How to avoid a normal shockwave
We have indicated that the manner by which we increase the efficiency of propulsion is by removing the normal shockwave that resides on every propeller, rotor and fan blade that is used to derive thrust. To achieve this, all we have to do is stop compression occurring at the point which results in the formation of the normal shock. The compression results as the flow over a surface of the blade decelerates to a point where the hitherto sonic flow (V0/Va >1.0, where Va is the local speed of sound) is no longer greater than M1.0. There are two manners to achieve this condition,
- Don’t exceed M 1.0 in the first place;
- Don’t slow down below M 1.0, if the flow velocity has exceeded M 1.0 upstream.
Don’t exceed Mach 1
Supercritical airfoils attempt to follow this proposition in part. By avoiding accelerating the flow on the suction side to velocities above Mach 1.0, a normal shock is avoided. (Note: Richard Whitcomb is considered the inventor of the supercritical airfoil, and as a design to achieve a specific outcome that is probably reasonable. However, the German aerodynamicist K. A. Kawalki penned a section that is consistent with supercritical airfoil designs of today, back in 1940 at Deutche Versuchsanstalt für Luftfarht, (“DLR”)).
In practice, the supercritical airfoils still result in a normal shock, but it is attenuated and in principle moved rearwards thus causing less adverse impact to performance. As an aside, that is what is supposed to happen; however, the suction face flow is very sensitive to boundary layer conditions, so the placement of an aft facing step in the form of the slat trailing edge disrupts the boundary layer to the extent that the resultant shock structure is to be found far further forward on an airfoil than anticipated. Details matter.
Don’t let the flow decelerate below Mach 1.0
This is what we do. If the flow velocity is greater than Mach 1.0 across the whole of the suction face, no compression occurs, there is no normal shock. Yes, you can have oblique shocks at the leading edge, and there is some interesting stuff going on at the rear, but it turns out, that is relatively straight forward.If the “normal shock” is removed, what happens?
The below happens. The images show the result of the left engine having been modified with a demonstration-level material, enough to show entertaining results. The right engine was standard.
Note that the left engine was due to have modules retired, it was end of life, and a ssuch, fuel flow for equal thrust was 5% higher than the right engine, EGT was 22C higher in the range 600-750C. The image is a screen capture of a continuous video made during this flight, with some annotations:
The rest of this story is, removal of shock reduces vibration. In the case above, the engine had high vibration, at scale limit, until modified, where the vibration reduced by slightly more than 50%. This is not an aberration, it is an effect that occurs on propellers, helicopter rotors, and fan blades.
Back to:
A Shallow Wade Into Propulsion
A Deeper Dive into Propulsion
An Even Deeper Dive into Propulsion
Further reading:
A Really Deep Dive into Propulsion
