About the Author

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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Monday, October 18, 2010

How High Can You Fly?

I was recently reading a thread on an online forum (How high can You Fly? at www.aviationbanter.com).

The question asked by the original poster was basically, “If engine limitations disappear, how high could an aircraft fly, given aerodynamic and airframe limitations?”. Interesting question, I thought. Unfortunately, the thread degenerated into personal sniping and, in many cases, a demonstrated lack of understanding of the problem(s) being discussed. So I thought I’d take a run at the question here.

First of all, engine limitations will never really disappear. That is, unless someone (who will subsequently become very rich) finds a way to circumvent the Second Law of Thermodynamics. However, with enough technological advances, these limitations can become practically insignificant in comparison to other limitations we face. So let’s look at the question from the idealized position of no engine limitations other than the fact that the engine must be air breathing (for combustion or some other chemical reaction) or at least air processing (for a propeller or fan).

The fact is, for currently existing airframes, eliminating engine limitations wouldn’t significantly improve maximum altitudes. It would almost certainly improve cruise and climb performance within the existing altitude range—consider an engine that produces more power from a the same weight, or one that doesn’t lose power as altitude is gained. It might also lead to small improvements in maximum altitude. But big changes in maximum altitude would also require changes to the airframe design.

Two non-engine problems that show up at high altitudes are critical Mach (resulting in the so called "Coffin Corner"), which is due to compressibility effects, and a decrease in the damping of flutter, which is the result of reduced air density.

Critical Mach (Mcrit) is the lowest speed at which a portion of the airflow over the top of a wing travels at or above the speed of sound. As the airflow slows down again, it forms a shock wave, which causes airflow separation—leading to a loss of lift and a massive rise in drag. As a general rule, thinner wings have a higher Mcrit than thicker wings. This is one reason why high altitude aircraft tend to have thinner, more lightly cambered wings.

Mcrit becomes an issue because at higher altitudes, a given indicated airspeed corresponds to a higher true airspeed (due to reduced air density). At the same time, the speed of sound is getting lower (due to reduced air temperature). So for a given indicated airspeed, Mach number gets higher as altitude increases. Conversely, for a given Mach number, indicated airspeed gets lower as altitude increases. This means that Mcrit, which is a fixed function of wing design, corresponds to a progressively lower indicated airspeed as we climb higher.

Sooner or later as we climb, Mcrit enters into the cruise speed range. At some altitude, stall speed and Mcrit meet, and there is no speed at which controlled equilibrium flight can be maintained. No matter how good our engine is, we must remain below this altitude at all times. We must also be cognizant of the narrowing range of useable airspeeds available as we approach this altitude. As an example, the U-2 high altitude spy plane has a tiny speed margin at it’s operational (read “spying”) altitude. 5 knots too slow and it stalls, 5 knots too fast and it overspeeds! Airline pilots, flying aircraft that are designed for high altitudes, must be very aware of the reduced speed range available at higher altitudes. The problem shows up at much lower altitudes in aircraft that are not designed for high Mach number flight.

Airframe problem number 2 is the effect that altitude has on Vne. One of the defining features of flight at speeds over Vne is the presence of flutter, which is an unstable and destructive vibration in the airframe which is aggravated by high speed airflow. This vibration is damped at low altitude in the higher density air. The damping effectively increases the value of Vne. To a first approximation, we can treat Vne as a constant true airspeed v-speed.

So the bottom line here is that the indicated Vne actually drops as altitude increases—it isn’t constant, as we normally assume. Manufacturers account for this in many cases by adding a carefully determined safety factor that allows for reduced damping at the aircraft’s absolute ceiling. Some manufacturers choose instead to publish multiple Vne’s, which are lower at higher altitudes—up to the aircraft’s absolute ceiling. Either of these approaches works fine, but what happens if we strap a new engine onto the airframe and enable flight at higher altitudes? In the absence of additional manufacturer’s Vne data, flying to higher altitudes is a gamble. We could inadvertently exceed the altitude-appropriate Vne and find ourselves in a world of hurt. This problem is easy to fix if we have data on the Vne change with altitude. But we will still eventually run into problems because the margin between Vs and Vne is too narrow—assuming we don’t run into Mcrit problems first!

So all in all, it fair to say that if-and-when far superior engine technology becomes available, airframe design will have to be adapted accordingly. Simply bolting newer and better engines onto existing airframes won’t allow us to realize the full potential of these engines. Alas, this is not a problem we’ll likely need to address for many years yet!

Happy Flying!


Paul Rooks said...

Great, so you're saying I can't take my C150 above 10 and dive at Vo lol. Oh well. Great article though, keep it up!

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