Will a leading-edge root extension (LERX) improve the maneuver performance of a WWII fighter?
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The way these LE extensions work is by forming a vortex that rolls across the upper surface of the wing at very high AOA values. The strakes added in front of the vertical stab of a number of WWII fighters that initially had lateral stability problems when they added bubble canopies (Eg. P47-51). Initially the P47 would depart controlled flight if a yaw angle greater than 11 deg occurred because of vertical stab stall. The strake causes the vortex to form and inject energy into the boundary layer, retarding stall.
Delta wing aircraft can also achieve very high AOA values without a "stall" developing.
Note all these devices work with low aspect airfoils. So with the wing planform of WWII fighters, no... Additionally the power loading of modern jet fighters is in a different league than WWII piston pounders. One disadvantage of high AOA maneuvering is energy dissipation. Run out of energy (speed or altitude) and one is in a tight situation.
Delta wing aircraft can also achieve very high AOA values without a "stall" developing.
Note all these devices work with low aspect airfoils. So with the wing planform of WWII fighters, no... Additionally the power loading of modern jet fighters is in a different league than WWII piston pounders. One disadvantage of high AOA maneuvering is energy dissipation. Run out of energy (speed or altitude) and one is in a tight situation.
wuzak
Captain
Supermarine tested leading edge extensions on the Spitfire wing, mainly trying to reduce drag, sort of mimicking a laminar flow wing.
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Do you know the result of that test? Does the LERX reduce the drag as the laminar flow wing does? Thanks
They as a HIGH AOA device do not reduce drag, however in the above instance they may act as somewhat of a wing root fillet. This area in a typical WWII aircraft could be problematical. For example the minor rounded filleting of the P38 where the inner wing meets the "car" eliminated a rather sever vibration of the tailplane.
A satisfactory wing planform will stall the wing roots first and the tips last, allowing roll control down to near the stall. With a fairly straight wing planform of medium to high aspect ratio the above mentioned devices would not be helpful.
On laminar flow wings, drag is reduced by moving the CL of the wing aft, delaying boundary layer separation at low AOA values. The cruise efficiency is bought at the expense of often poorer high AOA abilities.
A satisfactory wing planform will stall the wing roots first and the tips last, allowing roll control down to near the stall. With a fairly straight wing planform of medium to high aspect ratio the above mentioned devices would not be helpful.
On laminar flow wings, drag is reduced by moving the CL of the wing aft, delaying boundary layer separation at low AOA values. The cruise efficiency is bought at the expense of often poorer high AOA abilities.
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GregP
Major
The vertical fin fillet on the P-47 and P-51 was NOT really a leading not extension. It was a simple addition to vertical surface area to compensate to the loss of vertical area when the turtledeck was removed for the bubble canopies. The moment of the area of the added dorsal fin is the product of the dorsal fin area and the distance from the center of lift to the center of the fin area. There is a certain cumulative moment of vertical area aft of the CL and, if you lose some, you have to add it back in to avoid losing some yaw stability.
You have to do the same thing in RC aircraft when you put a front rudder or canard on a conventional aircraft for extreme aerobatics. The moment forward of the center of lift must be accompanied by a similar moment addition aft or you get uncontrolled flight.
Leading edge extensions work on vortex generation impacting the main wing in sustained forward flight when the LEX is more or less unbanked relative to the airflow. That is never the case for a vertical fin dorsal fin addition. The vertical fin of the P-51 or P-47 was never stalled in forward flight for normal yaw angles; it simply had some turbulence impacting it. The vertical fin of a WWII fighter is never stalled perhaps unless you manage to to a snap roll, which is not a combat maneuver, and is prohibited in most WWII fighters, especially later ones with higher speed capabilities.
While the dorsal extension might function similarly to an LEX for very small time increments, it's primary function was to restore stable flight at very small increments away from straight flight. In other words, to make the fighter fly essentially feet off the rudder in normal forward flight. The pilots had enough on their plates without having to continually use rudder when flying in loose formation, and it was getting tiresome for them. The dorsal fins extensions were made to simply reduce pilot rudder workload, particularly on long missions.
You have to do the same thing in RC aircraft when you put a front rudder or canard on a conventional aircraft for extreme aerobatics. The moment forward of the center of lift must be accompanied by a similar moment addition aft or you get uncontrolled flight.
Leading edge extensions work on vortex generation impacting the main wing in sustained forward flight when the LEX is more or less unbanked relative to the airflow. That is never the case for a vertical fin dorsal fin addition. The vertical fin of the P-51 or P-47 was never stalled in forward flight for normal yaw angles; it simply had some turbulence impacting it. The vertical fin of a WWII fighter is never stalled perhaps unless you manage to to a snap roll, which is not a combat maneuver, and is prohibited in most WWII fighters, especially later ones with higher speed capabilities.
While the dorsal extension might function similarly to an LEX for very small time increments, it's primary function was to restore stable flight at very small increments away from straight flight. In other words, to make the fighter fly essentially feet off the rudder in normal forward flight. The pilots had enough on their plates without having to continually use rudder when flying in loose formation, and it was getting tiresome for them. The dorsal fins extensions were made to simply reduce pilot rudder workload, particularly on long missions.
The point, as made in the P-47 flight manual is that the aircraft will depart controlled flight at a yaw angle of 11 degrees. A typical rudder should be capable of about 15 deg yaw angle creation. Why is so much authority needed? Torque and P factor at low speed and high power. Whether or not most WWII fighter pilots could fly in formation with feet on the floor is open to question. I wouldn't fly the 185, Beaver, Herk or even 747 with feet on the floor (unless it was on autopilot).
Happy New Year all!
Happy New Year all!
MIflyer
1st Lieutenant
The Ercoupe has a wing to fuselage leading edge fillet that has a built in stall strip to help ensure the root stalls first.
I know of someone who owns an Ercoupe where a previous owner apparently thought that stall strip was a manufacturing imperfection and smoothed it out.
I know of someone who owns an Ercoupe where a previous owner apparently thought that stall strip was a manufacturing imperfection and smoothed it out.
With regards to Vertical fin stall and strakes:
This means that at some sideslip (around 15° for most configurations) the vertical tail stalls and its stabilizing side force decreases with further increase in the sideslip angle, whereas the destabilizing influence of the fuselage increases further. The aircraft becomes laterally unstable pretty soon.
To buy a little more stability around the point where the vertical tail stalls, these strakes have been added. They have a high leading edge sweep, so they will produce vortex lift just like a highly swept delta wing, only sideways. The suction force of this vortex will act on the root of the vertical, just were it has the biggest chord. In fact, this helps to keep most of the vertical active, because now it becomes part of that delta wing ahead of it.
This means that at some sideslip (around 15° for most configurations) the vertical tail stalls and its stabilizing side force decreases with further increase in the sideslip angle, whereas the destabilizing influence of the fuselage increases further. The aircraft becomes laterally unstable pretty soon.
To buy a little more stability around the point where the vertical tail stalls, these strakes have been added. They have a high leading edge sweep, so they will produce vortex lift just like a highly swept delta wing, only sideways. The suction force of this vortex will act on the root of the vertical, just were it has the biggest chord. In fact, this helps to keep most of the vertical active, because now it becomes part of that delta wing ahead of it.
Shortround6
Major General
Maybe I am way wrong but here is a picture (I think ?) of an LERX in action
Notice the contrails coming off the wing tips vs the contrails coming off the LERX
Yes the LERX is helping the lift but as someone said earlier, the chances of a WW II fighter being able to pull that kind of angle of attack ( wing tip contrails compared to the wing showing angle of attack?) for very long without bleeding of speed and stalling are pretty slim.
Notice the contrails coming off the wing tips vs the contrails coming off the LERX
Yes the LERX is helping the lift but as someone said earlier, the chances of a WW II fighter being able to pull that kind of angle of attack ( wing tip contrails compared to the wing showing angle of attack?) for very long without bleeding of speed and stalling are pretty slim.
Notice that both leading edge devices and flaps are deployed in the above photo. With a short low aspect swept wing considerable lift is retained, at the cost of high energy loss. Also roll control is still available via the elevons. For a high aspect non swept wing of a WWII aircraft the effect would be negligible, the energy loss hard to recover and lateral control bad and tendency to spin out of the maneuver quite high.
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- #14
Stoxm,
The jet in the video is generating enough AOA to create the vortices. When looking at a modern jet fighter, particularly those that are FBW, don't compare them too closely to WW2 metal as they are literally worlds apart performance wise.
The F16/22 both have vortices on the airframe while the Eagle would get them on top of the wings. The Fulcrum would get them on the engine louvers while turning hard / creating some serious lift.
Cheers,
Biff
Shortround6
Major General
AOA is angle of attack and refers to the angle the wing is at in relation to the direction the aircraft is actually traveling in. The airplane does not need to be flying horizontally.
Any plane that is doing a hard turn has a high angle of attack.
wuzak
Captain
I can't find the story at the moment. It was somewhere in Morgan and Shacklady.
IIRC it did make some improvement, but not enough to incorporate in production.
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- #18
So, the LERX can improve the turn performance? Thanks
The JF-17. A materialized doctoral thesis about common sense.
