Thoughts on Handling Under Power

The musings below were first posted as a message on the flphg list:
They were a more reasoned follow-up to my first post on the subject:

I originally had the entire text of the second message posted here.  Since then I have updated that with figures so that it is hopefully easier to understand.  It is now contained completely in a separate document:  FLPHGHandling.pdf (550 KB)

A brief summary: I make the case that thrust line is a significant part of the handling under power, and that many things a hang glider pilot is taught, like leading a turn with their feet, are counter-productive when flying with power.  I recommend reading the above pdf file for the complete story.  What follows here is a very condensed version consisting mainly of the figures with a small amount of explanation.  If something in the figures is not clear, I recommend reading the full version above.

Note: Some very good references for how hang gliders fly and factors affecting their handling, see the series of books by Dennis
Pagen (

Assumptions and Conventions

In the following I will use the convention that all forces are those acting on the glider. While it is pretty obvious that the Lift force acts upward, there has been some confusion about the direction of the Thrust force. My convention for thrust is that it acts on the glider in the FORWARD direction, as that is the direction of the force that the harness applies to the glider.

In order to avoid spending too much time trying to sort out "inside" and "outside" wings versus left and right turns, ALL TURNS WILL BE LEFT TURNS for the sake of this discussion. That way when "left" wing is used in one sentence and "inside" wing in the next one, it will be obvious that they are one and the same.

Yaw Stability

Hang gliders achieve directional (yaw) stability because of the swept wing design. If a glider is flying straight ahead, the air is meeting both wings at an angle (that angle being determined by the nose angle of the glider). If a bit of turbulence or something else causes one wing to get ahead of the other one, the airflow will be hitting the forward wing more directly, and it will also have a larger apparent frontal area (relative to the airflow). It will also have an increased apparent length which increases the effective moment arm of the drag forces on that side. The increased drag force on the forward wing (and reduced drag force/moment arm on the rearward wing) produce a yawing moment that turns the glider back towards being straight into the airflow.
Normal Unpowered Turn Stability
So far we have talked about yaw stability in straight-ahead flight, but how about that left turn? As we roll left the glider will start 'slipping' to the left as well. This is because the "lift" generated by an airfoil acts perpendicular to the wing - not necessarily "up". Thus the lift force is now acting up and to the left, while weight is still acting straight down. These unbalanced forces will cause the glider to move sideways (slip) to the left as well as continuing forward. This puts the left wing more directly into the airflow thus causing a yawing moment that slows the left wing and rotates it back. That is how our swept wing yaw stability comes into play to overcome the
adverse yaw effect from our roll motion.

Roll Instability

An unpowered aircraft inherently has a higher degree of roll stability than when flying with power. For this discussion we are talking about being in a constant turn, such as doing 360's. In our constant turn to the left our left/inside wing is moving through the air slower than the outside wing, thus we would expect it to have less lift. However, because we do not have any power, we must also be moving downward relative to the air we are in (even if that air is moving upwards, as in a thermal or ridge lift). Both wings will be moving downward at the same speed in a stable turn. Because the left wing is moving forward slower, this downwards motion causes it to  have a higher angle of attack than the right wing.

If we add these two things together we have the potential for very nice roll stability: the left wing is moving slower (less V) but has a larger angle of attack ('a') which increases CL(a). Putting both of those into the equation we see that it would not be hard to achieve a situation where these two effects exactly balance the higher V but  lower CL(a) of the right wing.
Gliding Turn Stability
Now we add power. To begin with, let's add just enough power that we are making our stable turn at constant altitude (air is not lifting or sinking). While the inside wing is still moving forward slower than the right wing, neither wing is moving downwards. Therefore the angle of attack is the same for both. The only thing in our lift equation that is different between the left and right wings is that V is smaller for the left wing. The right wing with the larger V will be generating more lift which creates a moment that wants to roll the glider ever more steeply.
Climbing Instability
In climbing flight the upwards motion reverses the relative changes in 'a' for both wings. That is, 'a' increases for the right/outer wing - so now the right wing has both a higher V and higher CL(a) than the left wing, and the roll moment becomes even larger. Thus we can expect to do more "high siding" to maintain a stable turn under power than we do when gliding.

Thrust Line Direction

When we have power attached to our bodies we control the direction of the thrust force by our body orientation, so orientation becomes much more important than when just gliding. Let's look at all three possible orientations in turn (and recall the convention that thrust is acting on the glider as a *forward* force).  Note that these figures show only the effect of thrust line forces.  Many other forces also come into play, which are not shown here.  For a more indepth discussion, see the full write up: FLPHGHandling.pdf (550 KB)  Each glider will have it's own way of combining these various effects - for some the thrust line effects may hardly be noticeable, while for others the thrust line force may dominate the handling.

Body Parallel to Keel

The thrust line remains parallel to the keel, however it is now offset to the left. I have thought about this in a lot of different ways, but the simplest one is to simply think of a twin-engine aircraft with the right engine dead - the thrust acting to the left of the centerline will tend to yaw the glider to the right, just the opposite of what we want.
Powered Turn - Body Parallel to Keel

Feet First Roll

This is the standard method used by many hang glider pilots, myself included (for unpowered flight) - the feet are moved farther in the roll motion than the shoulders. One good reason for doing this while free flying is that we are both moving the body sideways and rotating it about the hang point - the rotation causes a yawing torque on the glider which tends to turn it in the direction we wish. It is thought that this helps to produce "flatter" turns.

Now let's look at the thrust line direction. The pilots CG is displaced to the left, but the feet/prop are further to the left, causing the thrust line to be twisted clockwise to the glider (when viewed from above). This clockwise rotation of the thrust line causes a component of force acting to push the glider to the right. However, in order for the yaw stability of our swept wing to come into action we need to be slipping to the left. Thus under power our feet-first roll technique becomes counter-productive.
Powered Turn - Leading with feet

Head First Roll

One of the first things new hang glider pilots learn is that if you simply move the upper part of your body sideways you will have very little control. That is because instead of moving your CG to produce a roll input all you have really done is twisted your body about the hang point. After a few panicked episodes of "the glider wouldn't turn and I was as far over on the bar as I could go!" the new pilot learns about keeping their body parallel to the keel (or even leading with their feet). Experienced pilots simply don't consider using a head-first rolling motion.

Let's take another look at that, except with thrust added to the picture. The pilot shifts his CG the same amount as with either the parallel or feet-first method, but rotated in the opposite direction from the feet-first method. That is, his head has moved the most to the left and his feet the least. The thrust  line now has a sideways component to the left, which is the direction we want to turn and also the direction the glider needs to slip in order for the correct yaw to take place. It is my contention  that this head-first method is the easiest way to initiate (or roll out of) a turn.
Powered Turn - head first

Experiments and Observations

On a number of occasions I have proven to myself that I can successfully turn the glider using only a yawed thrust line with no roll input. The way I have done this is to grasp the control bar with only 2 fingers on one hand, which I use to rotate myself about the hang point. Using those 2 fingers I am not able to create any significant sideways movement of my CG, thus I am not creating a rolling moment. I have been able to do a series of reversing turns using this method. What I believe is happening is this: when I yaw the thrust line (let's saw CCW viewed from above) I am introducing a sideways force to the glider. This sideways force creates a side slip which then causes the wing to yaw to left because of the swept wing yaw stability. In this case the mechanics work backwards because the yawing motion causes the left wing to slow (decreased lift) and right wing to speed up (increased lift) - thus creating the rolling moment to coincide with the yaw.
Yaw Only Turn

Here is a video clip that demonstrates a series of reversing turns using nothing but the thrust line:  ThrustTurns.wmv (UPDATE: higher quality video on Vimeo) In the video I do a series of 5 reversing turns (5 to left, 5 to right) using thrust line only.  Not that my right arm is dangling down the entire time, and I am using only my left arm to rotate about the hang point - and I can assure you, my wrists are not strong enough to make any significant one-armed roll inputs.

Because of the relatively mild turns, the turn direction at times may not be immediately obvious, but you can easily tell it from the position of the left wing (the camera is mounted on the right wing tip).  If the visible wing tip is below the horizon, the glider is turning left, if it is above the horizon, the glider is turning right.

Note that the turn reversal does not always happen immediately - it sometimes takes as much as 10 or 15 seconds after I have rotated my body before the glider follows.  But I believe it is pretty clear that whichever way my head is pointed is the direction the glider will turn in.

There are definite limitations to the thrust-only turn method - I have only been able to get it to work in smooth conditions and at shallow bank angles. It seems that at steeper bank angles the roll instability of flying under power is too great for this method to overcome. Once I am past more than about 5 or 10 degrees I have to add some roll force to the yawed thrust line in order to reverse the turn.

There are also limitations on how far I can yaw my body before I am blocked by the downtube. And if you are trying this with a glider that is not high aspect ratio (i.e., long root chord) - you will want to be very careful that you are keeping the prop clear of the sail!!! That does not appear to be a problem with my glider, but you should satisfy yourself that that is the case with your own glider before trying anything too radical!

Turning with only a yawed thrust line is a cute trick (and very handy for staying on course when you have one hand busy, such as while zipping up your harness) but useful mainly for demonstrating the effect of the thrust line on turns. However, if we can turn only with a yawed thrust line, then it should be apparent that we can use the angle of the thrust line to either help us or hinder us when turning. If the glider will turn under nothing but a roll input, and will also turn by simply yawing the thrust line, then it should be very easy when the two are combined. This has been my experience.

My hang glider will coordinate a 360 very nicely when flying without power (that is, no roll force required to maintain a constant bank angle), but tend to spiral-in when flying with power. I have found that by simply yawing my body, so that the thrust line points slightly towards the upper wing, I can achieve a stable coordinated turn under full power without high siding. And the effort to roll out of the turn when directing the thrust towards the high wing is also greatly reduced.

Rigid Wings

Rigid wings have a different means of coupling roll and yaw. While a flex wing glider is controlled by rolling the glider (which creates a yawing motion because of the swept wing design), a rigid wing works just the opposite. A rigid wing is turned by inducing a yaw motion (creating higher drag on the inside wing via a control surface) which then creates the roll. That is because a rigid wing does not depend on having swept back wings to maintain yaw stability. Instead it uses dihedral to maintain roll stability (dihedral is what you get when the wings form a V, instead of being flat across the top - instead of sweeping the leading edges backward at the tips as with a flex wing, they are moved upward).

To demonstrate how this works, take an envelope or something similar and fold it into a V. Hold it in front of you so that you are looking directly down the fold, and imagine that it is flying directly at you. Now turn it about the vertical axis in the CCW direction (as if it were yawed to it's left) and you will see that it's "right" wing (on your left) now has a much greater angle of attack, while it's left wing has had it's angle of attack greatly reduced - thus a moment has been created that tends the roll the glider to it's left.

I have no personal experience with rigid wings, but I can offer two observations that suggest how they will handle under power. The first is that the control surfaces out towards the tips of the wings are capable of producing large yawing moments by virtue of their long moment arms. This suggests to me that whatever effect a yawed thrust line has is very minor compared to the control afforded.  The second is that the comments I have heard from rigid wing flphg pilots indicate that they seem to think handling is very easy, even under full power.

If you still have questions, I recommend the full article: FLPHGHandling.pdf (550 KB)

(updated July 11, 2013)

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