Flying the Mosquito
Wind Drifter
Technical Info

Effect of FLPHG Engine Thrust on Bar Position

Most flphg pilots are familiar with the apparent change of control bar position when using a powered harness.  Nothing about the glider changes, rather the pilot now has additional weight located below the feet.  In order to balance pilot and engine, the harness suspension point must be moved to the rear, which effectively moves the glider forward with respect to the glider.  See Illustration of Control Bar Position Change  for clarification if you are still unclear on this point.  Up to this point we are not considering the engine - that is only the weight distribution of the harness, with the engine off.

Adding Thrust

This is where the picture becomes less clear for many.  What happens to bar position when you start the motor and add the prop thrust to the picture?  Before going on, think about this for a moment first.  Does the bar (relative to the pilot) move back, stay in the same place, or move forward?  Does the glider speed up?  What happens to sink/climb rate?

The Basics

For this discussion the pilot will apply no pitch pressure - that is, the pilot is flying at the glider's trim position.  This is not a limitation, simply a way of removing some variables from the equation.  The glider can be trimmed at any speed - min sink, best glide, top speed, etc.  But we will always let the glider determine the pitch angle, with no bar pressure applied by the pilot.

In addition to the aerodynamic forces on the wings, there are two other forces which act on a hang glider: the weight of the glider, and the force in the hang strap.  Note that I did not say the pilot's weight.  Think about a pilot at the top of a loop - the glider knows nothing about the pilot's weight, only the force in the hang strap that is attached to the pilot.  As long as the strap is in tension, the glider continues flying around the loop.

The glider could care less about gravity or what is right side up.  It will attempt to return to a stable flying angle of attack relative to the direction of the forces acting on it.  A glider would be perfectly happy flying upside down or straight up, if the forces acting on it were in the right direction.

Think about a kite - it has the same two forces acting on it - the weight of the kite, and the tension in the string.   It can fly at many heights and angles to the ground.

The Forces on a Glider

Of course we have aerodynamic forces (lift and drag), but they are only present when other forces cause the glider to move relative to the air.  And those other forces are the glider weight and hang strap tension, as shown in the figure.
Glider and Pilot's Weight

I'll pick round numbers for all the weights:

Glider - 70 lbs (this probably includes a fuel tank mounted on the top of the downtubes)

Pilot - 180 lbs

Harness - 70 lbs (this includes parachute, etc)

Flying along, power off, there is therefore a 250 lb load in the hang strap.  The total load being borne by the glider is 320 lbs.

We are looking only at level flight, no turns, so this total load is acting straight down.  The pilot is letting the glider fly at trim, as we already mentioned, so there is no pitch pressure.

CG Locations

The Center of Gravity (CG) of the pilot must be directly below the hang point for this case.  We are going to assume that the CG of the glider is at the hang point.  This is a reasonable approximation, although it may actually be slightly above or below it.

Now Add Thrust to Hang Strap Load

Now, let's add prop thrust to the picture.We are going to keep this simple and assume that the pilot is hanging perfectly level with the engine off (although many pilots actually fly with the tail of the harness a bit lower than the head). This means that the pilot forms a 90 degree angle to the hang strap. We are also going to use a nice round number of 100 lbs of thrust.

The thrust pushes the pilot forward, and this causes the pilot, harness, and hang strap to rotate about the hang point, as shown in the figure to the left. The thrust vector and the hang strap maintain the 90 degree angle that existed with power off.  Note that the total force in the hang strap has decreased, to 229 lbs.
Hang Strap Forces

Does this make sense?  Sure - think about the harness tail pointing straight down - sort of like an upside down helicopter - in that case the thrust would subtract directly from the pilot's weight.

Also notice the hang strap angle is 23.6 degrees from vertical.  If we looked only at this figure, we would conclude that the pilot has moved a long ways forward through the control bar.  In fact, we can use the figure and numbers at left to figure out just how far.  Let's say the pilot CG is 50 inches below the hang point - then we can use the figure at left by simply dividing all the numbers by 5. The Pilot Weight Vector would now be 50, which is his distance below the hang point.  The Thrust vector would now be the distance the pilot has moved forward, which would be 20 inches!   But this can't be right, because that is as long , or longer, than most people's arms.   But this is assuming the glider has not changed position at all, which is obviously not the case.

Total Glider Load

Now we can take a look at the whole picture.  I've taken the Hang Strap Load vector from the previous figure and copied it to the figure at the left.  To this I've added the Glider Weight Vector.  This is, of course, assuming steady state, non-turning flight.  We now see that the Total Load on the Glider is 294.5 lbs,  and the angle from the vertical at which it acts.

That's it - that one quantity, the Total Glider Load, is all that the glider knows.   The glider does not know anything about gravity, engine thrust, or any of the other values we used to get to this point.  The glider will fly based solely on that one load vector.

Total Load on Glider
Now, let's look at what we can conclude from that.

1. The glider will fly exactly the same as if it had a total load of 294.5 lbs on it.  Forget about the thrust and everything else - we now just have a glider with a total load of 294.5 lbs, instead of the 320 lbs we saw earlier.  So the glider will be flying slightly slower (remember that the velocity change is only the square root of the weight change).  It will also have exactly the same angle, relative to the total load, that it has unpowered.  So, because the angle of the total load has changed, so will the glider pitch angle.  That is, in our example, the glider will be pitched up by 18.1 degrees.  But the glider does not know it is pitched at a higher angle, as far as it is concerned, it is flying exactly the same as if it were unpowered and flying with a 294.5 lb total weight.

2. Now let's look at what the pilot is experiencing.  Remember that the pilot is angled 23.6 degrees forward from the hang point.  But the glider control bar has angled up 18.1 degrees.   So the pilot has an angle, relative to the glider, of only 5.5 degrees.

If we continue to use the hang strap length of 50 inches, then this means the pilot has moved 4.8 inches further forward than the control bar has. (simply muliply the strap length by the rotation angle, in radians).

Summary and Conclusions

When we take all the above pieces and put them together, this is what we get:

1. The glider responds only to the Total Load Vector acting on it.
2. Adding thrust to the pilot changes the angle of the hang strap tension load, and slightly reduces the tension from when the power was off.
3. The total load on the glider is reduced, and angled forward.  The glider will continue to fly at the same angle, relative to the Total Load Vector, that it did before, but at a slightly slower speed.
4. Because the glider weight and hang strap tension loads are in different directions, under power, the control bar position relative to the pilot will change.  The heavier the pilot, and the lighter the glider, the less change in control bar position there will be.  Conversely, the lighter the pilot and the heavier the glider, the more change will be experienced.

We can use this information to make an approximate Rule of Thumb:

Approximate Rule of Thumb:
Changing thrust changes sink/climb rate, changing bar position changes speed.   As we have seen, changing thrust also changes speed and bar position slightly, but not by much.  If a pilot changes the bar position, he changes the speed.  The effect, on the glider, of pulling in or pushing out on the control bar is no different from simply moving the hang point forward or aft.  In the first case the pilot must maintain the bar at the new position by exerting a force on it, in the second case the glider trims at the new speed with no bar pressure.  The difference in bar pressure between those two cases is the only difference there is, and the glider does not know which of those two cases it is responding to.  Now, changing speed will also change the sink/climb rate, but not by nearly as much as the speed changes.  So, to a first approximation, the rule of thumb above works very well for thinking about how the glider responds to the two control inputs of throttle setting and control bar position.
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(updated December 23, 2023)