LEF and LERX for F-15

Military aircraft - Post cold war aircraft, including for example B-2, Gripen, F-18E/F Super Hornet, Rafale, and Typhoon.
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garrya

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Unread post20 Jul 2017, 03:07

How come leading edge flaps and leading edge extensions never implemented on any version of the F-15? Even some F-4 versions has leading edge flaps to improve close in combat capabilites , the F-5 has both LEF and LERX, the technologies must be available when F-15 was designed.
So what is the reason for not putting those things on F-15?
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There was a version of F-15MTD with both canard and TVC, how come things like LEF and LERX were never tested?
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basher54321

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Unread post20 Jul 2017, 21:09

Think you need F-15.net :D

http://www.joebaugher.com/usaf_fighters/f15_2.html

The F-15A has a very large, cantilever, shoulder-mounted fixed-geometry wing, swept back at a 45 degree-angle. The wing area is 608 square feat, offering a low wing loading and providing excellent combat agility. The wing is set at zero incidence, and has a slight amount (one degree) of anhedral in order to reduce stability in the rolling plane. The wing is a multi-celled, three spar structure with multi-stiffened skins. In contrast to some other modern fighters, the F-15A has conventional outboard ailerons and conventional flaps, and no other control surfaces. In particular, no spoilers or leading-edge extendible slats are fitted. The wing area is sufficiently large that no slotting or blowing is needed to achieve acceptably low landing speeds. The use of variable camber with movable surfaces on both the leading and trailing edges of the wing was ultimately rejected by the design team, since a design with a fixed leading edge employing conical camber offered only slightly higher supersonic drag and only marginally reduced subsonic performance, both of which were more than offset by increased advantages in terms of reduced weight, simplicity of manufacture, and ease of maintenance.

Early in the development program, the builder removed three square feet of area from the trailing edge of wing tip on each side beginning with the 4th aircraft in order to cure a problem encountered with severe buffet experienced above 30,000 feet at speeds between Mach 0.9 and 0.95 and at 6g or more. This created the characteristic raked wingtips of the F-15
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botsing

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Unread post20 Jul 2017, 21:35

The front part where the wings are attached to the body are some kind of thick LERX.

Check how they (and also the intake ramps?) produce vortices:
Image

Image

I would also think that putting anything in front the variable intake ramps of the F-15 would complicate the design a lot beyond what it currently is:
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garrya

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Unread post21 Jul 2017, 02:17

basher54321 wrote:Think you need F-15.net :D

http://www.joebaugher.com/usaf_fighters/f15_2.html

The F-15A has a very large, cantilever, shoulder-mounted fixed-geometry wing, swept back at a 45 degree-angle. The wing area is 608 square feat, offering a low wing loading and providing excellent combat agility. The wing is set at zero incidence, and has a slight amount (one degree) of anhedral in order to reduce stability in the rolling plane. The wing is a multi-celled, three spar structure with multi-stiffened skins. In contrast to some other modern fighters, the F-15A has conventional outboard ailerons and conventional flaps, and no other control surfaces. In particular, no spoilers or leading-edge extendible slats are fitted. The wing area is sufficiently large that no slotting or blowing is needed to achieve acceptably low landing speeds. The use of variable camber with movable surfaces on both the leading and trailing edges of the wing was ultimately rejected by the design team, since a design with a fixed leading edge employing conical camber offered only slightly higher supersonic drag and only marginally reduced subsonic performance, both of which were more than offset by increased advantages in terms of reduced weight, simplicity of manufacture, and ease of maintenance.

Early in the development program, the builder removed three square feet of area from the trailing edge of wing tip on each side beginning with the 4th aircraft in order to cure a problem encountered with severe buffet experienced above 30,000 feet at speeds between Mach 0.9 and 0.95 and at 6g or more. This created the characteristic raked wingtips of the F-15

So basically, the F-15 didn't have LEF and LERX to reduce weight and maintenance? Seem like a bit of a waste to me, it could be much better dogfighter than F-16 at low altitude if LERX and LEF was implemented, since it already have very low wing loading
Last edited by garrya on 21 Jul 2017, 02:59, edited 1 time in total.
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garrya

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Unread post21 Jul 2017, 02:23

botsing wrote:The front part where the wings are attached to the body are some kind of thick LERX.

Check how they (and also the intake ramps?) produce vortices:
Image

IIRC Johnwill said that LERX need to be sharp to create vortex, F-15 wing gloves are rather blunt. It may be able to create vortex but much inferior to proper LERX such as the one on F-16
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eloise

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Unread post21 Jul 2017, 03:02

garrya wrote:So basically, the F-15 didn't have LEF and LERX to reduce weight and maintenance? Seem like a bit of a waste to me, it could be much better dogfighter than F-16 at low altitude if LERX and LEF was implemented, since it already have very low wing loading

F-5 had both leading edge slot and leading edge extensions but still lose to F-15 in dogfight, i think F-16 strength came from negative stability tail lift and T/W
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sferrin

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Unread post21 Jul 2017, 11:52

Apparently those "nodding" intakes on the F-15 are also used as control surfaces, similar to PAK-FAs "levcons".
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Unread post21 Jul 2017, 13:02

garrya wrote:So basically, the F-15 didn't have LEF and LERX to reduce weight and maintenance? Seem like a bit of a waste to me, it could be much better dogfighter than F-16 at low altitude if LERX and LEF was implemented, since it already have very low wing loading


If Boyd had had his way it might have been more like that but what they did probably met USAF requirements so was deemed good enough - I remember a comment from Harry Hillaker after meeting their design team that they went with a lower risk conventional design because of the cost and time involved.

Also about compromise - I had read its size was dictated to some degree by the radar antenna they wanted to get in the nose - and if they added LEFs they might have had to ditch putting AIM-9s on the side of the drop tank pylons.
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Unread post21 Jul 2017, 13:46

basher54321 wrote:Also about compromise - I had read its size was dictated to some degree by the radar antenna they wanted to get in the nose - and if they added LEFs they might have had to ditch putting AIM-9s on the side of the drop tank pylons.

This would have been a consideration on the F-4E too as well. In the end the F-15 always had great turning performance. Adding LEF would have added weight which would have reduced acceleration and climb performance even if we assume the added lift overcomes the added weight for turning.

I would have liked to see the switch to -229 motors in the C to improve sustained performance, but that is an expensive pipe dream. I did get to talk to the F-15 ACTIVE team once and I asked if they put the -229s in, they just smiled and said "bigger."
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sferrin

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Unread post21 Jul 2017, 14:58

sprstdlyscottsmn wrote: I did get to talk to the F-15 ACTIVE team once and I asked if they put the -229s in, they just smiled and said "bigger."


The -232 with TVC, which was bench tested up to 37,150lbs thrust back in the 90s, might be what they're referring to. :twisted: :shock:

http://aviationweek.com/awin/pws-229a-e ... -500-hours

https://www.flightglobal.com/news/artic ... ing-32888/

A brief write-up of that test used to be on the P&W but it was long since removed. The same engine was tested with TVC.

(This also makes one wonder what the F119 is truly capable of.)
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sprstdlyscottsmn

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Unread post21 Jul 2017, 21:48

sferrin wrote:The -232 with TVC, which was bench tested up to 37,150lbs thrust back in the 90s, might be what they're referring to. :twisted: :shock:

That is what I took it to be as well. Just thinking of an Eagle with those motors.... You would have to be very gentile with your afterburners or else you will be out of gas in a hurry.
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Unread post21 May 2019, 22:46

Basher points out two correct reasons:

1. Complexity & maintenance reduction
2. LE slats or flaps would leave no room for missiles over the drop tanks

So instead the designers went for a large wing area and a relatively thick cambered wing profile. But whilst this did help, a lot more was needed to make up for a lack of automatic leading & trailing edge high lift devices. So the next step was to improve the amount of lift that could be gained via the fuselage itself, and the addition of all moving intake ramps was one of the main solutions to this = as AoA increases these deflect downward to delay boundary layer seperation over the covered area of the top fuselage, thereby greatly increasing the lift over what would've otherwise been generated with fixed intakes.

These three things combined, 1) low wing loading 2) thick cambered wing & 3) all moving intakes helped the F-15 achieve a very respectable lift to weight ratio despite its lack of wing mounted high lift devices.

That said the penalties of lacking any automatic LE & TE slats and/or flaps wasn't completely negated, and as a result the F-15 is best suited to keep the fight fast, as the other teen fighters to a varying degree all feature better low ro medium speed ITR & STR.
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Unread post22 May 2019, 03:06

It would be interesting to see the variable intakes have an attachment to reshape airflow that goes over them to wrap more over the midline of the body. Shouldn't alter the airflow through the intakes.
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Unread post22 May 2019, 13:03

basher54321 wrote:Think you need F-15.net :D

http://www.joebaugher.com/usaf_fighters/f15_2.html

The F-15A has a very large, cantilever, shoulder-mounted fixed-geometry wing, swept back at a 45 degree-angle. The wing area is 608 square feat, offering a low wing loading and providing excellent combat agility. The wing is set at zero incidence, and has a slight amount (one degree) of anhedral in order to reduce stability in the rolling plane. The wing is a multi-celled, three spar structure with multi-stiffened skins. In contrast to some other modern fighters, the F-15A has conventional outboard ailerons and conventional flaps, and no other control surfaces. In particular, no spoilers or leading-edge extendible slats are fitted. The wing area is sufficiently large that no slotting or blowing is needed to achieve acceptably low landing speeds. The use of variable camber with movable surfaces on both the leading and trailing edges of the wing was ultimately rejected by the design team, since a design with a fixed leading edge employing conical camber offered only slightly higher supersonic drag and only marginally reduced subsonic performance, both of which were more than offset by increased advantages in terms of reduced weight, simplicity of manufacture, and ease of maintenance.

Early in the development program, the builder removed three square feet of area from the trailing edge of wing tip on each side beginning with the 4th aircraft in order to cure a problem encountered with severe buffet experienced above 30,000 feet at speeds between Mach 0.9 and 0.95 and at 6g or more. This created the characteristic raked wingtips of the F-15


Damn I love those raked wingtips... made it look so much more bada$$... :)

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