mike andresenClimb Speeds 

 

By Mike Andresen 

 

I was having lunch in Payson on a warm day when I observed a C-172 with four people on board taxi out. What really caught my eye was that the pilot deployed flaps for his take-off. I started thinking to myself, does the temperature today make Payson a short field to a max gross weight C-172? What climb V speed will he use? How are the climb V speeds affected by flap settings? If he has an engine failure after take-off will he be glad or sorry he has flaps out?
Let's start with a quick review of Vx and Vy. One is best angle of climb and one is best rate of climb. I'll start with an analogy to explain what that means. One can drive from Apache Junction to Lake Roosevelt by either taking Apache Trail or US60. Apache Trail is shorter distance but takes a long time to get there. US60 is a longer distance but faster. Apache Trail would be the Vx of driving and US60 would be the Vy. Vx gets you to altitude in the shortest distance but longer time while Vy gets you to altitude in the longer distance but shorter time.
Choosing Vx or Vy has nothing to do with the length of the runway. It has to do with clearing obstacles after leaving the runway. If you are staring at a pine tree on take-off you want to get above it in the shortest distance possible and will choose Vx. If there is no obstacle then choose Vy. Remember that there are short field no obstacle and short field obstacle take-offs and they are handled differently. (Landings too).

climbspeeds1

Effect of Density Altitude
Climb rate, which is maximum at Vy, is a function of excess power. Climb angle, which is maximum at Vx, is a function of excess thrust. Vy is derived from the power required for level flight curve of the airplane and Vx is derived from the thrust required for level flight or the lift-to-drag ratio curve. I'll go into detail on this in a future article. As density altitude is increased, the normally aspirated airplane has less excess thrust and power for climb. The minima of the power and thrust curves will converge and Vx will equal Vy. This occurs at the absolute ceiling where there is only one angle of attack that will maintain level flight.
How big a difference does this make? The graph on the previous page is performance data from flight testing I performed on an RV-10. Over a practical range of airport density altitudes, it only makes a 2 to 3 knot difference.

Effect of Flaps
Significant flap deployment (15 deg or more) will drastically alter the aircraft power and thrust curves and hence the optimum climb speeds. The addition of drag will certainly negatively impact climb performance. Flap deployment does have an advantage of getting you off the runway a little earlier and lowering your stall speed but generally a clean airplane has the best climb performance. Here is an excerpt from the Cessan 172 POH: "The use of 10 deg flaps will shorten the ground run approximately 10%, but this advantage is lost in the climb to a 50-foot obstacle. Therefore, the use of 10 deg flaps is reserved for minimum ground runs or for take-off from soft or rough fields." Getting back to our hot day in Payson pilot, it would have been better to leave the flaps up.

Cruise Climb
Now that the pine trees are no longer at eye level, we relax a bit and glance at the cylinder head temperatures. Yikes! That Vx climb sure did heat things up! So now the objective of our cruise climb speed is to create enough positive air pressure in the baffle air dam above the cylinders to push cooling air down through the cylinders into the lower part of the cowl and out of the engine compartment. The cruise climb speeds are found in both the Lycoming engine operating manual and in the aircraft POH. Using the published cruise climb speed +5/-0 kts and a rich mixture should keep the cylinder head temperatures in a tolerable range.

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