Sunday, July 15, 2012

Flight Test Follies: Climb tests

The other night I noticed the plane was taxiing like garbage and when I felt the inside of the right wheel pant bracket, I burned the finger. So I realized -- after some thought -- that I had tightened the axle nut too tight a week ago. So, yesterday, I fixed that and today I restored the wheel pants to their previous grandeur and went flying.

This is a lousy time of the year -- at least this year -- for flying in Minnesota. It's ridiculously hot and the air is awfully dirty. It's not IMC, but you better keep looking down to keep ground contact, and that's only at 1500 AGL.

I went out this morning -- my plan was before it got too hot, but it was already too hot -- to do some more climb tests which I've been doing for the last few weeks. I climb at 70, 80, 90, 110, and 120 knots and time how long it takes to climb 1,000 feet. Then I average it out and I plot it on this here graph paper.

This show the feet-per-minute climb rate (known as Vy) which is the most FPM gained over a given time. And, with a fixed pitch prop and only wheel pants on, it's pretty easy to calculate Vy -- it's about 90 knots, at which I climb at roughly 1275 FPM.

But I also need to calculate Vx, which is the speed at which the plane will gain the most altitude over a given distance

The Van's instructions say to calculate it using this graph...
"Draw a straight line from 0-0 beginning of the chart up to a point where it is tangent to the curve."

This is the part where you find out why I'm a writer and not an engineer. I have no idea what "tangent to the curve" means.

The FAA's flight testing handbook (available here)
(1) Best angle of climb speed can be found
by using the same chart developed for the best rate
of climb tests. Draw a line (tangent) from the zero
rate of climb feet per minute (see figure 4) outward
to a point, on the rate of climb airspeed curve. Where
both lines touch, draw a line straight down to the
airspeed leg of the chart.
(2) The airspeed that the line intersects is
the best angle of climb airspeed.

And it provides this example:

I can draw a line as well as the next guy, but when it says, "draw a line out to the airspeed curve," well, to where, exactly?


  1. Bob--

    The tangent line to a curve is where it will just "kiss" the curve at one point. I bet it's too hot to test, but more data points at slower speeds will give you a better curve.

  2. You are going to have to redraw your graph in order to draw the tangent line. Vertically, you start at zero, then jump to 700, then 750, 800, etc. You are going to have to remove that "jump" from 0 to 700 and make each interval a constant value starting at zero. The same goes for the horizontal axis where you jump from 0 to 50.

    Once you have it redrawn so you don't have those jumps, then you can follow the FAA example.