Page 3 of 3

Re: Why the small wing?

Unread postPosted: 01 Apr 2019, 17:50
by johnwill
madrat wrote:Johnwill-

The LERX on F-16 has always intrigued me. I notice the F-16 has a very narrow LERX similar to the Hornet. Was a wider LERX, like on a Super Hornet, counter productive? The F-35 seems to use a decoupled LERX with the chine from the upper intake forming sort of a bi-plane effect not really dissimilar than how Rafale shapes the canard shoulder over the leading edge of the main wing. I have to imagine again, the net effect just wasn't a benefit or we would have seen something similar backfir to the F-16. Was the LERX in the F-16 changed much from the concept to the final product, e.g. to get to its current shape?


LERX on a twin vertical tail airplane can be a can of worms due to interaction of the vortices with the tails. The Hornet found that out the hard way, so ever since then, designers have been very careful to avoid that problem while still meeting the RCS, lift, and drag needs. As always, the final design is a compromise between several competing requirements.

Several different shapes were studied during design of YF-16, but once the shape was selected it wasn't changed, even to the current airplanes.

Re: Why the small wing?

Unread postPosted: 02 Apr 2019, 23:29
by n3sk
So it seems the wing design of the F-16 is the best compromise. Any change causes issues that are not worth the effort or negatively affect another aspect of performance.

The CTF’s do cause minor yaw control issues and high speed. They do not and much meaningful lift.

Power is the answer to make up for the added weight.

One place I commonly see vapor vortices is the lerx and wings leading edge meet. If there is enough pressure to squeeze moisture out of the air, is this also a sign of turbulence and drag??

This question is a bit off topic but relates. These vortices mentioned which cause problems for twin tail jet, is that why the Su-57 has levcons. Is it an attempt to alleviate this issue and smooth out the airflow over the top of the fuselage. Will we see these in future design?

Re: Why the small wing?

Unread postPosted: 03 Apr 2019, 02:23
by madrat
Levcons seem to act like Handley-Page slats. keeping the front edge inline with airflow then smooths laminar airflow over the wing I reckon. Just from an eyeball guess, it probably helps smooth airflow at high AoA.

Instead of levcons, I think any future changes on a design should be to cut drag. I wonder how a Y-tail like on F-117A would function on an F-16 by eliminating both the vertical stabilizer and horizontal tail planes. Maybe the ventral side of the tail section could be slat free with some sort of aerodynamic package that more closely conforms to the body. Stuff like that. The downside is it's money thrown into the wind since we already have F-35A.

Re: Why the small wing?

Unread postPosted: 03 Apr 2019, 05:20
by saberrider
n3sk wrote:So it seems the wing design of the F-16 is the best compromise. Any change causes issues that are not worth the effort or negatively affect another aspect of performance.

The CTF’s do cause minor yaw control issues and high speed. They do not and much meaningful lift.

Power is the answer to make up for the added weight.

One place I commonly see vapor vortices is the lerx and wings leading edge meet. If there is enough pressure to squeeze moisture out of the air, is this also a sign of turbulence and drag??

This question is a bit off topic but relates. These vortices mentioned which cause problems for twin tail jet, is that why the Su-57 has levcons. Is it an attempt to alleviate this issue and smooth out the airflow over the top of the fuselage. Will we see these in future design?

The vortices is a s presense from the medium AOA in flight on F16 LEX's. Sweept angle (more 60 degree's) create them at lowAOA. They're being useful at high pitch angles when WINGS start to loose lift to drag ( increase)

Re: Why the small wing?

Unread postPosted: 21 Apr 2019, 09:53
by saberrider
The wings are small (wingspan wise) to accomplish minimum drag request for the payloads .

Re: Why the small wing?

Unread postPosted: 21 Apr 2019, 20:49
by vilters
The production F-16's wing is 10% bigger then the YF-16 wing, and many of us want to torture the clowns that shelved the XL-F-16 WIng.

Re: Why the small wing?

Unread postPosted: 19 Aug 2019, 13:18
by secretprojects
madrat wrote:Johnwill-

The LERX on F-16 has always intrigued me. I notice the F-16 has a very narrow LERX similar to the Hornet. Was a wider LERX, like on a Super Hornet, counter productive? The F-35 seems to use a decoupled LERX with the chine from the upper intake forming sort of a bi-plane effect not really dissimilar than how Rafale shapes the canard shoulder over the leading edge of the main wing. I have to imagine again, the net effect just wasn't a benefit or we would have seen something similar backfir to the F-16. Was the LERX in the F-16 changed much from the concept to the final product, e.g. to get to its current shape?


The answer to the last part is - yes, very much changed.

In the beginning, the LERX/strake was a wide forebody with blunt leading edges:

The aerodynamic design concept of obtaining high-lift coefficients at transonic speeds by the use of wide forebody shapes has been an integral feature of most General Dynamics fighter designs since before the FX (F-15) competition. The concept was initially wind-tunnel-tested in 1966. Sharp, narrow forebody strakes were also investigated briefly at that time; however, it was then considered an advantage to produce the lift with a blunt leading edge in order to maintain attached flow and greater leading-edge suction for lower drag.


When wind tunnel tested, this configuration (401F-0 with twin vertical tails) did give high lift, but had very poor directional stability at moderate to high angles of attack.

At this point, NASA/Langley Research Center aerodynamicists were consulted. They suggested that the lift of the wide forebody could be increased by sharpening the leading edge to strengthen the vortices rather than weaken them as
our earlier attempts had done. The point was that forebody separation is inevitable at very high angle of attack; therefore, the lift advantages offered by sharp leading edges should be exploited. This also would allow the forebody vortices to dominate and stabilize the high-angle-of-attack flow field over the entire aircraft, even improving the flow over the outboard wing panels.


Lots of different strake shapes were tested - I count more than 133 variations in the data tables.

Two series of parametric forebody strake tests were initiated. A series of delta planform strakes were designed for testing on the conventional forebody aircraft Configurations 785 (single vertical tail) and 786 (twin vertical tail). The second series had curved planforms (gothic and ogee) and were designed for testing on the blended configuration, 401F-5. These two parametric transonic tests provided the basic data for all the other evolutionary forebody strake tests, which throughout the YF-16 and F-16 development as various design changes required re-evaluation of the strake effectiveness.


However, once the final shape was settled, it hasn't been altered since.

Source:
NASA Contractor Report 3053
Aerodynamics of Forebody and Nose Strakes Based on F-16 Wind Tunnel Test Experience
Volume I: Summary and Analysis
C. W. Smith, J. N. Ralston, and H. W. Mann


https://ntrs.nasa.gov/archive/nasa/casi ... 019972.pdf

Re: Why the small wing?

Unread postPosted: 19 Aug 2019, 15:35
by sprstdlyscottsmn
Excellent find!

Re: Why the small wing?

Unread postPosted: 20 Aug 2019, 04:20
by zero-one
I always wondered why wasn't the F-35C design chosen as the base form F-35.

We've seen engine thrust added without an increase in fuel consumption. And the F-35 is expected to have this as well.

So wouldn't the C model eventually reach F-35A levels of acceleration and E bleed. The A on the other hand will never reach the C model's lift generation, fuel capacity, range etc.

Re: Why the small wing?

Unread postPosted: 20 Aug 2019, 13:29
by secretprojects
sprstdlyscottsmn wrote:Excellent find!

Thanks!

I've been researching the evolution of the YF-16 design for a while. Here's thumbnails of an 18 page PDF I've put together so far. Long term goal is a comprehensive history of the LWF program including design history of all proposals.

F-16DesignEvolution.jpg