Best Rate-of-Climb and Best Glide Tests

I continued testing my Bede BD-4C airplane this week. I went out one evening and began flights to determine the best rate-of-climb speed, which would allow me to determine the best angle-of-climb speed. I got to fly and see a gorgeous sunset but I ran out of time and could not complete the tests. I went out again today, finished the tests, and then ran a series of tests to determine the best glide speed.

Best Rate-of-Climb Tests

There are two climb speeds that are interesting for airplanes.

• Vx or best angle-of-climb is the speed you fly to get the highest while moving the shortest distance over the ground. If you take off and there is a tree at the end of the runway (and you do not want to trim the foliage any shorter) then you climb at Vx until you are higher than the tree.
• Vy or best rate-of-climb is the speed you fly the get the highest the quickest. If you take off and it is hot and bumpy and you want to get to where it is cooler and smoother as soon as possible then you climb at Vy.

Testing for best rate-of-climb means taking off, climbing at a certain speed for a minute or more, landing, refueling, and doing it again at a different speed. I ran tests at 90, 85, 80, 75, and 70 knots. Because I ran out of daylight, I had to spread the tests over two days. When all was flown and done, I reached two conclusions:

• Vy for my Bede BD-4C is about 80 knots
• Vx for my Bede BD-4C is probably 75 knots

It was too turbulent (bumpy) when I flew the 80 and 70 knot tests and I did not get good data. Because of that, I was able to estimate Vy but not draw the graph that I need to draw to determine Vx. I do know that Vx is about 5 knots less than Vy so, for now, I will use that estimate.

Graphs, Graphs, and More Graphs

I got a whole bushel of data out of my EFIS and wrote some software to plug in behind Jupyter Notebooks and let me analyze and plot it. Here is a sampling, from the rate-of-climb test at 75 knots. We start with a graph of the entire 25 flight. This graph shows airspeed against altitude and vertical speed. The actual test is the little section where I hold the airspeed at 75 knots and see how fast the airplane climbs.

The interesting part of the flight is where I hold the airspeed at 75 knots. Zooming in and adding pitch angle:

I wanted about a minute of data where the airspeed and the vertical speed were pretty stable. I got that from about 7.5 to 8.5 minutes. Perfect. The airplane climbs at about 850 feet per minute at 75 knots airspeed at 5000 feet density altitude.

There are some other factors worth looking at, starting with engine power. For the test to be valid, I should fly at 75% power (150 HP) for the entire time. That means 2400 RPM and about 24.5 inches of manifold pressure. Here is the graph.

I was not perfect on the manifold pressure but I was pretty close. The RPM is controlled by the propeller governor and it is not perfect, either. Since the RPM crept up while the manifold pressure crept down, it probably balanced out and I ended up with about 75% power throughout the test.

The engine is air cooled. At 75 knots with a high angle of attack, the airflow through the engine compartment is reduced. This graph shows the effect on the engine.

You can see that, as I reduced airspeed to 75 knots, both the cylinder head temperatures (CHT) and the oil temperature began to rise. The hottest CHT peaked out at about 420 degrees, which is hotter than desired but within the limits set by Lycoming. This indicates that I need to work on the baffling, which routes cooling air past the engine. I can continue to fly but I need to monitor the CHT.

Oil temperature peaked at about 220 degrees Fahrenheit, well within the 245 limit. Oil pressure dropped to 74 PSI, well above the 55 PSI limit.

You can see that CHT, oil temperature, and oil pressure all recovered to normal values within a couple of minutes of the end of the test. Increased airflow is a good thing.

Best Glide Tests

For best glide, I am looking for the airspeed which will keep me in the air the longest in the unlikely event that the engine quits in flight. Testing is the opposite of what I did above. I climb to altitude, pull the engine back to idle, set the propeller for low RPM, and glide at a certain airspeed for a minute or so. Then I climb back to altitude and repeat for another airspeed. Here is the overview graph.

At 75 knots, the airplane was sinking at about 500 feet per minute. It is pretty clear that that is much better than the sink rate at any other speed so this test is complete.

Interestingly, the tests at 90 and 85 knots did not give me much valid data. The air was so turbulent today that, even though I held a pretty steady airspeed, the vertical speed never stabilized during either of those tests.

What’s Next?

All of these numbers are dependent on the weight of the airplane and the density altitude. I have run these tests at just one weight, about 1700 pounds. I will return to these tests later and rerun them at the maximum gross weight of 2400 pounds. I may also pick an intermediate weight of about 2000 pounds. (Empty weight for my airplane is 1281 pounds.)