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?

Bob--

ReplyDeleteThe 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.

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.

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