Brian Abraham
13th Jun 2010, 10:28
The following is an edited article doing the rounds in the home build circles. Would anyone with credentials care to comment. Ghengis, JT? Thanks in advance.
Why isn’t every piston airplane turbo-normalized? This is a good place to remember Robert Heinlein’s wonderful acronym: TANSTAAFL. There Ain’t No Such Thing As A Free Lunch. As attractive as it appears at first, there are several mechanical arguments against turbocharging airplanes. One of the biggest is heat. If the engine is making full rated power, it must reject a certain amount of heat to stay with operating limits. This is exacerbated by the fact that compressing air makes it hotter. This is manageable if the airplane is in the lower atmosphere where there is plenty of cooling air, but if the engine is operating in very thin high-altitude air, there is a lot less mass to absorb heat. Soon cylinder head temperatures are beyond limits and oil is cooking. But these are mechanical details and people can devise mechanical solutions. They may be heavy, complicated and expensive, but they work.
No, the real problem is not mechanical. The real danger is exceeding the Never Exceed Speed, noted as Vne.
Many pilots assume that operating at high altitude (greater than 12,500 ft, say), even with the increased power supplied by a turbocharger, will not be a problem if the mechanical problems are solved. Sure, they can go faster, but not so much faster that they exceed the limitations marked in living color on the airspeed indicator. How, they ask with apparently perfect logic, can the airplane be exceeding Vne if the needle is in the green arc?
Because the airspeed indicator is The Gauge That Lies. Despite its name, an airspeed indicator does not measure speed. It measures “q” – dynamic pressure caused by packing air molecules into a tube. Now, several limiting speeds like stall speed (bottom of the green and white arcs), gust loads (top of the green arc), and maneuvering speed (blue line) are also functions of q, so they may be read directly off the dial. In these cases, the logic is true.
This logic is NOT true for the very important red line at the top of the yellow arc. Here’s why:
Consider an aircraft flying in smooth air at cruise speed. The aircraft structure is then slightly disturbed (such as by turbulence). In response, the aircraft structure will oscillate with amplitude decreasing until the oscillation stops altogether. This dynamically stable response is due to damping acting on the system, either from the aircraft structure and/or air. If the cruise speed is incrementally increased there will be a particular speed at which the amplitude of structural oscillation will remain constant. The speed at which constant amplitude oscillation can be first maintained is defined as the “critical flutter speed”, or more generically “flutter speed”. Flutter is almost a pretty word.
You’d associate it with butterflies and silk handkerchiefs. But in the engineering sense, it can be highly destructive. Once flutter has started, the amplitude may quickly become so large that a structure will disintegrate, literally shaken to pieces.
Remember, as the airplane climbs, there are fewer air molecules and less air pressure, so the needle on The Gauge That Lies reads a lower speed, even though the airplane is actually going just as fast. That’s why True airspeed is faster than Indicated. But flutter does not depend on Indicated Air Speed/dynamic pressure. It is directly related to True Air Speed — the velocity of the air passing by the airframe. The velocity of the excitation force is the prime concern, not the magnitude. It is very possible to exceed this critical “flutter speed” without encountering flutter if there is no initial disturbance. But if the critical flutter speed is exceeded and then a disturbance is encountered, the aircraft structure will begin to oscillate in response to the velocity of the passing air. This is not a typical resonance, where either increasing or decreasing the speed will move the aircraft away from the critical frequency and the vibration will stop on its own. Going faster merely pumps more energy into the system, increasing the amplitude of the flutter. Go faster, flutter harder. Only going slower and lowering the velocity of the air over the airframe will solve the problem.
RVs are designed presuming the installation of naturally aspirated engines (and pilots). Van’s flutter analysis is conservative, but not so conservative as to allow for the true airspeeds that might occur using an engine that can develop 75% of rated power up to altitudes of 20,000 feet or more.
Interestingly, airplanes without engines – let alone engines with turbochargers -- can encounter the same dangers. Sailplanes often fly at quite high altitudes. Those long, long wings tend to be flexible structures which makes them, potentially, quite susceptible to flutter. Sailplanes may not have engines, but they certainly have the equivalent of a lot of power in the Earth’s gravity. They also have very little drag. The combination means that they can accelerate very quickly indeed. A sailplane pilot who points the nose down at altitude could find himself in a grave situation very quickly. It is not uncommon to see charts in sailplane cockpits correlating the Vne to indicated airspeed.
The margin of safety narrows with altitude, and actually goes negative in some cases. A negative margin of safety is not considered desirable by passengers or insurance companies. Pilots, too, although they are superior beings with greater intellectual capacity, should be concerned. Superior intellect hits the earth just as hard, although it tends to be more surprised when it happens.
If you must hurtle through Mother Nature’s atmosphere at a speed higher than the Vne of the RV -10, it would be best if you found another airplane to do it in. Preferably one designed for the job.
We anticipated – hoped for, actually – a firestorm of discussion over the article in the 6 issue of 2004 concerning Vne. Well, we got a smoldering match head anyway. As I suspected, the idea that a major reason for establishing Vne was based on a True Airspeed number was news to some pilots – it certainly was to me. A couple of correspondents expressed concern, evidently thinking that somehow the safety margins of our airplanes had been narrowed.
A caller questioned using True airspeed when FAR Part 23 (the regulations governing certified aircraft) uses indicated airspeed. Again, the answer is about margins. If, for instance, an airplane with a normally aspirated engine is flying above 10,000 feet, the diminishing power will offset the increasing true airspeed by an amount that will make it impossible reach Vne. Whether the pilot is reading true or indicated doesn’t matter – he’s still within the margin of safety. It’s possible that knowing this, the writers of Part 23 decided to keep things simple.
Why isn’t every piston airplane turbo-normalized? This is a good place to remember Robert Heinlein’s wonderful acronym: TANSTAAFL. There Ain’t No Such Thing As A Free Lunch. As attractive as it appears at first, there are several mechanical arguments against turbocharging airplanes. One of the biggest is heat. If the engine is making full rated power, it must reject a certain amount of heat to stay with operating limits. This is exacerbated by the fact that compressing air makes it hotter. This is manageable if the airplane is in the lower atmosphere where there is plenty of cooling air, but if the engine is operating in very thin high-altitude air, there is a lot less mass to absorb heat. Soon cylinder head temperatures are beyond limits and oil is cooking. But these are mechanical details and people can devise mechanical solutions. They may be heavy, complicated and expensive, but they work.
No, the real problem is not mechanical. The real danger is exceeding the Never Exceed Speed, noted as Vne.
Many pilots assume that operating at high altitude (greater than 12,500 ft, say), even with the increased power supplied by a turbocharger, will not be a problem if the mechanical problems are solved. Sure, they can go faster, but not so much faster that they exceed the limitations marked in living color on the airspeed indicator. How, they ask with apparently perfect logic, can the airplane be exceeding Vne if the needle is in the green arc?
Because the airspeed indicator is The Gauge That Lies. Despite its name, an airspeed indicator does not measure speed. It measures “q” – dynamic pressure caused by packing air molecules into a tube. Now, several limiting speeds like stall speed (bottom of the green and white arcs), gust loads (top of the green arc), and maneuvering speed (blue line) are also functions of q, so they may be read directly off the dial. In these cases, the logic is true.
This logic is NOT true for the very important red line at the top of the yellow arc. Here’s why:
Consider an aircraft flying in smooth air at cruise speed. The aircraft structure is then slightly disturbed (such as by turbulence). In response, the aircraft structure will oscillate with amplitude decreasing until the oscillation stops altogether. This dynamically stable response is due to damping acting on the system, either from the aircraft structure and/or air. If the cruise speed is incrementally increased there will be a particular speed at which the amplitude of structural oscillation will remain constant. The speed at which constant amplitude oscillation can be first maintained is defined as the “critical flutter speed”, or more generically “flutter speed”. Flutter is almost a pretty word.
You’d associate it with butterflies and silk handkerchiefs. But in the engineering sense, it can be highly destructive. Once flutter has started, the amplitude may quickly become so large that a structure will disintegrate, literally shaken to pieces.
Remember, as the airplane climbs, there are fewer air molecules and less air pressure, so the needle on The Gauge That Lies reads a lower speed, even though the airplane is actually going just as fast. That’s why True airspeed is faster than Indicated. But flutter does not depend on Indicated Air Speed/dynamic pressure. It is directly related to True Air Speed — the velocity of the air passing by the airframe. The velocity of the excitation force is the prime concern, not the magnitude. It is very possible to exceed this critical “flutter speed” without encountering flutter if there is no initial disturbance. But if the critical flutter speed is exceeded and then a disturbance is encountered, the aircraft structure will begin to oscillate in response to the velocity of the passing air. This is not a typical resonance, where either increasing or decreasing the speed will move the aircraft away from the critical frequency and the vibration will stop on its own. Going faster merely pumps more energy into the system, increasing the amplitude of the flutter. Go faster, flutter harder. Only going slower and lowering the velocity of the air over the airframe will solve the problem.
RVs are designed presuming the installation of naturally aspirated engines (and pilots). Van’s flutter analysis is conservative, but not so conservative as to allow for the true airspeeds that might occur using an engine that can develop 75% of rated power up to altitudes of 20,000 feet or more.
Interestingly, airplanes without engines – let alone engines with turbochargers -- can encounter the same dangers. Sailplanes often fly at quite high altitudes. Those long, long wings tend to be flexible structures which makes them, potentially, quite susceptible to flutter. Sailplanes may not have engines, but they certainly have the equivalent of a lot of power in the Earth’s gravity. They also have very little drag. The combination means that they can accelerate very quickly indeed. A sailplane pilot who points the nose down at altitude could find himself in a grave situation very quickly. It is not uncommon to see charts in sailplane cockpits correlating the Vne to indicated airspeed.
The margin of safety narrows with altitude, and actually goes negative in some cases. A negative margin of safety is not considered desirable by passengers or insurance companies. Pilots, too, although they are superior beings with greater intellectual capacity, should be concerned. Superior intellect hits the earth just as hard, although it tends to be more surprised when it happens.
If you must hurtle through Mother Nature’s atmosphere at a speed higher than the Vne of the RV -10, it would be best if you found another airplane to do it in. Preferably one designed for the job.
We anticipated – hoped for, actually – a firestorm of discussion over the article in the 6 issue of 2004 concerning Vne. Well, we got a smoldering match head anyway. As I suspected, the idea that a major reason for establishing Vne was based on a True Airspeed number was news to some pilots – it certainly was to me. A couple of correspondents expressed concern, evidently thinking that somehow the safety margins of our airplanes had been narrowed.
A caller questioned using True airspeed when FAR Part 23 (the regulations governing certified aircraft) uses indicated airspeed. Again, the answer is about margins. If, for instance, an airplane with a normally aspirated engine is flying above 10,000 feet, the diminishing power will offset the increasing true airspeed by an amount that will make it impossible reach Vne. Whether the pilot is reading true or indicated doesn’t matter – he’s still within the margin of safety. It’s possible that knowing this, the writers of Part 23 decided to keep things simple.