Friday, September 22, 2006


And Speaking of Flying...

Getting a private pilot's license has always been "on my list". My dad had his when he was about my age, and flew my mom around on some of their dates. I know I'd love it. I've always had a natural feel for what makes airplanes tick, and natural talent at handling any vehicle I've ever tried, from bikes to powerboats.

So when one of my relatives last weekend was complaining about airport security, and declaring that if she ever flew again she'd be her own pilot, it made me start thinking about small planes again. I'm a compulsive researcher (just ask my friends; one of the most common complaints about me is my know-it-all tendencies) and set off to find out what kind of small airplanes (lightplanes) are available today. Answer: the majority are disappointingly the same as when I was a kid looking through my dad's old flying stuff.

The few new-age designs out there have specs that tell me something about their designers' intent. For example, the useful load (weight capacity, incl. fuel) of a Piper Saratoga and a Lancair IV are within 100 lbs or so, but the Lancair cruises almost twice as fast and is 20%+ more fuel-efficient. If you compare any two old and new designs with similar seating capacities, you'll find about the same thing. New airplanes carry the same load as old ones, but generally faster and with lower fuel consumption--using the same Continental or Lycoming engines.

That's a whole box of cookies for modern aerodynamics and materials science, but it leaves me scratching my head in one way. The two aircraft linked above, which are fairly typical, have a max load of about 1200 lbs including up to 60-70 gallons of fuel. At a little over 6 lbs/gallon, that's 400 lbs or so out of that 1200, leaving 800 to divide among pilot, passengers and luggage. Even 6-seat lightplanes come with numbers like that, so unless I'm missing something pretty much every single-engine lightplane on the market can't fill all its seats without being gigantically overloaded!

You're right about full fuel = reduced passenger load or full passengers = reduced fuel capacity (i.e. range). When you're full of passenger lbs you can't also "fill it up" with fuel (only partially full). At first this might seem stange. However, small plane's (and most large ones) are designed that way to maximize the planes mission flexibility. If the fuel tanks were sized for 4 passengers (i.e. smaller tanks), and you only had two passengers on board, then you would be limited to under-utilizing the plane's load/range capabilty. Thus, taking off with partial fuel is not uncommon - as long as the fuel onboard will get you to your destination. Calculating a plane's weight and fuel needs (with a buffer) before each flight is a common pre-flight task and taught during pilot training. Hope you follow-up on your "list" someday - flying is great fun! :-)
The tradeoff between fuel/passenger weight makes sense. I think what I was finding strange was the the slimness of the overall useful load which in some cases seems to be less than the number of seats in the cabin even with no fuel.

For example, I think a Beech Baron has six seats standard, but a useful load of around 1300 lbs. I'm guesstimating that number, but I know it was under 1500 lbs. That would work out to around 215 lbs per seat. If you assume that your six seats are divided between three couples (including the pilot) then 180 lbs per person is a reasonable (probably optimistic for modern Americans) average. That leaves about 220 lbs for fuel and luggage. Six people probably have close to 100 lbs of luggage for a long weekend, so that's 120 lbs of fuel. Which works out well if you polish off a pot of coffee before taking off :-)
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