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View Full Version : Probably a very stupid question, but...........



Banger2004
08-17-2007, 03:32 PM
Can someone explain to me in simple terms, the following:

The appearance of the Fw190 over the skies of western europe was a bit of a shock to Spitfire pilots, partly due to its ability to roll very effectively.

So how come (and this is where I really show my considerable ignorance) the ailerons on the Spitfire (or any aircraft I suppose) could not be enlarged to give greater surface area, and so presumably more, and faster deflection?

I suppose if it was that simple, it would have been done. I know the Spitfire ailerons early on were fabric covered, changing to metal skinned to improve performance, but why not a little larger? Would it compromise other areas?

Any airframe designers out there to explain this to me?

Daiichidoku
08-17-2007, 03:58 PM
http://img.photobucket.com/albums/v479/Daiichidoku/0568.gif

p-11.cAce
08-17-2007, 03:59 PM
Others will come along here and flesh this out - the simple answer is that an aircraft requires a balance of all its parts as a cohesive whole. If you increase the area of the ailerons significantly you will also need to stregnthen the spars, which in turn may require adding wing area because now you've increased the weight. You will probably need more vertical area to counteract the increased adverse yaw so the vertical stab will need to be bigger, and probably the rudder as well. Now you need to strenghten the fuselage to carry that larger tail and maybe lengthen it to regain your cg. Now you need a bigger engine to carry that extra structure, and perhaps larger fuel tanks to keep the range you need with the bigger engine. Now you are back to strenthening the wing again to carry the weight of the additional fuel. By that point, you probably have lost any roll advantage. Vicious circle - glad I don't design them http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

LEBillfish
08-17-2007, 03:59 PM
Well, the simple answer is change one thing then everything else must to match it.....

Roll is not simply ailerons alone. Wing length and width, torque from the engine, vertical and horizontal stabilizer size as well.....Aditionally, increase the control surface and you have to increase the force to move them.

Now to solve that you could:
A. Get bigger stonger folks to fly...Naturally the cockpit might then need to be enlarged to acomodate a gorilla changing the whole plane again.
B. adjust the linkage (not knowing if it was a mechanical or hydraulic system assuming mech....Yet the change in mechanical movements ratio then demands more movement of the stick to gain more force....Also, all the mounting points, pivots, cable, etc....So again the plane redesigned.
C. if mechanical go to a hydraulic system demanding more equipment (primative in the day) meaning more weight, so changes to the entire plane again to keep the same performance otherwise...
D. etc...

Lastly, as you move a plane closer to instability, making it manuever better...You also lose that stability that makes it an accurate gun platform especially at slower speeds.

In the end it's all about trade-offs....Gain this lose that, improve this you must improve other aspects...Balanced design based upon available tech. is the key...Yet whatever the balance found is then must be coupled with tactics to suit that planes quirks...Quite often, it's simply using your planes advantages, and forcing the others disadvantages that wins the fight.

To a point............

Banger2004
08-17-2007, 04:11 PM
Thanks, I am beginning to see the light. Nicely explained too. Certainly it seems designing a thoroughbred aircraft is indeed a black art.

The Spitfire controls were operated with cables and pulleys, I guess that to change to hyraulic power would involve too much weight gain and other complications (probably a redesign like with bigger ailerons)? Did the Fw190 benefit from technology such as this? Or did the German designers just get it 'right'?

Zeus-cat
08-17-2007, 04:12 PM
There are no stupid question, but there stupid people.

Actually, this is a pretty good question. There are only two ways to enlarge the ailerons. One is that make them longer and the other is to make them wider. Both involve changing the wing significantly. Also, by enlarging the ailerons you are probably reducing the lift characteristics of the wing by a considerable amount.

I don't think longer would work very well as your options are to leave the wing as it is and simply lengthen the aileron. Now one part of the wing is considerably longer than the rest which will certainly result in poor aerodynamic performance. The other option here would be to push the front of the aileron further forward into the wing to accommodate the longer length. I believe this would require the entire wing be redesigned as you have compromised the structural integrity of the original design. I don't think lengthening the aileron looks like a good option.

So the other option is to widen the aileron. Again, I think you compromise the structural integrity of the wing by doing this. The wing was designed to have an aileron of X meters and so the cabling and mechanical devices controlling the aileron would have to be repositioned inside the wing to accommodate the wider aileron.

On other thing you have to consider is that a larger surface area for the aileron means the pilot will have to work harder to move it to get the deflection you need to get the greater maneuverability you desire. The pilot may simply not be strong enough to perform this action.

I'm no expert, the above is just my 2 worth.

Viper2005_
08-17-2007, 06:02 PM
Aileron authority wasn't the limiting factor in the Spitfire's roll rate at high speed. The limiting factor was pilot strength, since the controls were manually operated.

Initially, fabric ailerons were used. These tended to balloon at high dynamic pressures, which acted to increase the control forces.

Metal ailerons retained their profile at high dynamic pressure and this resulted in lower control forces, and thus higher available roll rates.

Further improvements were possible by changing the design of the ailerons. However, this was delayed for 2 basic reasons:

1) As originally designed, the Spitfire offered reasonably "care-free" handling characteristics. It was the fastest diving aeroplane in the world when it came off the drawing board, with a limiting dive speed of 450 mph IAS or Mach 0.85. Obviously, application of full aileron at the limiting diving speed would have imposed unacceptable loads upon the structure. It was therefore quite convenient that the high control forces served to limit the amount of aileron that most pilots could apply; in today's terminology this would be called envelope protection.

The Spitfire's wing was designed when understanding of transonic effects was limited at best. As such, although the theoretical aileron reversal speed* was outside of the flight envelope, it was considered that the theory might not be entirely trustworthy.

Therefore, despite the fact that alterations to the aileron design, changing its profile, improving sealing and adding spring tabs could have significantly reduced the control forces in roll, it was decided that such modification would only be embarked upon once the wing was strengthened. The Spitfire VIII featured an improved wing with a higher aileron reversal speed; the Mk. 22 featured a totally re-designed wing; the changes being such that it was at one stage planned to rename the aircraft!

2) Any change in design would have necessitated a cessation of production whilst new tooling was produced. It was considered better to press on with a less than perfect aeroplane than to pause production. Anyway, it would have taken a considerable amount of time for any improved aircraft to replace the previous generation. For this reason the logical development of the Spitfire was greatly disrupted, and the marks III and IV never saw the light of day. Instead, modification of existing airframes was undertaken to improve performance. There were literally hundreds of "mods", ranging from fitting rear-view mirrors to fitting different engines. Minor mods were embodied at Squadron level; major mods required a return to the factory.

It was theoretically possible for a Mk. I or II to be converted into a Mk. V and thence into a Mk. IX, thus remaining a frontline aircraft for much of the War - and it probably happened in a few cases, though as my books are at home and I am at university I cannot cite an actual case.

The other obvious method of improving roll rate was to clip the wings. This was easy enough to do as the wing tips were manufactured as separate units. The Spitfire III was designed with a shorter span wing, but it never saw the light of day for various reasons, not least among which was the fact that in 1941/2 the average altitude of combat was increasing (culminating in Ju-86P raids at 46,000 feet). To combat this, an increase in wingspan was required in order to minimise induced drag; the Spitfire VI, VII and the earliest production Spitfire VIIIs therefore featured extended wingtips, which would have made roll performance even worse.

However, once the Spitfire had demonstrated its capability to intercept the Ju-86P at extreme altitude, both sides seemed to decide that combat at 46,000 feet was a bad idea, and the average altitude of combat progressively decreased.

Meanwhile, in North Africa, the RAF was faced with relatively low altitude combat against high performance aeroplanes, yet because ADGB was the priority, it was equipped with outdated Spitfire V aircraft. A modification scheme was therefore drawn up in order to make the best of a bad job. This modification scheme clipped the wings, and cropped the supercharger impeller, swapping full throttle height for power. This resulted in the "LF" version of the Spitfire V. Due to the nature of the modifications and the age of the aircraft, they were referred to as "clipped, cropped and clapped" in service.

The modification was also embodied in many UK Squadrons as the Spitfire IX replaced the mk V in high altitude fighter role.

Since the early Spitfire IX (Merlin 61 or 63) wasn't well matched to the Fw-190 at 20,000-25,000 feet, a similar set of modifications were embodied to create the LF.IX. This time a new engine was fitted (the Merlin 66) which instead of a smaller supercharger impeller featured lower gear ratios, which amounts to the same thing - a lower supercharger impeller tip speed, and thus a lower charge temperature). Clipped wings were optional, and it appears that their popularity varied in inverse proportion to the average altitude of combat.

*The speed at which the force applied by the aileron to the wing structure causes it to twist to such an extent that the nett result is a roll in the opposite sense - the aileron effectively starts acting as a tab. Analysis of this sort of aeroelastic behaviour is extremely complicated, even with modern computers.

VMF-214_HaVoK
08-17-2007, 09:09 PM
Originally posted by Daiichidoku:
http://img.photobucket.com/albums/v479/Daiichidoku/0568.gif

I hope you brought enough to share!

Choctaw111
08-17-2007, 09:17 PM
I have been sitting back to watch this thread unfold. It is not the original question but all the responses that may come along http://forums.ubi.com/groupee_common/emoticons/icon_rolleyes.gif .

VMF-214_HaVoK
08-17-2007, 09:19 PM
Originally posted by Choctaw111:
I have been sitting back to watch this thread unfold. It is not the original question but all the responses that may come along http://forums.ubi.com/groupee_common/emoticons/icon_rolleyes.gif .

I love to watch self proclaimed experts debate. Dont you?

Avont29
08-18-2007, 01:28 AM
my teacher always says there is no such thing as a stupid question

Friendly_flyer
08-18-2007, 02:41 AM
Thanks for an interesting read, Gents!

So, to sum up:

The easiest way to reduce roll-rate was to clip the wings (which was done). Changing the ailerons was a bit more work, and wasn't done until the Mk.22 version, when Britain no longer had a pressure to keep up high production runs.

Banger2004
08-18-2007, 03:58 AM
Thank you for you're answers, made things much clearer for me.

As for the one or two less helpful comments, this thread was always intended to be a serious question about something I knew very little about. I am pleased that some people chose to enlighten me, rather than turn this into another 'my aircraft is better than.........' slugfest. Whew!

Blutarski2004
08-18-2007, 05:41 AM
Originally posted by Banger2004:
Can someone explain to me in simple terms, the following:

The appearance of the Fw190 over the skies of western europe was a bit of a shock to Spitfire pilots, partly due to its ability to roll very effectively.

So how come (and this is where I really show my considerable ignorance) the ailerons on the Spitfire (or any aircraft I suppose) could not be enlarged to give greater surface area, and so presumably more, and faster deflection?

I suppose if it was that simple, it would have been done. I know the Spitfire ailerons early on were fabric covered, changing to metal skinned to improve performance, but why not a little larger? Would it compromise other areas?

Any airframe designers out there to explain this to me?


..... I'm no aeronautical engineer but, from what I've read on the subject, design of efficient ailerons is extremely touchy. Long and narrow ailerons are much more efficient than short and broad types. And aileron efficiency is highly influenced by apparently minor details such as hinging configuration, aileron cross-sectional profile, and gap between aileron and wing along the hinge line.

Wingspan also is a major factor, with (other things being equal) roll rate varying inversely as the 4th power of the wingspan.

p-11.cAce
08-18-2007, 09:00 AM
I love to watch self proclaimed experts debate. Dont you?

I'm not sure where the debate is here - seems like everyone is in agreement....what am I missing?

Jediteo
08-18-2007, 12:38 PM
Originally posted by Avont29:
my teacher always says there is no such thing as a stupid question

"There are no stupid questions, Only stupid people" - Mr/Mrs Garrison from South Park


(no offence)

stathem
08-18-2007, 01:48 PM
Originally posted by Friendly_flyer:
Thanks for an interesting read, Gents!

So, to sum up:

The easiest way to reduce roll-rate was to clip the wings (which was done). Changing the ailerons was a bit more work, and wasn't done until the Mk.22 version, when Britain no longer had a pressure to keep up high production runs.

Pretty much, although the Mark VIII (redesign) had smaller (shorter) ailerons, coupled with a slightly stronger wing. Which is why it had a higher aileron reversal speed (as Viper mentioned) and, I believe, a better roll performance at higher speeds (possibly at the expense of slower speed roll rate?)