Read the forum code of contact
By: - 24th November 2013 at 19:05 Permalink - Edited 1st January 1970 at 01:00
Supersonic jets are designed to a fineness ratio which typically exceeds a numerical value of 3.5. Bigger numbers are better. Fineness ratio is area in the X/Z plane divided by area in the Y/Z plane. But there are other factors which must be considered such as wing sweep, wing thickness and propulsion performance.
By: - 24th November 2013 at 20:56 Permalink - Edited 1st January 1970 at 01:00
This link may be interesting in the considerations, written for layman
http://www.mach-flyg.com/utg80/80jas_uc.html
By: - 24th November 2013 at 22:10 Permalink - Edited 1st January 1970 at 01:00
Supersonic jets are designed to a fineness ratio which typically exceeds a numerical value of 3.5. Bigger numbers are better. Fineness ratio is area in the X/Z plane divided by area in the Y/Z plane. But there are other factors which must be considered such as wing sweep, wing thickness and propulsion performance.
Fineness ratio, yes. That's where aircraft gains there nasty pointy shapes. But, do you remember the X3 Stiletto, the absolute king of the Pointy? That one was a hog to pass Mach 1.
So slenderness, yes. But all abt Whitecomb experimental works and Hayes theory is abt the according variation of plane section relative to the mach wave. The transition btw one section to another are linked by a governing equation in sinus of the mach number.
But what's about the coca bottle neck so famously shown in 70's, 80's fighters?
See it like this:
- Imagine a slice of air traveling around the body shape of the airplane. When it reaches the section containing the wings, there is a sudden increase in the value of the section. To compensate for that increase and stay in the limits defined by the area rules (a sinus something increase), the fuselage shape have to be narrowed to balance for the section increase (ex. the F106).
- On the contrary, when the slice passes trough the rear of plane (beyond the wing trailing edge), there is a brutal overall section decrease that must be balanced by a bump (see the Buccaneer) or by the positioning of a vertical stabilizer.
The most interesting and vivid description of the area rules is by looking at pictures of the F102 and Buccaneer (both were adapted to the newly discovered theory). On the Dagger, you can trace its design along the years and be able to see the work of Whitecomb, just like an archaeologist (what Engineering sciences needs urgently in our troubled times).
I encourage you to read the according theory in a book of aerodynamics (didn't hve mine with me at the time writtng this here and that comment would hve been much better if I did). Many are of easy access and way better than extracts hooked randomly on the internet. But as this is an internet Thread, here is my try:
http://naca.central.cranfield.ac.uk/reports/1956/naca-report-1284.pdf
Hve a good read!
EDIT:
Forgot the main question (wing on top)!
Basically, wing placed at the middle of the fuselage are the most aero efficient. However, for structural issues the wing might hve to be put bellow the fuselage in order to benefits from a carry-trough spars (the cantilever beam). This lead generally to a Von Karman shaped being added at the wing root to smooth the shape transition at the trailing edge of the wing and prevent the creation of a vortex street. On today airplanes with trapezoidal/Delta, this can be achieved by the use of an angled trailing edge (M2K <> Mig21) and chines (F16).
Wing mounted on top, on fighter aircraft, usually results more of a trade off with structural concerns. A fighter is a small aircraft that needs to be turned around quickly without expensive heavy and cumbersome ladders and handling equipment for the maintainers while still be able to carry external loads under its wings. Hence it has to have a low landing gear that does not makes it seat like a crane out of reach of the ground personnel and limits the weight of the aircraft (landing gears are heavy parts and necessitate strengthened attachment points). There is also the take off and landing distances that factor in. The more AoA you are able to generate during the take-off roll for example, the more lift you can generate, hence shortening your take-off distance (and similary your landing distance).
So, to resume the above digression, you need a tall landing gear, but don't want your plane to seat on the tarmac like a crane out of maintainers hands reach or see its performances crawl under its undercarriage weight .
One simple solution to deal with that is to place the landing gear on the lower portion (and most rearward as structurally possible) of your aircraft and put the wing on the top of the fuselage (or shoulder mounted). It has some dynamics drawbacks (less kinematics agility) but that can be outpaced with structural and aero benefits (the staggered wing concept and the carry trough spars)) etc...
So, as you can see, wing on top, bellow or in the middle hve mostly nothing linked with area ruling (yes there is a frontal effect for wings on top but see it as a second round effect (something tailored to smooth choice alrdy made).
By: - 29th November 2013 at 11:41 Permalink - Edited 1st January 1970 at 01:00
See it like this:
- Imagine a slice of air traveling around the body shape of the airplane. When it reaches the section containing the wings, there is a sudden increase in the value of the section. To compensate for that increase and stay in the limits defined by the area rules (a sinus something increase), the fuselage shape have to be narrowed to balance for the section increase (ex. the F106).
Which is one of many CFTs design problemss... (apart from being positioned on the extrados, therefore dgrading aerodynamics))
Posts: 162
By: logical1 - 24th November 2013 at 13:52
Probably most everyone here knows about the area rule effect on transonic flight. So here is my question. Does wing placement have anything to do with the amount of area rule that is designed into a supersonic plane, and trasonic drag?
I look at an F-100, and the F8U Crusader. The F-100 has a mid placed wing, and the F8 has the wing placed on top of the fuselage. To the eye the F8 doesnt have nearly as much visible area rule. All of this centers around the fact that both aircraft weight about the same, and both have a J-57 engine. To be fair the J-57 in the F8 is a later dash model and does have more power. But-------------the F8 is almost 300 mph faster than the F-100.
Further, most of the latest jets starting with the F-14 and F-15 and a lot of Russian fighter have the wing on top. So------are there any aero engineers here that can comment on this?