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I've never flown a PP but found this response very interesting and informative. The writer experienced an engine seizure just after take off.

 

 

 

Re: From the flying chair

 

by Xplorer » Tue Feb 22, 2011 2:31 pm

 

richard C wrote:You came down rather quickly in the Efato - could you have stretched the glide - or is that realistically how quickly it is all over ?

 

Hi Richard, it might look a bit hectic to a non-paraglider pilot, but it was a non-event, other than the expensive repair to the engine.

 

If I remember correctly, I was back in the air minutes later with my other paramotor which happened to be in my van too.

 

To explain the dynamics of EFATO on a paramotor for non-paramotor pilots:

 

Unlike other aircraft, we do not have elevators to control pitch, nor can we weightshift like a flexwing-trike for pitch control.

 

We rely on Pendular Stability to keep the pilot below the wing which has a stabilising effect on the pitch (and roll too).

 

The thrust line is several meters below the center of drag (which is approx 10cm below the wing), so under power, the pilot gets pushed forward of the wing.

 

Now, viewing the aircraft from the side, the wing is a triangle comprising the front lines, the chord of the wing, and the rear lines. This triangle pivots around the bottom point, where the lines attach to the motor by means of a carabiner.

 

Under thrust, the entire triangle pivots back slightly around this attachment point, which increases the Angle of Attack.

 

As this AoA increases, the airspeed reduces accordingly due to the higher drag... so more power translates into LESS airspeed which I guess is quite unusual as far as aircraft go.

 

For pitch control, we have Four tools at our disposal:

 

1) Trimtabs (aka Trimmers) are buckles on the rear risers which adjust their length length, changing the geometry of this triangle.

 

Lengthening the rear risers, raises the tail of the wing, reducing the AoA. In reality, it essentially shifts the weight of the pilot and motor slightly forward. This is a set-and-forget means off accelerating the wing. For most landings, we trim right back, much like conventional aircraft lower flaps to land at a lower airspeed. We tend to launch with the wing "slightly" accelerated on the trimmers, to help the wing lift off the ground easier, and to offer less drag while we run to takeoff speed in zero wind. Though the takeoff speed is raised by trimming faster, we can reach it in fewer steps using a shorter runway. This is foot-launching, so the pilot's legs are the undercarriage.

 

2) The Speedbar is a 35cm lengh of aluminium tube, suspended at each end by a line attached to the front risers through two pulleys on the risers, giving a 3 to 1 gearing to reduce the effort required when one kicks down on the speedbar to shorten the front risers (the two pulleys on the front risers are pulled towards each other, shorting the front-riser's overall length). This achieves the same result as the trimmers, i.e. pulling the nose down to reduce AoA to accelerate, by the same mechanism as the trimmers, i.e. pulling the pilot & motor's weight forward under the wing. The difference, is this is a more dynamic means of accelerating and the pilot can bend his knees at any stage to ease off this "foot accelerator", whereas changing the trimmers requires the use of the pilot's hands which might be buy elsewhere.

 

3) The brake toggles, one held in each hand, attaches to a line which cascades into multiple lines which attach all along the rear of the wing. The left hand pulls down the port-side trailing edge while the right-hand pulls down the starboard trailing edge.

 

This has two effects on the wing... pulling the brakes increases the AoA to slow the wing down, but it also adds more curvature to the wing and acts like Flaps, to increase the lift. Pulling harder, much more drag is created which slows the wing down substantially, to the point where it will eventually stall. This is used to flare at touchdown. The brakes are also used assymetrically as the primary steering mechanism on the ground and in the air.

 

4) Power control affects the pitch, as mentioned earlier. More power increases the AoA resulting in less airspeed, but more climb.

 

Increasing the power increases both the climb-rate as well as the climb-angle (due to reduction in horizontal velocity).

 

OK, so here we go... for takeoff, trims are opened approx 15mm on my wing. Brakes on ZERO while we build up airspeed on the takeoff run, then as we approach takeoff speed, I tend to apply some (limited) brake input to lift-off a bit earlier, hold that brake until positive departure from the ground is achieved (as dumping the brake after takeoff would result in immediate touchdown again) then ease off the brake once I have about 2 meters or more bellow me. While easing off, the climb rate and angle is reduced as the wing adjusts to the new airspeed, then resumes again albeit at slightly reduced levels.

 

The speedbar is never used during or immediately after takeoff, though its a comfortable footrest, so most pilots get it onto their feet shortly after takeoff, as soon as they are comfortable and seated properly in the harness. In this case, I had not even started to get the speedbar out yet.

 

So there I am, just airborne, just got into the harness seat, just starting a left-hand turnout, not yet pulled my trimmers in for a better climb-rate, when the engine suddenly starts seizing. First thing is to hit the engine-kill switch to prevent further damage, then... where to land?

 

There is no time to pull the trimmers in for a lower descent rate and a lower airspeed and softer landing, so we land as-is and make the best we can with the situation...

 

As the engine dies, so the AoA suddenly reduces from its thrust-induced high - this causes the wing to surge forward and dive in an attempt to build up airspeed. When this happens close to the ground, things get very busy, very fast.

 

On a paraglider, every AoA has an associated airspeed (all else remaining equal). Whenever there is a surplus of airspeed, the wing will climb to bleed it off. In fact, this is what the motor does, creating a constant surplus airspeed so the wing gains altitude under power.

 

However, whenever there is a deficit of airspeed, the wing will dive, steeper then its normal glide-angle, then as the airspeed once again matches the "trimmed speed", it will revert to normal glide. Momentary changes in AoA and/or Airspeed due to turbulence, causes the wing to bob up and down trying to balance these forces out.

 

So, the engine is suddenly out, transiting from a powered climb, to a glide, with a dive in midst of this transition... you don't want to hit the ground in this dive, so as the engine dies, you need to compensate by adding some brakes to prevent the surge and dive... at altitude, you would not bother, just let it dive and re-establish glide at its leisure. But near the ground, you hit the brakes very quickly, but gently, just enough to hold the overhead and not dive at the ground.

 

Now comes the problem...on a normal landing, you keep the speed up by keeping off the brakes, to maintain enough kinetic energy to convert to potential energy at the last moment in the form of a flareout, by careful but firm application of the brakes just before touchdown...

 

In this case, I have been forced to use the brakes early to arrest the dive, with not enough altitude to ease off the brakes to prepare for a landing... it is going to be tricky... the best approach in this case is to hold the brake, and add much more just before touchdown, which will result in a near vertical touchdown which is normally the hardest impact on the undercarriage and suspension, namely the pilot's legs and his spine...

 

With the rail coming up on my left and already in a left-turn, trying to get back to the field would be too risky... so, cross over the rail holding the turn, then use the swing-back to help turn into wind at the last moment.

 

I have an injured back and not keen to take a hard landing, so let's look for the biggest bush to fly into, i.e. go for the softest landing I can find (not enough to call it a "crash-landing")...

 

Not enough height to reach that bigger bush up ahead, and a touch too high for the near bush, so deteriorate the glide-angle further by adding even more brake to descend steeper into the nearer bush, but this makes the landing much more vertical, which is why it looks so dramatic on the video... but in reality, there was no stress nor worry, it was a relatively soft landing with low impact velocity albeit at an unusually steep angle.

 

It takes a long time to write this and a long time to read, but at the time there was no time to think when it happened.

 

Most pilots react by instinct gained from experience. A split second after I realised the engine was going to die, I had already evaluated the amount of energy available (altitude and speed), considered the options, formulated the plan and initiated the plan, all without really thinking it through, it just happens... the last four seconds was merely carrying out the rest of the plan without any pressure at all.

 

This is really a non-event by paragliding standards and is not even a reportable incident.

 

I made the video just because I had it on camera and it was one of the first times I had the camera mounted on the rod above the motor showing this particular perspective, and to show to the company that reconditioned the cylinder... it turns out that a small piece of aluminium welding they used to fill the groove from the previous failure (ring-pin came free and gouged the cylinder), had broken free where it runs into one of the ports and jammed between the piston and cylinder.

 

 

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