Plane & Pilot
Tuesday, October 23, 2012

Say When

When you should retract gear, reduce power, stop climb, initiate descent and more

If you're like so many of the rest of us who fly four-seaters with the rear seats empty, one way you CAN increase your cruise performance slightly is to slide the front seats all the way back to generate a more aft CG during cruise and reduce the down force on the tail. That may generate a knot or two of extra cruise on most general aviation singles.

5] When should you close the cowl flaps for cruise?
Some aircraft checklists suggest closing the cowl flaps as the last item on the cruise checklist. In fact, it should be the first item. Some manufacturers forego cowl flaps altogether to keep things simple for pilots; others install the cooling doors primarily to help control engine temperature during climb and for extreme hot weather operation. Aircraft cowlings are nearly always an aerodynamic compromise between the conflicting demands of climb and cruise. The late Roy LoPresti once told me he designed his cowlings almost exclusively for cruise—then, he mounted the largest cowl flaps he could to keep the engine cool during climb.

If you're operating on a normal day with reasonable cylinder head temperatures, you should close the cowl flaps just BEFORE you level the airplane for cruise. Engine overhaul shops suggest that shock cooling or heating of an aircraft engine is anathema to long engine life. If you close the cowl flaps during top of climb when the airplane is operating at Vy or slightly faster, you won't be subjecting the engine to any major shock because of the relatively low climb speed. Conversely, wait until the airplane has accelerated to full cruise, and closing the cowl flaps will cause shock heating. The difference in ram air pressure inside the cowling will be far more dramatic.

6] At what point before your destination should you initiate your descent?
These days, it seems every GPS manufacturer offers a built-in VNAV program that allows a pilot to program his descent with precision. The automatic tendency is to select a vertical speed such as 500-1000 fpm and reduce power to keep airspeed consistent with normal cruise.

Assuming you're flying VFR below 18,000 feet and there are no obstacles in the way, you might be money ahead to settle for a slightly gentler descent at normal cruise power, minding the mixture as you fly into more oxygen rich air. Depending upon your airplane's drag profile, you might be able to save both time and money by dialing in a descent at a minimal 300 fpm. If you're flying with a tailwind, you'll want to maximize benefits from the free push, and a more gradual descent will offer that more favorable wind longer.

If much of your flying is in the Southeast or Southwest as mine is where low level heating can become uncomfortable in summer, you might want to consider a two-step descent, 300 fpm until you reach about 5000 feet AGL, then, increase to 700 fpm or more to minimize time in the hotter, lower altitudes. Most reasonably clean airplanes will accelerate disproportionate to their fuel burn, and you'll wind up arriving at your target altitude and distance having burned less fuel and spent less time in the descent.

A comparatively high drag design such as a Skyhawk or Cherokee may not realize any benefit from such a practice, but a Mooney or Bonanza could actually turn a slight profit in both time and money. (Pretty obviously, you'd need to temper your choices based on low level turbulence.)


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