"PPC Stalls"
Article written for EAA Sport Pilot magazine
February 2005 by Scott Hughes and Frederick Scheffel
Note: We sincerely appreciate your consideration in remembering that all rights are reserved for this article. So please get the written permission of the author before reprinting this article in any way, shape or form. THANKS!
Let me begin by saying I have personally seen nine PPC chute stalls in varying stages. I did four of the stalls myself. Three were intentional to test recovery methods and the forth occurred when I was hit from behind with a strong gust while under full power and in full flare, oops. (Recovered from the stall… no damage) This is not a good position to be in. The other five we have seen were other pilots at various fly-ins and competitions over the past 8 years that proved our recovery methods do work.
When a chute stalls it will fall backwards behind the cart. If you keep the power on and the flair in… the chute will stay back and become a metastable stall. If you do not reduce your power to allow the chute to go back overhead to a flying position and let the excess steering impute back out, the cart will basically fall until it hits the ground with what looks like an inflated chute.
If you reduce the power but keep the flare input you run the risk of full stalling the chute. The dynamics are that the chute will first fall back and stall. If the flair stays the same the chute will fly the outside tips backwards together forming a "U" shape. If you still maintain the flair it will then ball up and you are about to go for a heck of a ride at that point, (hang on). At any time during the stall you could release your flare impute to ¼ brakes and the chute will recover with varying degrees of an opening shock. If you go as far as to have a complete collapse, hold on to your teeth because it is going to be one heck of an opening shock. Unlike skydivers we do not have a slider on our lines to slow down the opening.
I wanted y’all to know that I didn’t arbitrarily pick ¼ brakes. Think of a skydivers chute when it opens. During that violent opening process you want the chute to pressurize as quickly and smoothly as possible in a controlled fashion. Skydivers want to open as straight as possible so as not to run into other jumpers during deployment. Most of the skydiving chute manufactures set their chutes up for opening at ¼ brakes. This provides the most stable position possible.
With that said let’s move on to the information provided by Fredrick that can be found in his training manuals from Skytrails Ranch.
T
here are two reasons why a powered parachute wing may stall.The internal air pressure (maintaining the shape of the wing) is reduced to (or below) the external air pressure.
You exceeded critical Angle-Of-Attack. (For a PPC wing, this is usually a condition called a parachutal-stall or a meta-stable stall.
This article is primarily addressed to the second reason for a PPC wing stall.
Now, "Why a PPC wing flies or doesn’t fly" during a stall begins
by addressing the critical angle-of-attack of the ram-air wing. The following
two pictures show airflow over a typical PPC wing: The first with good smooth
– lift generating - airflow, and the second with problems…
(Please note that a real picture of a PPC wing in flight would show a lot more wing arc and the cells of the wing would be pointed more down – instead of pointed ‘straight-ahead’ as depicted in the graphic. But this is the extent of my artistic abilities.)
Wing stalls due to excessive AOA (Angle Of Attack) are really more of a – non-pendulum - fixed wing problem, where Weight is not the constant and primary airspeed engine. Wing stalling is a huge aspect of a fix wing aircraft-training program - one that takes constant surveillance! Prevention of a fix-wing stalling takes constant awareness of airspeed, engine rpm and angle of attack.
We know there are places & demands for fixed wings (how else could we get from point ’A’ to point ‘B’ without them. But we just love our powered parachutes! Yea buddy! How can you not love this fantastic safety feature of the PPC: The resistance to wing stalls!)
So why is the powered parachute wing resistant to stalls? Well, for this explanation we again go back to the primary airspeed engine - weight! Since weight will not vary during straight & level flight, airspeed will not vary, as long as wing drag remains constant.
Once again… since your weight wants to remain constant, then your primary airspeed engine also wants to remain constant. And if your primary airspeed engine tends to be steady then your airspeed will tend to be steady – as long as there is NO change in the second factor of your PPC’s airspeed: wing drag.
Remember, there are only two basic factors that determine your PPC airspeed: Weight (your airspeed engine) and Wing Drag.
So, if you only increase your wing’s drag during a turn or to flare in preparations of a landing – then you should have no major concerns (like fixed wings) for stalls.
(Please also note that PPC aerobatics is totally outside the range of this article. So aerobatic maneuvers and their effects on ram-air wing stalls will not be considered here.)
A PPC’s rectangular wing will perhaps, stall (on average) at around 18 mph (ranging from 16 to 21 mph depending on individual specifications of your PPC, i.e., wing loading, wing shape, etc) and your vehicle’s weight (cart & occupants) will maintain an average airspeed of around 26-34 mph. So, normally, there is no reason to be concerned with PPC ram-air wing stalls.
Under normal flight conditions for a powered parachute with a rectangular ram-air wing, it would take usually take ALL THREE of the following factors to create a wing to stall or collapse.
A huge increase in wing drag – for instance inducing a full flare – which the PPC pilot uses to temporarily increase lift, increase drag and to reduce air speed. This maneuver also increases the AOA. In other words pulling in to much line.
A steady FULL throttle - which also puts the wing back, (relative to the position of the cart), and thus increases the AOA. This is the normal procedure for gaining altitude in a powered parachute.
A gust of air – from the front, which Mother Nature uses to push the wing back and thus increase the AOA, or a gust from the back which will in effect reduce the effective air (flying) speed.
If all of three of these factors are NOT simultaneously present while flying a standard rectangular ram-air wing, then the likelihood of a PPC wing stall is extremely low! – Perhaps even NON-existent! And it follows that since the pilot controls 2 of the 3 stall reasons, that if the pilot does not perform both pilot factors that can cause a PPC wing stall at the same time, then we do not need to be concerned with wing stalls during normal PPC flights. Therefore, if you do not want to experience a PPC full stall or a metastable wing stall, do NOT simultaneously use full-throttle and full-flare while flying.
The Lock-out during wing inflation (i.e., Meta-stable Stalling or going Parachutal)
A wing ‘lock-out’ occurs when there is not enough initial forward momentum to inflate the canopy and move it through the prop-wash and up & above the PPC cart. During this phenomenon the wing will hang ("stall") at about a 45-degree angle behind the cart (actually this is a ‘metastable wing-stall’ on the ground). And no matter how much you increase the PPC ground speed, the wing stays ‘locked’ in that 45-degree position until you change the suspension line loading – like hitting a bump in the road. Therefore, the best solution in this situation is to change the suspension line tension to the wing. To do this, simply, lower your ground speed (reduce your throttle setting) until the canopy begins to actually fall down behind the cart (this removes the suspension line tension), and then smoothly & rapidly increase your ground speed to "pop" the canopy up & above the cart.
Another technique to solve wing inflation ‘lock-out’ for the rectangular wing would be to pull the canopy back via a little "popping" flare. A quick flare will pull the canopy back at first then quickly releasing, or popping-off the flare will cause it to then ‘sling-shot’ the canopy up & above the cart.
NOW
, if one was to believe a TOTALLY different view for recovering from a stall or chute collapses, for instance, that going to full-flare and full-throttle would be the best way to recover from a stalled or collapsed wing this is the time to try your recovery technique. So, the next time you get a "lock-out" during wing inflation on your take-off roll, try going to full-flare and full-throttle, and see if this puts your wing above your cart and back into a normal flight or take-off position!!! We think not.The Parachutal lock out is also affectionately called the wall. The prop blast just shoots right into the middle of the chute and it just sits there shaking like a big stubborn dog on a leash. Most instructors recommend that you just start over and set up again.
Recovery from a meta-stable ram-air wing stall or wing collapse
The primary objectives in recovery in a PPC are to:
Reduce or Change the tension on the suspension lines. Go to 1/4 to 1/3 brake should resolve the situation.
Get the wing back over your cart
(Remember the meta-stable stall is keeping your wing back behind your cart – similar to the ‘lock-out’ during take-off roll/wing inflation.) You can’t keep pushing the cart forward with full throttle and expect it to go backwards to get under the chute. Reduce throttle to Idle.
Therefore the solution is to:
1st – MOVE YOUR THROTTLE TO IDLE
2nd – REMOVE YOUR FULL FLARE by reducing your Flare to ¼ (Note: In a semi-elliptical or ‘shaped’ wing your will need to maintain ¼ flare to keep your wing from over flying your cart during a stall recovery.)
Note: In some high aspect ratio chutes you might need to input side to side rudder control to help break the metastable stall.
One final note:
The above solution is very easy to state and to give as the correct answer to a metastable stall recovery. HOWEVER, in real life, moving the throttle to idle when you are falling is totally contradictory to your human instincts. Your normal instinct is to increase throttle – hec, in the past anytime that your PPC was descending and you wanted to climb, you quickly learned to increase throttle. Well, now you have to fight your instinct – you must remove all thrust from your propeller – you must move your throttle to idle to allow your cart to fall back under your wing and to get your wing flying again. So, think about this the next time you are ‘sailing-the-skies’ and try to set into your mind, that if you are falling (not descending & flying) that you must move the throttle to idle to recover from your fall and start flying again!
We sincerely hope this has cleared up any questions you may have on stalls, meta-stable stalls, chute collapses and the appropriate recover technique.
Frederick Scheffel
, CEO, AFISouthwest Regional Director - NAPPF
Frederick is the author of the "PPC Guide & Training Manual" and the lead AFI of SkyTrails Ranch, Inc. – a full-time, year-round PPC training center in the beautiful color country of southern Utah.
Scott Hughes,
AFI, CFII, MEI, EIEIOCEO, Hughes Air Aviation, Home of the New "Predator"
South Central Regional Director NAPPF
Scott is Co–Producer of "Flying101", a video magazine and the long a waited video, "Learning from our Mistakes or Crashing 101". He is also chief flight instructor for Chute the Breeze training center in Wharton, TX.
PS We would like to thank, Bill Gargano, (Quantum) and Betty Pfeifer, (High Energy Sports), Tim McCord, (Performance Design) along with Tim Stiff, (Hawkwing) for their input and open discussion with us.
"UL-PPC Guide & Training Manual" are