February 2, 2018, ©. Leeham Co: In the last Corner, we discussed the basics of supersonic flow, to prepare for a supersonic and transonic drag discussion.
We will continue the supersonic aerodynamics discussion, however, as there are some further areas needing an explanation before we move on.
In the last Corner, I characterized what happens when the airflow passes through a supersonic shock wave. What I didn’t describe is an additional supersonic flow phenomenon called supersonic expansion fans. These occur when the air flows over a convex shape like the first part of an airliner wing with transonic flow over the top (Figure 3).
Luckily, a kind reader mailed me about this after last week’s Corner. So, let’s get a more complete picture around supersonic flow and what the primary phenomenons are and how these affect airplane flight before we move to the drag part.
There are two types of supersonic shock waves, oblique (or slanted) and normal shocks. These have slightly different characteristics. We start with the oblique shock.
The oblique shock is created when the flow is compressed by a sharp object or a concave area on the aeroplane. Figure 2 shows the shock which is created by a T38 jet trainer’s nose when flying supersonic (taken in a wind tunnel with special photo techniques).
The oblique shock normally results in a slightly slower supersonic flow behind the shock (in special cases the flow behind the oblique shock can transfer to subsonic flow). Flow density, pressure and temperature increases past the shock (we talk the normal static values here), the Mach number decreases (but stays supersonic in the normal case) and the flow changes direction away from the object after the shock.
A normal shock, like the end shock of the supersonic flow in our transonic wing from the last Corner (Figure 3), has the flow passing from supersonic to subsonic flow. Like for the oblique shock, the static density, pressure and temperature increase across the shock.
What I didn’t describe last time is that there is a third way the supersonic flow can change state when flowing around an object. If the flow passes a convex surface, like the first part of the top surface of the wing in Figure 3, it will change state through expansion fans (not shown in the figure, but the fans are inside the supersonic flow area).
Across an expansion fan the Mach number increases, the static density, pressure and temperature decreases. On the transonic wing in Figure 3, the pressure decreases in the area before the normal shock and lift is produced.
The supersonic shocks are used in aviation to condition the airflow during supersonic flight. Supersonic airliners (SSTs) are hot right now. We will cover the most worthwhile case of shock use for such aircraft. It’s for the engine inlets.
Turbojet and turbofan engines can’t work with supersonic inlet air hitting the fan or compressors. The ideal condition is air with ~M0.5 passing into the engine.
As we have seen, shocks slow the speed of air while increasing the static pressure. So, shocks can be used to condition the engine inlet air for an SST.
The normal straight or pitot inlet is blunt and therefore creates a normal shock. This takes the flow from supersonic to subsonic speed while increasing the pressure, as we want. But it’s a strong shock and we lose energy in the flow passing the shock. The pressure recovery in a normal shock inlet suffers as the shocks initial speed increases, Figure 4. The normal shock inlet is inefficient for high supersonic speeds.
For aircraft flying above M1.5, a multi-shock inlet gives less pressure losse in the intake. Figure 5 shows the increasing static pressure and slowing Mach in a multi-shock inlet.
Observe the direction change of the flow through the oblique shocks. This is used in the F35 “bump inlet” or SDI (Supersonic Diverterless Inlet) to direct the boundary layer around the ends of the intake lips.
The Concorde inlet (Figure 6) uses four weaker oblique shocks to slow the air down, giving an intake pressure recover like the F15 curve in Figure 4.
The shocks in a multi-shock inlet must be managed to direct the flow after the shock in the right direction and to keep the shocks weak. Therefore, advanced supersonic inlets have variable shock creating areas. Like the movable ramps used for the Concorde, F15, Su27, Typhoon inlets or the cone shock areas used for Mirage, Mig21 or SR71, Figure 7.