Area Rule

[GLX]

I was reading an article about the Global Express and it stated that a feature associated with its advanced aerodynamics was the tail design with area-ruled aft fuselage. I believe that it results in lower drag at high speed but my books don't explain how - can anyone enlighten me with simplified explanations.

[Keith.Williams.]

The best (most simplistic) way to explain area rule is to imagine what happens when an aircraft flies through a thin vertical slice of the atmosphere. Upon first reaching the slice its nose pushes the air apart to produce a very small hole. This hole then expands to accommodate the increasing cross section of the fuselage. The rate of expansion suddenly increases as the leading edges of the wings pass through. This is followed by a sudden contraction as the trailing edges pass through, leaving only the rear fuselage. The passing of the fin and tailplane then produce similar, but smaller expansions and contractions.

Expansion of the hole requires the air to accelerate outwards and contraction of the hole requires the air to accelerate inwards. The rate of acceleration is proportional to the rate of change of the cross section of the aircraft. So very large rates of change of cross section require very large rates of acceleration. But the forces necessary to generate these accelerations are provided by the aircraft. This adds to the total drag affecting the aircraft.

For minimum drag it is necessary to minimise the rates of change of the cross section of the aircraft. In the case of a complete aircraft this would involve narrowing the fuselage to offset the sudden increases in cross section caused by the wings, fin and tailplane. The degree of narrowing required in these areas can be reduced by increasing the cross section in the area between the wings and fin/tailplane. It is this narrowing and broadening of the fuselage that gives area ruled aircraft their coke bottle shape. (The term coke bottle shape has been used in JAR POF exams)

The benefits of area rule are greatest for aircraft flying close to the speed of sound. This is because at such speeds, even moderate accelerations will cause the airflow to go supersonic, producing additional drag due to the formation of shockwaves. At very low speeds the extra weight and skin friction can increase the total drag, so (as always) it isn't all good news.

[John Farley]

Super answer Keith

(and without mentioning the Buccaneer!)

[Wee Weasley Welshman]

Keith - that really is a fantastically pitched explanation. The best I have ever read and I have read a lot.

Blackburn Buccaneer - taxi'd in one once and its the regret of my life that I never got a trip in one sat at 500kts in ground effect...

WWW

[411A]

One of the first airliners to use arearule was the B747SP...have a close look at the tail end sometime.

[quid]

GLX-

Many moons ago, I flew F105s for the USAF. That was one of the first to use the "area rule" concept. The F106 also used it. Search the www to find phots of them, and it's readily apparent.

[Joe Bolt]

Was it not an 'Area Rule' issue that caused structural problems on the Gloster Javelin?

[GLX]

BIK_116.80 & Keith. Thanks to you both for your answers.

Keith - after your explanation I thought that you ought to be teaching the subject as the explanation was so well delivered. Then I read your profile - wish I'd had teaching like that when I went through my fixed wing P of F. Thank you

[Transition Layer]

Super response Keith. I read a lot of stuff on Tech Log which simply does not sink in, but I think I managed to understand most of what you were saying. Very interesting stuff indeed.

As an aside, I just completed my Australian ATPL exams, and can't remember coming across this "Area Rule" in the Aerodynamics syllabus. Or maybe I just wasn't paying attention ;)

Cheers,
TL

[Dave Incognito]

Don't worry TL,

You will get to sift through all those aerody notes again when we take the inevitable resit next year. :D

[Pontius' Pilot]

Agreed, Keith what a great explanation. I have been lecturing high speed aero-d for many years and will definately add this explanation to my notes. Thanks.

[RedUnderTheBed]

Good explanation KW but from memory it only applies trans/super -sonically, which I think you forgot to mention.

Someone once told me the Mirage III WITH Matra missile was faster than the clean machine because it was 'area-ruled' for the missile and that the T-Bird was faster again because it was area-ruled clean and not with the missile - which may not hang off it anyway!

Any comments? :rolleyes:

[Ex Douglas Driver]

Its a similar case for the Skyhawk. While not specifically being area ruled, the TA-4K has better performance in the transonic arena, due to the large canopy area forward of the wing.

Offsetting this very marginal aerodynamic capability is that in the TA-4K the engine is down-rated, weighs 1000lbs more, carries 1000lbs less gas, the nose is 2.5 feet longer, and has a large lead weight in the tail to balance it out. All in all, the T-birds fly like a pig! http://www.stopstart.fsnet.co.uk/smilie/darkvisor.gif

[ 03 December 2001: Message edited by: Douglas Driver ]

[Seloco]

411A,
I think you'll find the reason for the shape of the rear end of the B747SP is somewhat more prosaic than area rule considerations. It was simply the desire to attach the standard 747 tail assembly to a considerably shorter fuselage, which as a consequence had to be narrowed somewhat more abruptly than normal. The end result may look a bit like area-ruling, but I bet it is not!