Effect of downwash on tailplane [Archive]

Mad (Flt) Scientist

24th Oct 2003, 02:19

You're on the right track.

The main wing generates downwash as a by-product of lift generation; the amount of downwash is broadly proportional to the lift created. (Think of the momentum change in the airflow as being the 'equal and opposite reaction' to the lift force; a crude but useful way of thinking of it.

If the tailplane were not behind the wing, it would be in the undisturbed flow field, and the angkle of attack at the tailplane would be the angle of attack of the flow (alpha) plus the angle of the tail relative to the fuselage.

Behind a wing, however, you have to allow for the downwash in determining the flow field. So the tailplane angle of attack is equal to the fuselage angle of attack, plus the tailplane angle, minus the downwash angle.

As the downwash is a function of the fuselage angle of attack, this creates a situation where the effect of a (say) one degree shift in angle of attack for the plane overall may only cause a half degree shift in tail angle of attack, as the downwash cancels some of the change.

Allowing for this effective change in tail angle of attack is important, as it is often a large effect (50%, or so, is not unreasonable for quite normal configurations).

It's a lot easier with a diagram!

Keith.Williams.

24th Oct 2003, 02:41

In the vast majority of aircraft the C of G (where the weights acts downwards) is forward of the wing C of P (where the lift acts upwards). This produces a pitching moment which tends to make the aircraft pitch nose downwards.

To maintain any given attitude the tailplane must produce a balancing nose-up moment, by generating a downward lift force. This downward tailplane lift depends upon the tailplane area, its angle of attack and the dynamic pressure of the airflow over it. In order to generate donwward lift the tailplane must be either of negative camber or at a negative angle of attack, or both.

The angle of attack of any aerofoil is the angle between its chord line and the relative airflow. If we start by imagining that the airflow approaches the tailplane in a horizontal direction, the tailplane will need to be set leading edge down (or elevators up), in order to give a negative angle of attack.

But the airflow rarely approaches the tailplane in a truly horizontal direction. Ths is because the generation of lift by the wing causes the airflow to be deflected downards behind its trailing edge. This downwashed airflow increases the negativity of the angle of attack of the tailplane.

The greater the lift being generated by the wing or the lower the aircraft speed, the greater will be the downash. So for any given elevator or tailplane angle, the angle of attack of the tailpane will vary with the amount of lift being generated by the wings and the aircraft speed. So when trimming the aircraft at any given combination of weight, C of G position and speed, the angle of the tailplane or elevators must take account of this downwashed airflow.

Downwash from the wings will also affect the way the aircraft responds to flap deployment. Deploying trailing edge flaps increases wing lift and downwash. Most of the extra lift is generated at the rear part of the wing, so the C of P moves aft. This tends to cause the aircraft to pitch nose down.

But the increased downwash increases the negativity of the angle of attack of the tailplane. This in turn increases tailplane down force thereby generating a nose-up pitching moment. If this increased nose-up moment is greater than the nose-down moment caused by the shifting of the wing C of P, then the aircraft will pitch nose-up when the flaps are deployed. This effect is most common in light aircraft where the tailplane is only a short distance behind the wings.

Another effect of the wing downwash is to reduce the effectiveness of the tailplane in providing stability in pitch
If an aircraft suddenly pitches nose up, the tailplane down force must reduce (or be changed into an up force) in order to return the aircraft to its original attitude. These changes in tailplane lift are produced by changes in its angle of attack.

As the aircraft pitches nose-up, the leading edge of the tailplane also pitches up (or at least less down), which tends to reduce its negative angle of attack. This tends to reduce tailplane down force, thereby allowing the aircraft to pitch nose-down back to its original attitude.

But the pitching up of the wing also increases wing lift and downwash. This increased downwash tends to reduce the changes in angle of attack of the tailplane. Try to visualise this situation looking at the aircraft from the left side. As the aircraft rotates nose-up (in a clockwise direction), both the tailplane chord line and the downwashed relative airflow also tend to rotate in a clockwise direction. So the changes in tailplane angle of attack are less than they would have been if the relative airflow had remained unchanged. The overall effect of the changing downwash is therefore to reduce the stabilising effects of the tailplane.

This is of course all a gross simplification whch (I suspect) will add to.