout gives the aircraft extra lift at the tip, and hence can go to higher incidences giving better turning performance.

I don't know which was the first type to introduce washout, but the Spitfire had 2 degrees, and I suspect that the Fw.190 had similar. Other types of this period (eg Bf.109) used outboard slats but this had other drawbacks such as increased drag.

The same principle was behind the poor stall behaviour of the Dragon Rapide, with its narrow pointed wingtips, but in this case the effects showed at low speed rather than in combat turn.

[rof120]

"...the Dragon Rapide, with its narrow pointed wingtips, but in this case the effects showed at low speed rather than in combat turn."

Thanks God! I would have been scared out of my wits (if any) by a combat turn performed by a good ol' Dragon Rapide (quite a scary beast). Parachute jumps from a Dragon Rapide - you had to get out first without jumping and stand on the lower wing - were simply wonderful.

No bad feelings, hey?

[Broncazonk]

I'm about to make Graham Boak and rof120's day...

https://www.platinumfighters.com/inv...-dragon-rapide

Bronc

[schwarze-man]

The leading edge "stall strips" should perhaps be called stall warning strips as they are usually used for that reason. At higher angles of attack the sharp point of the strip makes an increasingly strong disturbed stream of airflow rearwards. In cases where the horizontal tail surfaces fall within that disturbed flow, this technique can be used so that the tail surface is vibrated by the turbulent airflow and the vibration is felt by the pilot through the controls.
Some aircraft will show satisfactory warning without this method. Overall, the improvement is purely in the aerodynamic warning of high angle of attack, not the performance.
The presence of the strip does usually make a small reduction in the aerodynamic performance of the wing in normal flight.
Refering to Bf109 l/e slats, There will be a small reduction in efficiency due to the slight effect of the slat spoiling the wing shape. However, the stowed position is very clean and, when the slat deploys itself at high AoA the performance of the outer wing is greatly improved.
Washout on wings is not a perfect panacea, there may be adverse effects, including increased zero-lift drag and drag at high speed.
Overall, these points are generalised and individual aircraft will have their own characteristics.
Cheers

SM