April 13, 2018, ©. Leeham News: In the last Corner we discussed the pressure distribution on a conventional airfoil and compared it with a modern Supercritical airfoil. The Supercritical airfoil (which is used on all modern airliners) achieves a higher cruise Mach and a lower transonic drag by accelerating the air over the wing to a lower supersonic speed than conventional airfoils.
What conventional and supercritical airfoils share is a pressure distribution making them unstable. We need to stabilize them on an aircraft.
We saw the airfoil pressure distributions on our conventional and supercritical airfoil had most of the lifting force at the front. This front heavy lifting force increases when the angle of attack of the airfoil is increased.
The wing on its own is unstable. It risks tipping over when subject to gusts (coming from below, they will increase the angle of attack) or when a pilot commands to high angle of attack against the air.
This is what happened to Northrop’s original flying wings before artificial stabilization could stop the pilot from raising the nose too far in the B2 bomber.
For an airliner, one will not trust artificial stabilization to save the aircraft from tipping over (for a Military bomber like the B2 and forthcoming B21 it’s OK). The aircraft must be naturally stable, meaning if all control systems froze, the airliner shall not tip over backwards like a B2 could do when subject to a strong gust.
To configure the aircraft with a canard stabilizing surface, like the Rutan VariEze (or Beech Starship) requires the canard surface to be highly loaded to guarantee it stalls before the main wing stalls (the aircraft then lowers the nose, increases speed and regains stable flight).
The high loading of a limited span canard will create unacceptable levels of induced drag. This is why canards are mainly limited to military aircraft which can be designed as semi-stable or unstable, taking away the high loading of the canard wing.
When the stabilizing surface is placed behind the wing as a horizontal tail, it works better. The horizontal tail can be designed with both positive, neutral or negative lift in normal flight. The designs with positive or neutral lift are trickier to get right and therefore uncommon. We will focus the common horizontal tailplane type, the ones with negative lift in normal flight.
Figure 1 shows an aircraft with a normal airliner arrangement of the wing and tailplanes. We can see the centre of weight is configured so lift has its centre a little behind the centre of weight. This is deliberate; otherwise, the aircraft would not be controllable in pitch with a simple horizontal tail set at a fixed incidence. The nose down tipping moment produced by the difference in centres for lift and weight is called the aircraft’s (static) stability margin.
To counter the tendency for the aircraft to dip the nose, the horizontal tail is working as a small wing flying upside down; it produces a small down-force. By it, it bends the aircraft nose up by the lever caused by distance to the aircraft centre of gravity.
When the wing experiences a gust, which increases the angle of attack of the wing and lessens the (inverted) angle of attack of the tailplane, the downward force of the horizontal tailplane will reduce. The centre of gravity will now tip the aircraft forward, as the tail is no longer as strong in keeping the tail down.
The tipping of the aircraft forward will increase its speed. The wing’s increased lifting force then tips the aircraft nose up, as does the increased downforce from the tail. The aircraft gradually finds an equilibrium, called the trimmed speed.
In the next Corner, we will discuss aircraft stability further, as the trimmed speed will not be fully stable.