Notes for new hang glider and trike pilots--on sideslips
August 20 2005 edition
steve at aeroexperiments.org
(Note: this article has evolved from a much longer pair of articles that originally appeared in the February and July 2000 issues of Hang Gliding magazine, entitled "A new perspective on hang glider turns" and "More on slips, skids, and spirals".)
There are several excellent flight training guides available for new hang glider and trike pilots. As you read these guides, and as you talk to your instructor and to other pilots, you will encounter the idea of a "sideslip".
Here is the prevailing view among most hang glider and trike pilots, instructors, and authors about sideslips: if a pilot rolls a glider or trike from wings-level
into a banked attitude without adequately moving the control bar forward to
"coordinate" the turn, the glider or trike will "slip" or "sideslip", moving sideways through the air as it "falls" toward the low wingtip.
I suggest that you take this idea with a large
grain of salt.
During a long series of careful experiments (begun in 1998 when I was a brand-new hang glider pilot fresh off the training hill) where I used tufts of yarn to observe the airflow around a hang glider, I've found that there is no truth to the idea that an inadequate pitch "coordination" input will make a hang glider "slip" sideways through the air toward the low wingtip. If a hang glider or trike pilot does not make a firm enough pitch "coordination" input as he rolls his aircraft into a turn, the nose will drop and the aircraft will dive and accelerate, but it will not slip sideways through the air.
I suggest that whenever you read or hear about a "sideslip", you should mentally substitute the idea of a "diving, accelerating turn". If you don't give a firm enough nose-up pitch "coordination" input as you bank a glider or trike into a turn, the nose will drop and the aircraft will dive and accelerate as it begins to turn. Increasing your forward pressure on the control bar--so long as you are not too close to the stall angle-of-attack-- will bring an end to the accelerating dive, stabilizing the airspeed and restoring to the flight path to a more reasonable glide angle (or if you have an engine, to level flight or to a climb).
Similarly, if you are flying a hang glider or trike in a banked attitude, and you abruptly pull in the control bar, the nose will drop and the glider will dive and accelerate. If your control input was quite abrupt, the dive and the acceleration may be quite dramatic. This is another case where the prevailing view among pilots, instructors, and authors is that the hang glider or trike will "sideslip" sideways through the air. Again, I've discovered that this is not actually the case--the glider or trike does not slip sideways through the air as it accelerates and dives. Again, when you read or hear about this situation, I suggest that you mentally substitute the idea of a "diving, accelerating turn" for the idea of a "sideslip".
If you happen to be coming to hang gliding or triking with 3-axis experience, these ideas may be especially helpful to you. In this case you will have a very definite idea of what a "sideslip" is, and you will be used to using the rudder in a very specific way to prevent slips and skids. It will be confusing to you if get the idea that that your pitch "coordination" inputs that you'll be making with the control bar have something in common with the "coordination" inputs that you made with the rudder in order to prevent to slips and skids. They do not.
Since these ideas are a bit in conflict with the "conventional wisdom" among hang glider and trike pilots, it's quite obvious that you don't need to understand any of these ideas in detail to be a safe pilot or even an expert pilot. I don't recommend that you spend too much time thinking about the details of the physics while you are learning to fly a hang glider or trike. But if you avoid incorporating the idea of preventing "sideslips" into your thinking about your pitch "coordination" inputs, you'll end up seeing things more clearly.
For those readers who are already experienced hang glider or trike pilots: if you are of a technical mindset and are interested in learning more about how the physics of a diving, accelerating turn differ from the physics of a sideslip, or if you are interested in learning about the situations where a hang glider or trike will in fact exhibit a (brief) sideslip, explore these related links on the Aeroexperiments website:
"What makes an aircraft turn?"
Looking for the "slipping" turn while hang gliding--overview