A few years ago I submitted this article to MAN. It was selected and published in the first issues of On The Flight Line.
Density Altitude
We all have visions that our planes will fly wonderfully simply because we took the time to carefully build them. We may also believe our new ARF or RTF will also fly with little cause for concern because they were engineered with great attention to detail. In both cases we see our visions to fruition at almost all times except during the heat of summer. The late summer scenario though begins to unfold like this:
I was at the flying field one hot, dry Saturday afternoon and noticed planes using a lot more runway then usual, and I unfortunately witnessed two planes stalling on approach. To thoroughly confuse me our best 3D pilot was complaining his hovering maneuvers were not what they were used to seeing. So why would these events be occurring and what was the influencing factor I had experienced: Density Altitude. Let’s take a quick look at a weather factor many RC pilots do not take into account.
Density altitude is basically a measure of actual altitude conditions you will be flying under with factors that include mean sea level barometric air pressure, temperature, and dew point. Going back to my basic flight training in a Cessna 150 I remember well that there were times when under a high density altitude environment I could not fly the plane with an instructor and full fuel tanks. Many may think a four seat-plane is designed to carry four people, but the reality is, yes it will carry four people, but even in fairly optimal conditions you can’t put a full tank of gas in the plane. One of the culprits is density altitude.
With these factors present every time we fly our radio control airplanes in the hot summer we have to realize we are limited by density altitude. In essence, because we have high temperature, low barometric pressure and high dew points we just do not have as much for the prop to grab onto with each revolution. In the winter with low temperatures, high barometric pressures and when dew points don’t really count the air is very dense and we have lots of molecules to slice the prop through. So on a cool or cold day our planes perform like rockets and on a hot summer days they are sluggish.
Let’s look at another scenario I have heard many times that occurs in the mountains above 5,000 feet. Some flyers I have talked to complain that their planes will not fly and in fact even if they lift off the runway any simple maneuver promotes a stall. They return to the hanger and commonly think they need more power, while the same plane flies perfectly at sea level. Even worse they believe their plane was poorly manufactured. It, therefore, is possible, when we incorporate density altitude, my flying field at 814 feet will act like a field at 5,000 feet. All of a sudden the scenarios I witnessed at the flying field make sense.
Recently I hand launched a plane on a day that was 98 degrees, 28.94 inches of mercury and a dew point of 70 degrees and thought I would never get the plane high enough to get it back to the runway. The same plane had flown many times through the winter and spring with all the characteristics I desired. But that hot summer day when I calculated the density altitude from 814 feet above sea level, the density altitude was 4890 feet! No wonder.
So next time you fly on a hot day think about what you must do to avoid problems. First allow plenty of room for take off, second keep your airspeed up on final approach and third if you are under powered to start with, consider an early morning cool air flight. Remember, it’s not your plane that is misbehaving, but rather the forces of nature beyond your control. Being aware of density altitude might just save you a crash that frustrates you because you don’t know why it happened.
