Why Fly High?

gains from high altitude flight- Ex . perimental flights above 30,000 feet later by Wiley Post in his homebuilt Why were being made as early as 1920. pressure suit and famous super- That year "Shorty" Schroeder charged Lockheed Vega "Winnie th , e climbe Liberts dhi y engine Pera dL e Mae". biplan world'a o et s record heighf o t What' magio s c about high alti- 36,020 feet. The plane's engine con- tude flight? The secret is that an air- Fly craft can actually fly much faster than taine theturbochargee w dth nne - rde velope Sanfory db d Mos f Generao s l it thinks it is going without using any Electric. Oddly, the tremendous additional engine output. Thus, both breakthrough made possible by the true airspeed and range may be dra- High? matically improved. What happens is turbocharge e knowledgth y b d ean r gaine thin do s fligh largels wa t - yig that the density of the air, or weight per cubic foot of the air is reduced at HoveyW. ByR. (EAA 64958) e aviationoreth y b d n world until World War n — twenty years later. high altitude. A given applied horse- Box 1074 Schroeder encounterew no e th d power will, however, e resulth n i t Saugus, California 91350 lacfamiliaa o kt e t strearje du t mbu wings, prop, etc. doin same gth e giv- of pressurization was unable to ex- en amount of work regardless of the D'ESIGNER, F O experimenS - plore its potential benefits for high change in air density. For the wings to tal amateur-built aircraft have yet to speed, long distance flying. This ex- do the same amount of work, they discover the potential performance ploratio s donwa ne som 3 year1 e s must move through a thinne t a r ai r higher speed e samTh . e reasoning holds true for the drag producing component aircrafte th f so . What this means is that the horsepower required t altitud a same th r ee fo indicate d air- 40,000 I speed is the same as at sea level. The big difference is that the aircraft is fooled into thinkin onls i t gi y moving 25,000 at its normal sea level speed, when it s actualli y moving througr ai e th h much faster to get the same dynamic 20,000 effect. Since the rate of fuel consump- LU tion per unit, of time has not changed, o your miles per gallon, or range is greatly improved. 15,000 O. K., just how much do we gain by all effort? Figure 1 shows the potential increas n trui e e airspeed, 10,000 (or ground speed with no wind) for UJ three hypothetical supercharge- den o gine aircraft capabilia firse s ha Th t. - 5,000 f cruisin o mp0 y 10 t h t indicatega d airspeed at rated cruise power. We assume tha powee th t r outpu hels i t d constant at rated cruise power by the 400 use of the supercharger, or blower 100 200 300 system. Note that the airspeed in- TRUE AIRSPEE MPD— H FIGURE 1 creases with altitude unti t 25,00a l 0 40.000 i 25,000 20,000 o 15,000 CO 10,000 5,000 10 12 CABIN PRESSURE RELATIVE TO- OUTSIDE AIR — PSI FIGURE 3 MARC2 2 H 1973 Fuel Injection Engine Safety Burst Diaphram LRelief Or Pressure Peculator Valve Waste Gate Valve Bleed Air Control Valve Controls Manifold Pressure Air Compressor PRESSURIZATION SYSTEM SCHEMATIC DIAGRAM FIGURE 2 Ram Air Inlet feet the cruise speed is over 150 mph. and 7,000 feet at night are the maxi- chargers use an exhaust turbine to get If we were to fly this airplane at mum safe density altitude limits. Eld- these high speeds, however, this ap- 40,000 feet, the cruise speed would erly or heavy smoker type pilots will proach is very complex. Recent im- be almost 200 mph. This is double find much lower altitude limits. There provements in belt drives should the speed at Sea Level with no in- are two approaches to solving this make the mechanical drive approach crease in power or rate of fuel con- problem. The first is to carry and use more attractive to the amateur build- sumption. The range would also be supplemental oxygen. This works O. er. Several blowers have been de- doubled with no increase in fuel used. K. if you don't run out of oxygen. veloped for automotive use that could The practical limit in cruise altitude Refilling supply tanks is a bother, be adapted to aircraft use. Simple for an amateur-built aircraft is 25,000 expensive and seldom available. butterfly valves of the type used for feet, the data is however continued to Masks must be worn by all aboard. carburetor heat can be adapted to 40,000 feet to show the dramatic ef- Tests have shown that if the mask manually control manifold pressure. fect of high altitude flight. These as- should fall off at higher altitudes you Cabin pressurization is obtained by sumptions do not take into account might not be able to get it back on bleeding compressed air off the the extra time spent climbing to cruise properly before losing consciousness. blower. Manual adjustment is re- altitude. This is, however, not nor- There are limits to the altitude where quired to hold a constant internal mally significant for flights of two it is safe to use oxygen masks. This cabin pressure with changes in alti- hours or more. The other two lines on is generally about 20,000 feet. If you tude. A relief valve, backed up with a the curve show the performance in- climb to above 36,000 feet, in an un- burst diaphragm set to dump internal crease for 150 mph and 200 mph pressurized system, you are apt to pressure at structure limits is es- cruise speed aircraft. form fatal bubbles in your blood sential. The curves in Figure 3 show Why aren't all aircraft equipped stream. (Your blood boils at over the difference in outside and inside to cruise at high altitudes? Well, 36,000 feeU The best approach to air pressure for two hypothetical ca- there are a few problems. Firstly, this problem is to seal up your cockpit bins. One is designed to maintain piston type engines can not suck in and bleed air pressure off of your equivalent sea level pressure, (14.7 enough air through the intake system supercharger to build up the inside PSI) as altitude increases. The other to develop full cruise power at den- pressure until it is equivalent to a shows the pressure differences in a sity altitudes much above 7,500 feet. 6,000 to 8,000 feet altitude. cabin designed to maintain an inter- But even this much altitude helps im- What kind of equipment does it nal pressure equal to 7,500 feet al- prove performance over sea level take to do all this? A schematic dia- titude. Note that the pressure dif- operations. That is why most light air- gram of a complete system is shown ference for the higher internal alti- craft manufacturers quote cruise in Figure 2. The heart of the system tude pressure is much less than the speeds and range at 7,500 feet alti- is the supercharger, or blower, which sea level cabin. This would result in a tude. In order to develop rated cruise supplies compressed air to both the lighter cabin structure. power, at higher altitudes, a super- engine inlet manifold and to the cabin The requirements for designing charger must be used to compress pressure control valve. A two stage and building a high altitude experi- the intake air down to the same den- belt drive takes power off from the mental aircraft may sound involved, sity you would get at 7,500 feet. In engine accessory section to drive the however, all of the technology and this way the engine thinks it is at only blower at high speed. Typical centri- capability is available and the ama- 7,500 feet when it can be much fugal blower rotational speeds range teur builder who comes up with such higher. from 30,000 to 60,000 rpm depend- a machine will realize a giant step The second problem is the pilot and ing on the diameter and pressure forward in the advancement of ex- crew. About 10,000 feet in daylight ratio desired. Most existing super- perimental homebuilt aircraft. Q SPORT AVIATION 23.

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