Thinking aloud about Spits...

[Colin Leighfield]

I designed a 1/7 scale version of the prototype K5054 many years ago, I don’t know if I’ll ever finish it. The colour should be cerulean blue, slightly lighter than this. Later on the prototype was re-painted in a light grey after problems with cracking of the original colour.

The wing section is scale, NACA2200 series and the wash-out also scale at 2.5 degrees. Had the Dynam Spitfire IX for a while and that was a pussy-cat. The current Durafly Spitfire 24 is also a very good flyer and doesn’t tip stall. My Black Horse 72” Piper Cub would drop a wing though if I slowed it down too much!

[eflightray]

My 72" Depron Spitfire came in at 7Lbs - 2oz flying. No worries about stall speed and tip stalling.

Ray.

[Foxfan]

The blue colour was used on K5054 after it was first flown. It's said it was make it attractive to the public at an air show, but nobody knows for sure and nobody knows for certain exactly which shade of blue it was, whereas I like the zinc chromate and polished ally finish of the first flight. Also the earliest wing was "planked", clinker style with surface rivets, spanwise. That was changed when it went to blue and the countersunk rivets filled to give a super smooth surface.

I don't think a Durafly Spitfire is current any more.

Good to know the 46" version is a good flyer.

Martin

[Foxfan]

Nope, HK still have them listed, albeit on back order, but at £132 very good value I must admit.

Martin

[Colin Leighfield]

There is a lot of mythology about the colour of K5054. We know exactly what colour it was originally. In his book “R.J. Mitchell - Schooldays to Spitfire” his son Gordon describes it. In fact he still possessed the aluminium ash-tray sprayed in the original Cerulean Blue colour that had been given to his father. After the original tests the plane was taken back into the workshops, the undercarriage leg fairings fitted and it was painted. There were issues with the quality of the filler and suitability of the paint, so that there were problems with cracking and flaking early on. As a result the airframe was stripped and re-painted within a few weeks of the first flight. The light grey paint that I understand was used then has been assumed by some to be the original colour, but it wasn’t. Later on the plane was re-painted again in standard RAF camouflage and late photos show that. The “planked” wing structure remained throughout its life until the sad accident that killed White, the pilot. A major part of the production engineering lead by Joe Smith to get the Spitfire into production included re-engineering the wing structure to increase stiffness, because originally it lacked torsional rigidity and aileron reversal occurred at too low an air speed. That is why the “planked” effect is only visible on K5054. 

Edited By Colin Leighfield on 12/09/2019 23:27:14

[Foxfan]

I didn't know it remained with planked wings, Colin. I'd assumed they were changed prior to the blue being applied as it would have been difficult to get a smooth finish with a planked surface.

I still prefer the original colour of the first flight. The blue always had a vaguely PR look about it and I like absolute prototypes any way (except the very first EE Lightning, which was hideous!)

Martin

[mrwood]

The Alpha Models Spitfire features stressed skin foam semi-monocoque construction and is remarkably light.

[Chris Freeman 3]

I built my first Spitfire nearly 30 years ago and did not expect it to last, great flying aircraft and was eventually sold. I have built many over the years and all have flown well. Points to consider is the weight, the cg, elevator movement and power set up. You will need nose weight so keep the tail light, do not have too much elevator movement as they are sensitive, set up rates to assist, keep light as the smaller airframes do not like weight. Have enough power that must be reliable. The small Adrian Britton speed 400 plan could be a great plan to enlarge to the size you want as the structure is light and good looking for its size. It has a nice fuselage shape to it, not the flat sheet sides that most have. Traplet published this plan as a free plan with quite a few editions in their mags.

[Foxfan]

Thanks for that info., Chris. I shall try to find that Adrian Britton drawing and tuck it away in the plans pile for later.

Martin

[Nigel R]

Stand way off, squint a bit...

**LINK**

36" span, flat sheet wing, sheet & block fuselage.

Think it would go pretty good on a 200W or 250W electric setup

[Foxfan]

Certainly an easy way round the Spit build.

Thanks, Nigel. I've downloaded it.

Martin

[Foxfan]

Chris, found a copy on ebay and the Mrs. is ordering it now.

Thanks again.

Nigel, free plan downloaded.

Cheers,

Martin

[Erfolg]

One or two have mentioned an aspect that a Spitfire must have to fly well at lower speeds, and that is wash out as mentioned by Colin, the full size had it. It had it because it needed it. Reflexing the aileron as mentioned has a similar effect in that the camber is changed,

Buying a ARTF all of this aspect will have been taken care of.

The Ripmax Spitfire always flew well, although the body had been narrowed a lot, it still looked good. A hand launch model though.

I did come across a paper on the design of the Spitfire, it read somewhat differently to much that has been written. A lot was learnt from the failure of the 224, not just the successful S6 types. The failures were a significant component of finding legendary success. Also a lot of aircraft designers played a part in creating the final concept, who had experience with elliptical plan forms, plus those with the design and production of wing structures of the type and thickness that the design moved to. As so much, when you want to find the document, you cannot. The important thing was that with respect to washout, it was recognised that there were advantages aerodynamically at speed, without wash out, yet at slow speed, not good at all, without washout. As perfect as the wing is,it still was a series of compromises, that were carefully and successfully judged.

Edited By Erfolg on 13/09/2019 21:05:01

[Foxfan]

Yes, Erfolg, washout would seem to be essential. Not a problem in the build if the correct measures are taken before the wing is set up.

Martin

[PatMc]

Posted by Erfolg on 13/09/2019 21:02:39:

One or two have mentioned an aspect that a Spitfire must have to fly well at lower speeds, and that is wash out as mentioned by Colin, the full size had it. It had it because it needed it. Reflexing the aileron as mentioned has a similar effect in that the camber is changed,

Reflexing the ailerons doesn't have the same effect as washout, at least not close to the stall when the effect is needed.
As the stall angle is approached the air over the wing uppersurface breaks away at a separation point that moves progressively further forward. This separation point will be forward of the ailerons before the stall is reached, therefore reflexed ailerons will be ineffective or virtually ineffective as washout.

[PatMc]

Posted by Erfolg on 13/09/2019 21:02:39:

I did come across a paper on the design of the Spitfire, it read somewhat differently to much that has been written.

Is this the paper you are referring to ?

Edited By PatMc on 13/09/2019 21:38:59

[Erfolg]

Pat Mac, yep, that is the paper.

The effect of reflexing the ailerons is that the camber is changed, in such a way that looking at a lift drag graph, that at the angle of attack that the section sees, moves the stall to occur at a lower speed than the rest of the wing, assuming that the section in use is pretty standard.

Another way of delaying a tip stall is aerodynamically, where the tip section graph indicates that the stall AoA is greater than the section chosen for the root. Again it works to some extent, although with the caveats of both parts of the wing are either laminar or turbulent at this point, within reason. A problem often is that the tip and chord are so very different, that the Re value can indicate that on a say 10" chord the wing in that area is transitional where the tip should be laminar.

I think personally that using washout has the least risk. Colin is using generally about 2 degrees of wash out, where the losses due to a less than optimal efficiency are both acceptable and probably not noticed. At higher values the tip may hardly be working at speed.

Of course the FF had a solution, in that they would deliberately trip the wing into turbulent, which ensured a consistent trim. Also often argued at high AoA the separation bubble would remain attached for a little longer or at a slower speed etc.

[PatMc]

Posted by Erfolg on 13/09/2019 22:03:14:

The effect of reflexing the ailerons is that the camber is changed, in such a way that looking at a lift drag graph, that at the angle of attack that the section sees, moves the stall to occur at a lower speed than the rest of the wing, assuming that the section in use is pretty standard.

The camber forward of the separation point hasn't changed at the stall & the camber behind the separation point is doing nothing. The effective AoA is the same as if the ailerons were not reflexed.

[Erfolg]

Patmac, I am not suggesting that the Aoa is changed, it is that the stall occurs at a lower AoA, in the reflexed set up, a higher AoA (due to the camber being reversed to the normal set up).

I guess we will not agree. What matters some have tried the reflexed aileron approach and have found it has worked for them. I guess others will say it made no difference. I have not heard that any suggest that it has matters worse, with respect to stall behavior. Looked at another way, crow braking is an extreme way of altering the camber of the wing. Both increasing drag and maintaining control at what is now near the stall region.

Non of it is worth dying in a ditch over

The link is a good find by you, just showing how many people are involved in design. from Pranl and Von Karman and others, whose papers were used etc and the members of the team and so on. It is rarely one man, often a collection of experiences and knowledge, some of which is used, some ignored or just discarded.

Edited By Erfolg on 13/09/2019 22:29:34

Edited By Erfolg on 13/09/2019 22:34:16

[Peter Christy]

PatMc: I don't think anyone is suggesting raising the ailerons as a substitute for built in wash-out. However, presented with a finished wing that exhibits tip-stalling, the options are either 1) build another wing or 2) find an alternative solution.

One alternative is raising both ailerons slightly (doesn't work with strip ailerons, though!). This is simple, and often surprisingly effective.

Aerodynamics - like my own speciality, electronics - can be something of a black art, especially at model sizes.

Sometimes effective solutions are weirdly counter-intuitive. And at the end of the day, even if it doesn't work on a specific model, you are no worse off for trying it!

--

Pete

[Erfolg]

On reflection, I can see that saying that the camber is reversed is at least misleading and in most cases wrong.

With the ailerons reflexed I was trying to say that is what we have done. The camber line has now changed to now be a reflexed form as used with many flying wings. If the tip were symmetrical then the camber line would be reversed, which almost certainly would be a bad idea.

The trouble with this sort of discussion, is that they are hedged with restricting caveats, which means that a concept is not applicable in some cases. A classic case of this is the general bending formula (the M/I =f/y=E/R one), there are a number of criteria that are assumed, one being that the materials properties are identical in compression as in tension. On that basis concrete is not suitable for analysis/calculation with the relationship.

I think I need to remind myself what we are trying to achieve. The first is that the root section of the wing stalls before the tip. Often we are trying to amend the AoA where the max CL occurs. We often see increased drag as useful in slowing the model down. It is often this increase in drag which requires a power on approach, to avoid slowing to much (then stalling).

On that basis the simple solution of wash out, with flaps works rather well.

Now that there are wing profile plotting/printing software like Profi, the aerodynamic solution is quite viable. No more tedious interpolating from one section to another. The down side is you need to study the data to make sure that the tip section stalls at a higher AoA than the root section, by a descent margin.

Another issue that I have to remind myself is that many of the profiles and data sets are produced for full size sections operating a much higher Re numbers. That is why i have favoured many of the Eppler profiles as many are at Re values for model aircraft. On that basis nasty surprises are avoided and consistent results are obtained.

A practical problem I have, is that I am not that accurate a builder. Foam cut or moulded wing types are normally very accurate.

Edited By Erfolg on 14/09/2019 10:44:05

[eflightray]

Alternative would perhaps be, -

don't add wash-out so the whole wing is lifting better, (you are not wasting the outer wing area and making flying inverted riskier), and most of all, learn to build lighter, , and don't fly near the stall speed.

Seems logical to me.

Ray, (72" Spitfire with no wash-out)

But I should add, I don't make 'scale' models either.

Edited By eflightray on 14/09/2019 13:29:50

[leccyflyer]

Thinking aloud about Spitfires - if it;s got clipped wings it isn't really a Spitfire, it;s something that is diminished, less perfect, with it's most precious, iconic feature debased.

To a lesser extent the later Mks, like the Mk 24, are also a bit less of a Spitfire - somewhat removed from the purity of the Mk 1A . The most effective Mk might arguably be the Mk IX, but even that isn't a Mk 1A .

[Erfolg]

Oh! Leccy, of course they are all Spitfires.

If it was just an semi-elliptical wing a He112 and the He70 would be a Spitfire.

If the Merlin made the Spit , then a Bf (Me) 109A would not be a 109 after the first model.

They are all attractive, particularly the latter marks to me eyes, far better balance of proportions for a flying model.

We just have to accept that semi-elliptical wing is not good at the stall, although excellent in other ways. Washout is the way to reach perfection.

Edited By Erfolg on 14/09/2019 15:37:13

[Foxfan]

For me, the later Mks are better looking, even though that wing changed a bit. And I like the bubble cockpits. But to me the painting in cowsh*t cammo ruins anything, anyway, so debating the finer points of the aircraft's development pales into insignificance. A Mk IX is sufficiently close to a Mk I to convert one to tother if it matters that much to you. The livery is probably what makes the most difference. My First Flight version couldn't be anything (at a glance, stand off) other than what it pretends to be as no other Spit. had that scheme. But I think, considering the OP, we have begun splitting hairs.

I recieved my mag. article and plan by Adrian Britton this morning and am resolved to make it, ready for when I can fly.

Martin