For decades we’ve made no significant gains in useful airspeed. The reason is nature’s speed limit, the speed of sound, Mach 1. But now there is some progress.
Exhibit A is the Gulfstream G650 which cruises efficiently at Mach .90, about 516 knots true airspeed. That’s more than 50 knots faster than the high speed cruise of other jets. Even the G650 long range cruise at Mach .85 is an advance of almost 30 knots over the high speed cruise of other jets.
Gulfstream has been able to push the speed limit forward through a better understanding of how air flowing at transonic speeds behaves, and how to design an entire airframe to minimize the drag of the inevitable shock waves.
Now we are seeing increased interest and investment in the dream of a supersonic private airplane. Aerion has been working on a supersonic airplane for more than a dozen years but has new capital, and new credibility, now that aerospace giant Airbus has made a significant investment. If Airbus believes an SSBJ is possible and is willing to invest should we believe?
Aerion’s chief aerodynamicist Richard Tracy holds patents on a natural laminar flow supersonic airfoil. That seems like a non sequitur. Supersonic flow is the opposite of laminar flow which is smooth and non-turbulent. As airflow reaches transonic speed a powerful shock wave builds which retards all airflow like a dam.
How could anything like smooth laminar flow be maintained at supersonic or even transonic speeds? That’s the Aerion secret, and it has worked in wind tunnel and actual airfoil testing in flight. And it’s what Airbus wants. Even if Airbus has no interest in building a supersonic airplane of its own it has huge stakes in reducing drag as airflow over its conventional airframes reaches transonic speeds even when the airplane itself is flying at Mach .80 or .85.
The speed of sound is also a very real speed barrier for propeller airplanes but we are seeing some progress there, too. Mach 1 places its speed limit on prop airplanes mainly by robbing the propeller of efficiency beyond a certain airspeed. The airspeed a propeller blade encounters is a combination of the rotational speed and the airspeed of the air entering the propeller disk. Once that airspeed goes transonic the same shock waves the hold back a jet airplane rob the prop of efficiency. No matter how much more power is applied to the propeller Mach effects restrict how much thrust it can deliver at higher airspeeds.
Unlimited Reno racers prove that a prop airplane can fly very fast, in excess of 500 mph. But they achieve those speeds at very low altitudes. Mach value increases as air temperature warms so a Reno racer flying at 500 mph in the warm low altitude air over the race course is flying at only about Mach .65. That’s fast, but still modest by jet speeds. And if the Reno racer climbed to an altitude where cruise can be efficient the speed would plummet because the airspeed value of Mach goes down and the propeller is laboring at a higher percentage of Mach.
But, again, we are seeing progress, namely in the very thin and swept prop blades Hartzell is making. It’s been known for decades that a thin airfoil and swept leading edge delay shock wave formation to a higher speed but until very strong composite fiber material was developed there was no way to make a propeller blade of the optimum shape. Now Hartzell can do it, and propeller airplanes like the TBM 900 are flying faster on the same amount of engine torque.
While we can see progress in pushing back nature’s speed limit there is an artificial limit that remains–laws against supersonic flight over land. The U.S. and many countries around the world forbid supersonic flight because of the unavoidable sonic boom. To be useful and worth the $80 to $100 million a supersonic private airplane would cost you need to fly supersonic over land, not just oceans.
At this point it’s difficult to know if nature or manmade laws are holding back development of a supersonic airplane. Gulfstream has invested in technology to suppress a boom but says it won’t go all-out until laws change and allow supersonic flight over land. Aerion believes it can quiet the boom and thus fly around the laws. Either way, legal as well as aerodynamic hurdles remain.
For those of us who marveled at the sleek models of supersonic jets built by Douglas, Boeing and others, and the exotic Concord built by the British and French, it’s sad to see those dreams of our youth still stuck on the shelf. Maybe nature has simply thrown up too big of a road block to fly efficiently with useful range at supersonic speeds. But maybe not. I didn’t believe I would be around to see an airplane cruise for more than 5,000 nm at Mach .90, but Gulfstream did it. Maybe our best minds can find ways to push nature back and resume our long delayed march toward ever faster and useful airplanes. I hope so.