Expect Left Traffic

Best speed for the money (1.31 * Vy)

lift-drag So last week this post came across the AOPA news feed, and since my first cross country was this week, it was quite timely.  The ballooning gas prices are on everyone’s mind now, especially people with hobbies or businesses that depend on some form of engine.  Now, as a renter I don’t pay for fuel so I could conceivably rip around at full throttle or maneuvering speed if it’s bumpy and not worry about how much it’s costing me in fuel.  However, as one who likes his club, it’s planes and conserving what’s left of our environment for my child(ren), I do what I can regardless of how small the effort may seem.  About two weeks ago I downsized my truck for a commuter card with over twice the mileage.  It helps that I’ve been eyeing the new Mini Coopers since BMW brought them over to the US, and that we can always hook a trailer up to the other car should I need to haul a crippled bike.  In the end I guess all vehicles regardless of the number of wheels, terrestrial or aerial use should be chosen for their primary mission.  Ah the things we learn when we start taking on new skills, motorcycles made me a better driver and now the perspective of a pilot is creeping in elsewhere as well.  Enough rambling, on with the lesson.

If you haven’t just skipped ahead to the AOPA post yet, here’s how fuel economy works in a plane.  While flying you generate lift to keep you aloft, and a byproduct of that lift is induced drag.  The faster you go the less of that drag exists, as you can fly at a lower angle of attack (basically, how high the wings and nose point in the air).  There is a tradeoff though.  Parasitic drag increases the faster you go, unless I suppose your plane is two dimensional.  The chart above shows this curve, the scale for speed and drag can change, but the curves for induced and parasitic drag are always the same regardless of the plane.  This is a fixed ratio, which makes sense but isn’t always glaringly obvious, even to seasoned pilots.  L/D max is the point that both of those curves meet, which gives you the most efficient lift and best rate of climb (Vy), but leads to a slow ground speed and sluggish control inputs.  If you need to get somewhere for the least amount of fuel, and time is no object, that’s the speed to do it.  Pilots though, like drivers, hate poking around at speeds we don’t have, especially if it make the plane fly like it’s in molasses.  Well it turns out the L/D curve isn’t a full ellipse, it has a flat spot in it.  In that flat spot it seems you can glean almost a third more speed, for only a fifteen percent increase in drag.  That’s probably the best trade off in the whole curve.  The formula works out to 1.31 * Vy and will work for every single plane out there, since the drag ratios never change, just they’re relation to speed and drag.

It also turns out, in the planes I fly, maneuvering speed with full tanks and the lightweights that my instructor and I are meet up nicely with this formula.  I won’t have to think much about it, clear air or not.  Now if only there were such an easy formula for my Mini.

September 1st, 2008 Posted by | Flying |

Tags: | | |