Why LERX.net ?

 

Being impassioned by aerodynamics of any flying machine, I seek actively since a few years any document concerning the subject. It is thus quite naturally I chose the name of a plane's aerodynamic part to baptize my Website...

 

 

What does mean LERX ?

 

It is an English abbreviation of Leading Edge Root Extension who wants literally to say : Extension of the root of the leading edges.

Word corresponding in French is: APEX.

 

 

Where are they on the plane ?

 

LERX are the natural prolongations of the wings towards the nose of the plane. In some kind, they make the joint between  wings and  fuselage.

Put your mouse on the pictures below to see where are located the LERX.

 

 

 

 

 

 

But for what are used the LERX ?

 

We find it quasi exclusively on the modern fighters.

 

As we have done with the pictures above, and in order to illustrate what will follow, we will use the F/A-18 Hornet as example.

 

This airplane have LERX of very great dimensions so much so that his manufacturer, Northrop (absorbed since by Boeing), had baptized "Cobra " the preliminary projetct  due to the fact that this airplane seen on top resembled the reptile with these two surfaces behind the head.

 

All aerodynamic surface forming an angle higher than 30˚ with the axis of relative wind passing around it stalls,  because the air passing on the upper face of  this part changes from a laminar flow to a turbulent flow. The turbulet airflow create a fall of lift power. The wing takes down and the

plane falls!

 

This phenomenon is even more delicate at low speed due to the fact that the speed of the air moving on the top surface of the wing decreases

with the proper speed of the plane. Lift obtained by the wings being directly in relation with the speed of the airflow of the upper surface of

the wings, when the plane slows down, the lift power decreases.

 

It is possible to counter this phenomenon by 3 main ways:

 

- To increase speed obviously, but it's not what one seeks at the time of the landing for example...

- To increase the angle which the wing and the airflow are forming, this one being limited to 30˚ as we saw above.

- To increase camber of the wing. It is what we do when the plane extends the flaps and the mobile leading edges at the time

   of landing or takeoff.

 

Militarily speaking, it is interesting for a fighter, involved in a dogfight for example, to decrease its speed as much as possible in order to let

a faster airplane go ahead. By this manoeuvre, the fastest plane of both go ahead of its adversary and then allows the slower airplane

stay behind him in position of shooting.

 

It is vital for the fighters to fly fast to be able to reach a zone of intervention as soon as possible, but it is critical too to be able to fly as slow

as possible while keeping the plane control in order to be able to ensure itself to remain behind the adversary and therefore in offensive

position.

 

In that way, the LERX provide a considerable supplement of lift at raised angles of attack, i.e. when the angle formed by the wings and the

relative wind direction due to the displacement of the plane increases.

Indeed, the LERX generate powerful swirls of air or Vortex which increase the air velocity on the roots of wings and around tails. This makes

it possible to keep the control of the plane at angles superiors of 30˚. These Vortices when are intenses are particularly visible,

materialized by the condensation of the moisture of air.

 

 

 

LERX also allow the correct air feeding of the engines during those flights at high angles.

 

For viewing a video or pictures made by NASA at testing time of high angles flights, choose the links below.

Notice as tails beat due to the swirls's force generated by the LERX.

 

      Links towards the vidéos of NASA here.

      Links towards the photographs of NASA here.