LERX/Chine and Wing/Body-Fairing Design Question

[KJ_Lesnick]

I'm looking at a picture of the Grumman-design concept that would ultimately become the F-14 (before it sprouted swing-wings), and the F-15, wondering what kind of LERX (chine) configuration is better for standard flying,aggressive maneuvering (and at high-mach, trim-drag benefits) at...
-Low airspeed?
-Intermediate Airspeed?
-High Airspeed?
-Transonic?
-Low Supersonic?
-Intermediate Supersonic?
-High Supersonic?

Additionally, I was looking at the wing-body fairings both planes feature. The Grumman Pre-F-14 concept features a pancake between the two engines (as does the F-14), and other than the chines, a little bit outboard on the sides of the engines (I think), where as the F-15 features much more pronounced wing-body fairings, it would appear, than the F-14, but possesses no pancake.

Which set-up works better for standard flying, aggressive-maneuvering (and at high mach-numbers, trim-drag benefits) at...
-Low Airspeed?
-Intermediate Airspeed?
-High-Airspeed?
-Low-Supersonic?
-Intermediate-Supersonic?
-High-Supersonic?


This isn't exactly something you can find on google, and can only find on an aviation-based forum. I have great respect for the knowledge members on this forum possess, even on very obscure aircraft and such, so I look forward to your replies.

Kendra Lesnick

 

[Sundog]

I'm not sure what you're referring to when you say "pancake." Do you mean the area of the fuselage between the engines on the F-14?

Anyway, separated engines like on the F-14 and Su-27 tend to be good in terms of isolating the engines from each other; meaning if one engine get's damaged it's not as likely to damage the other engine. However, having the separated engines increases surface area and hence, drag (Skin Friction Drag). It does tend to trap "air" between the nacelles, with the nacelles acting like large fences, so, IIRC, there is more lift generated by this configuration from the fuselage.

However, the separated nacelles also means all the mass of the engines are further away from the cg laterally, therefore, one would need larger flight control surfaces or greater leverage (span, or controls further out on the wing, which means a more stiffened wing, which means more weight, or larger flight controls, which does mean more weight and drag) to meet the roll rate requirements when compared with an aircraft that has the engines right next to each other in the fuselage.

However, if both types of approach are used, they can both meet the requirements. Looking at the YF-22 and YF-23 would be an example of this, in terms of LO type aircraft, where they both meet the mission requirements, but one with engines together and one with them apart.

As for the wing root fairings on the F-15, I would assume those are there for "volumetric considerations;" i.e.- they're probably there for area ruling and a place to put the cannon, fuel, etc., as much as for aerodynamic reasons.

The best chine, IMHO, is probably the one on the F-22, since it's the waterline around the forward fuselage and intakes and wouldn't add a lot of surface area, but still allows usable volume within the aircraft. i.e.- maximum aerodynamic efficiency, in cruise and high alpha, for minimum weight and drag.

 

[KJ_Lesnick]

Sundog,

I'm not sure what you're referring to when you say "pancake." Do you mean the area of the fuselage between the engines on the F-14?

Yup that would be it...

KJ

 

[F-14D]

Sundog,

I'm not sure what you're referring to when you say "pancake." Do you mean the area of the fuselage between the engines on the F-14?

Yup that would be it...

KJ

The area between the nacelles on the Tomcat contributed a significant portion of the aircraft's total wing area/lift especially when the wings were swept aft. The wings themselves are 565 sq ft., but the lifting body area contributes another 443 sq, ft, making the total lift area 1008 sq ft. This was one of the reasons the Tomcat turned much tighter and climbed better than most people expected (actually, better than Grumman itself expected). Of course, with the rotten TF30 engines, it couldn't sustain the really tight turns (even if the TF30s managed to keep running), but once the F110s were fitted and the Tomcat got the thrust it was designed for, there were many more who were surprised during ACM.

 

[KJ_Lesnick]

F-14D,

The area between the nacelles on the Tomcat contributed a significant portion of the aircraft's total wing area/lift especially when the wings were swept aft. The wings themselves are 565 sq ft., but the lifting body area contributes another 443 sq, ft, making the total lift area 1008 sq ft.

Wow, that's a lot of area!

Just to be absolutely clear, when you said the lifting-body area do you mean just the pancake, or the pancake and glove? Also, out of curiousity, does the F-15's 608-square-foot wing count the wing-body fairings (which to the best of my knowledge include the booms and the lifting area inboard of the booms which hold the vertical and horizontal stabs on)?

This was one of the reasons the Tomcat turned much tighter and climbed better than most people expected (actually, better than Grumman itself expected).

Are you serious?

Of course, with the rotten TF30 engines, it couldn't sustain the really tight turns (even if the TF30s managed to keep running), but once the F110s were fitted and the Tomcat got the thrust it was designed for, there were many more who were surprised during ACM.

Yeah, even F-15 pilots in mock dogfights against F-110 equipped F-14's were surprised at the improvement.


KJ_Lesnick

 

[KJ_Lesnick]

To Sundog,

I forgot to ask some questions...

Anyway, separated engines like on the F-14 and Su-27 tend to be good in terms of isolating the engines from each other; meaning if one engine get's damaged it's not as likely to damage the other engine. However, having the separated engines increases surface area and hence, drag (Skin Friction Drag). It does tend to trap "air" between the nacelles, with the nacelles acting like large fences, so, IIRC, there is more lift generated by this configuration from the fuselage.

Does the increase in lift outweigh the increase in drag?

However, the separated nacelles also means all the mass of the engines are further away from the cg laterally, therefore, one would need larger flight control surfaces or greater leverage (span, or controls further out on the wing, which means a more stiffened wing, which means more weight, or larger flight controls, which does mean more weight and drag) to meet the roll rate requirements when compared with an aircraft that has the engines right next to each other in the fuselage.

Does the Su-27 have a poor-roll-rate? How significant is the reduction in roll-rate?

However, if both types of approach are used, they can both meet the requirements. Looking at the YF-22 and YF-23 would be an example of this, in terms of LO type aircraft, where they both meet the mission requirements, but one with engines together and one with them apart.

What do you mean "both types of approach"?
-A mix of the widely-spaced engines and huge wings and big wing-body fairings?
-Or a mix of the pancake, combined with large wing-span for leverage and big flight-controls?

As for the wing root fairings on the F-15, I would assume those are there for "volumetric considerations;" i.e.- they're probably there for area ruling and a place to put the cannon, fuel, etc., as much as for aerodynamic reasons.

Well they definetly probably serve as a means to increase volume, although they do have aerodynamic functions as well.

The best chine, IMHO, is probably the one on the F-22, since it's the waterline around the forward fuselage and intakes and wouldn't add a lot of surface area, but still allows usable volume within the aircraft. i.e.- maximum aerodynamic efficiency, in cruise and high alpha, for minimum weight and drag.

Yeah, the chine is barely even visible yet it certainly produces a powerful enough vortex to do what's necessary. I don't know how well it would increase lift super-sonically to compensate for trim-drag, however the plane has a forward C/L (to provide it with super-maneuverability) so even when it shifts back, it shouldn't be too bad.


Kendra Lesnick

 

[weirc]

Hi, this report may prove usefull in the effects of intergrating the engines into the aft-end of fighter aircraft, it also provides some pointers for further reading in the references section.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890011555_1989011555.pdf

 

[KJ_Lesnick]

weirc,

Thank you for the NASA article, however at the time I'm working more at the front end (however this could come in useful).


KJ Lesnick

 

[KJ_Lesnick]

What are the advantages of each of these chine-shapes? What are their disadvantages?

1.) Chine starts out at the highest sweep angle, with the chine's sweep angle reducing the further aft you go. Eventually the chine meets with the much lesser swept wing?
2.) Chine starts out with the lowest sweep-angle, with the chine's sweep angle increasing the further aft you go. Eventually the chine meets with the much lesser swept wing?


KJ_Lesnick