Stretching Fabric Beyond Limits

Published by: Dave Schultz on 8th Mar 2013 | View all blogs by Dave Schultz

Stretching Fabric

Beyond Limits

BY J. MAC MCCLELLAN

 

 

WHEN MOST NONPILOTS LEARN that airplanes can be skinned in cloth

instead of metal, they are surprised. The idea that fabric can carry

the loads of flight is not intuitive. Most of us at one time or

another have ripped our pants in a way that leaves lingering conviction

that we wouldn’t want to bet our life on highly stressed

cloth and stitched seams.

But anybody with knowledge of aircraft construction knows

that fabric cover has a long, durable, and proven history. Fabric

cover is so strong that it is still used on a number of high-performance

aerobatic airplanes such as the Pitts and Eagle biplanes.

And many large and fast airplanes of the World War II era used

fabric covering on critical flight control surfaces.

In the early days of aviation, most airplanes were skinned with

fabric, usually made from cotton or linen. When the covering is

coated with dope or more advanced paints, it becomes smooth

and impervious to air penetration. In more recent decades, manmade

fibers have been created and woven into much stronger and

more durable fabrics. With proper installation and care, modern

fabric covering can fly safely for decades.

So it is a surprise that fabric cover failure would bring down a

light-sport aircraft, killing the two people onboard.

The airplane was a P&M Aviation Pegasus Quik 912S that

was manufactured in the United Kingdom. The Pegasus Quik

is a weight-shift-control (WSC) aircraft, meaning the pilot

pushes and pulls on a control bar to shift

the weight of the fuselage—pod, actually—

and occupants to control the aircraft

instead of using movable control surfaces.

The 912S refers to the Rotax 912 engine,

rated at 100 hp, that powers the Quik.

The most basic WSC aircraft are little

more than a wing with the pilot suspended

below. The pilot grasps a bar and

muscles his weight fore or aft to pitch up

and down. And he shifts his body left or

right relative to the bar to bank the airplane

to turn. There is no yaw control in

the normal sense.

A WSC is what most of us would call a

hang glider. Hang gliding is popular along

seashores, where a breeze can produce lift

and there are dunes or cliffs to launch

from to begin the glide.

The Pegasus Quik shares the same

basic control concept with a hang glider,

but is a much more advanced aircraft.

The Quik has a fuselage pod and tricycle

landing gear. The two seats are side by

side. And the four-cylinder Rotax

engine is mounted behind the seats

with a pusher propeller.

The swept wing of the Quik is

mounted on a pylon—mast is another

word that comes to mind—protruding up

from the fuselage. The wing is hinged in

such a way that it is free to pitch leading

edge up and down relative to the fuselage,

and to bank left or right. Dual

control bars that are attached to the

wing protrude down to a position in

front of each seat.

The Quik is no slouch, with a maximum

takeoff weight of only 903 pounds

for its 100-hp engine to push. Many

light-sport airplanes weighing almost

50 percent more have the same 100 hp

available. And with 114 square feet of

wing area, the Quik has low wing loading,

giving it a slow stall speed and

tight maneuverability.

The mission was to fly around a

small island in Hawaii on what was

billed as an “introductory instructional”

flight. Aircraft such as the Quik

that are approved under the light-sport

aircraft (LSA) rules cannot be flown for

compensation or hire to take a person

on a sightseeing flight as you can, with

some restrictions, in a standard category

airplane.

The 49-year-old pilot flying the Quik

held a sport pilot certificate and a flight

instructor rating for WSC aircraft. He

also had a repairman certificate for LSA

in the WSC category, was certificated to

pilot balloons, and held a private pilot

certificate for single-engine airplanes.

He was legally certificated to conduct an

introductory instructional flight, but

given that the operation was in Hawaii,

and the flight would be mostly over the

beach and near cliffs arising from the

ocean, one could wonder if instruction

was the primary mission for the flight.

The pilot’s logbook was not recovered

by the NTSB, but on a medical

certificate application nearly three years

before the accident, he reported having

1,800 hours’ total flying experience.

The Pegasus Quik left the factory in

Marlborough, England, about four years

before the accident, with an empty

weight of 464 pounds. With its

maximum takeoff gross weight of 903

pounds, there were 439 pounds of useful

load available. Maximum fuel capacity

was 17 gallons, so with the tank full, the

payload was down to 337 pounds for

people and any other items on board.

About a year after the Quik was manufactured,

a ballistic recovery parachute

system was installed, along with several

other pieces of additional equipment.

Maintenance records show that the BRS

and other equipment added 38.5 pounds

to the empty weight, lowering the fullfuel

payload to 298.5 pounds.

The NTSB reports that the pilot and

passenger together weighed approximately

420 pounds. If the fuel tank were

full, the Quik would have weighed about

1,024.5 pounds at takeoff, or about 121

pounds—13 percent—above the certified

maximum gross takeoff weight.

The Quik flew across the island to a

beach where there is an impressive rock

formation rising out of the water. The

weather was good VFR. Witnesses

reported seeing the Quik fly back and

forth along the beach and inland toward

the cliffs. The pilot reportedly flew

close to the cliffs, making steep turns

back toward the ocean. Because the

fuselage pod must remain suspended in

tension below the wing, all maneuvers

must be positive g. The manufacturer

restricts pitch attitude to a maximum of

45 degrees up or down and bank angle

to 60 degrees.

Several witnesses reported seeing the

pilot fly toward the cliffs, apply full

power, and nose-up steeply in an apparent

attempt to climb over the

450-foot-high rocks and enter a valley.

Witnesses said that the pilot didn’t clear

the cliff, but banked very steeply to the left

at the last moment, missing the cliff face

by about 50 to 100 feet. As the Quik nearly

completed the 180-degree turn away from

the cliff face, witnesses said the bank

angle increased to almost vertical. One

witness said he saw the airplane begin to

side-slip downward in the steep bank at a

rate he estimated to be 500 to 600 fpm.

The pilot managed to roll level, but was

only about 200 feet above the water in a

nose-down attitude.

The NTSB reports that witnesses had a

good enough view to see the pilot fully

extending his arms on the control bar,

pushing forward in an attempt to raise the

nose and stop the descent. One witness

saw the wing begin to buffet and said he

then heard a loud “pop” and the fabric on

the right side of the wing went slack.

Several witnesses then saw the Quik complete

a barrel roll to the right before it

impacted the water at an airspeed the

NTSB estimates to be 70 knots. The BRS

chute was not deployed. Both onboard

were killed by impact.

Divers raised the Quik from about 30

feet of water. The aircraft’s pod was

crushed and wrinkled in a way that is

consistent with impact forces from the

front left. The wing sustained severe

impact damage, and the fabric covering

had numerous tears and holes. NTSB

investigators could not be certain exactly

what fabric failure was caused by impact

with the water and what may have failed

in flight.

The NTSB also recovered two GoPro

compact video cameras that had been

mounted on the Quik. One camera was

aimed ahead of the airplane, and the other

pointed back toward the occupants.

Investigators were able to recover the

video images and, using the view ahead

and the background view of the rearwardfacing

camera, reconstruct the flight path.

The video record confirmed witness

reports of the Quik maneuvering along the

beach and cliffs, including the very steep

turn away from the cliff and dive during

recovery from the turn.

The fabric cover on the Quik wing is a

polyester material made by a company

that specializes in making fabrics for sailboat

sails. The loads on aircraft covering

and sails are similar, and in both applications

ultraviolet light is a major threat to

the strength of the fabrics over time.

That’s why sailors cover their sails with

UV-resistant canvas when the sails are

furled. The NTSB provides no information

on whether the Quik was stored

inside when not flying, or left out in the

tropical sun.

The Quik maintenance manual

includes the warning to “check your sail

for ultraviolet damage regularly. Flying

with a damaged sail could cause structural

failure, injury, or death.” The maintenance

manual directs that the fabric cover

should be tested annually or every 100

hours for strength degradation caused by

UV damage.

The typical fabric strength tester is

called a Bettsometer, which has a probe

that penetrates the fabric. The tester is then

pulled against a calibrated scale to determine

how much force is required to rip a

small area of the fabric or a seam. The manual

specifi ed 1,360 grams of force on the

Bettsometer is the minimum before the

fabric cover must be replaced. The logbook

of the accident airplane shows the most

recent 100-hour inspection occurred one

year before the accident. The Quik had

fl own a total of 380 hours at that time.

There was no record of the required

Bettsometer test being performed. The previous

100-hour inspection records show the

test was performed and the fabric strength

tested okay. NTSB tests of the fabric of the

recovered wing showed the Bettsometer

results averaged 950 grams for the right

wing and 800 grams on the left side.

A representative of the Quik manufacturer

told NTSB investigators that about

two weeks before the accident, the pilot

had contacted him to purchase a new

wing. The representative said the pilot

told him the aircraft had accumulated

about 500 hours of flight time.

The NTSB determined the probable

cause of the accident to be “the pilot’s continued

operation of the aircraft with

deteriorated wing fabric and his aggressive

maneuvering at low altitude, which

resulted in the right wing fabric’s failure

during fl ight.” The Board found that contributing

to the cause of the accident was

“the pilot’s loading of the aircraft in excess

of the maximum gross weight limit.”

The Board also points out that the

video of the flight shows that the “student”

never touched the flight controls,

nor were there any flight activities that

suggested flight training.

Another great article brought to you by the EAA:
http://www.sportaviationonline.org/sportaviation/201303?pg=86#pg86

Comments

24 Comments

  • Abid Farooqui
    by Abid Farooqui 5 years ago
    Unfortunately not a surprise to me. I saw this coming and warned the instructor about his attitude in respect to flying aggressively overloaded with an aircraft whose wing stayed exposed to UV for hours on end even when not flying. My comments were met with harsh words and denial. It is utter rubbish that these guys in Hawaii operate a commercial trike business with no hangers. I know the situation there about hangers and I know its hard to get one but if you are concerned about safety, operating trike instruction business without a hanger is a sure shot sign of getting way more UV exposure than you would otherwise have. I told this to NTSB and Honalulu FSDO as well when they asked me what can be done about improving safety of trikes in HI (Besides making sure that instructors actually carry proper amounts of gas with proper reserves for each flight and tame down the needless hot-dogging). They told me that is not something in their control about hangers but they will warn the participants about hot dogging and fuel requirements.
  • John Olson
    by John Olson 5 years ago
    All you have to do is bag your wagon when you're done flying. Simple, effective and cheap.
  • Rizwan Bukhari
    by Rizwan Bukhari 5 years ago
    >>you do it with little effort and rapido,

    That was funny :) You my friend should also get into book writing business
  • Abid Farooqui
    by Abid Farooqui 5 years ago
    Bagging the wing -does- do something to the sail. Its not a no effect item. Wrapping up the sail less carefully puts creases and they also diminish the sail's strength. They found a lot of creases on this wing in question above. How many of us own Bettsometer, seriously. A betts test every 100 hours or sooner depending of harsh usage is one correct way to judge the sail condition. This includes all stitching etc.
  • John Young
    by John Young 5 years ago
    Hi Monty,

    There's plenty to read and learn about a bettsometer. Rule of thumb - 2500 hours of UV which is about 800 hours of flying - the wing is poked.

    Regards
    John
  • Dave Schultz
    by Dave Schultz 5 years ago
    Testing Dacron Fabric
    http://www.trikepilot.com/members/profile/1871/blog-view/_558.html
  • John Young
    by John Young 5 years ago
    Hi Monty.

    Do not make "light" of this. A 19 year-old wing really needs a 6 month betts test. The saving grace is the very light wing loads that we encounter.

    There are plenty of videos online that demonstrate a betts test.

    Fly safe and keep well.

    Regards
    John
  • John Olson
    by John Olson 5 years ago
    Bagging the wing -does- do something to the sail. Well we found something to agree on Abid. Bagging the wing protects it nicely. I tend to double-bag mine, even tripple-bag it. One of the very real benefits of bagging your wagon is that you get hands-on with every part big and small, what amounts to an annual inspection for most trikers, on the regular basis. As for creasing the sail, this is one of the furst lessons taught to the newbie hangie. Don't crease it when you roll the sail- roll the sail when you roll the sail. Rusty-my amigo Rusty not my trike Rusty, my amigo Russty at Gunnison Gliders makes a great UV bag at a very reasonable price. It only takes a.few.minutes to pull it on and takes care of sun damage nicely. I have my wing in a Rusty bag right now. Flew it this morning, then bagged it.
  • John Olson
    by John Olson 5 years ago
    I forgot to mention tip bags, which every hang glider comes with but which I have never aeen supplied on a trike wing. Maybe Airborne supplies tip bags? I know them guys are hangies too. Well, the sail is rolled up and a tight-fitting tip bag is pulled over the tip before the wing is set on the ground. If you don't have tip bags they are simple to make. Just cut the legs off a pair of trousers and you have two tip bags and a pair of cutoffs. If they are not so tight to stay on the tips as you continue to break down them a couple of bungee balls works fine. There are a couple other pads I might recommend.too. Don't bake your wing. Bag it.
  • John Olson
    by John Olson 5 years ago
    In some cases cut-off pqnt legs work fine fot tit bags
  • Mark Pansing
    by Mark Pansing 5 years ago
    My gibbo came with bags. I cut a whole in them and wear them on my legs under my trench coat.
  • John Olson
    by John Olson 5 years ago
    That's because Gibbo is a hangie too.
  • Abid Farooqui
    by Abid Farooqui 5 years ago
    Every trike wing I have flown except one came with those tip bags.
  • Chuck Burgoon
    by Chuck Burgoon 5 years ago
    For many years Mylar has been use for the leading edge insert material. I don’t particularly like it due to its properties. In particular, the ease with which it is bent and permanently deformed (and no inherent padding traits). Otherwise, it works well for its intended purpose.

    In the 80’s there were three materials in use for leading edge inserts, one of which was same Mylar material used today.

    The other two were foam inserts. One foam type was used by Flight Designs on the Demon. It was a very soft white foam about ¼’ thick. It was not firm enough to maintain the leading edge profile very well, but was nice for folding and bagging.

    However, the best leading edge material I’ve ever seen was what Bennett used on the Streak. It was the about ¼’ yellow thick dense stiff foam that held leading edge definition and folded and bagged nicely. It was the best of both worlds. I always plan to switch to this foam when my Mylar gets deformed, but so far I haven’t had to…yet. I anticipate the need with my new strutted Orca if I take it on the road much.

    In the past my 103 *road trike* wing of choice has no insert and has straight battens. Quick easy fold-up with the battens rolled in the sail. But sadly…I sold my 103 trike last week…
  • Jake McGuire
    by Jake McGuire 5 years ago
    "also shoving a hook through the fabric and yanking on it seems primeval! sure is scope for missinterpretation!"

    no scope for misinterpretation with the bettsometer test. step 1: look up the required strength. step 2: pull until the scale shows that much force. if you have a giant tear in your wing, it failed.
  • Chuck Burgoon
    by Chuck Burgoon 5 years ago
    Monty, all battens were straight plastic sail-boat battens in the early days. Mine flies really well. Funny thing is many prospective trike pilots come up to me after watching it fly and say "that's what I want". Then I explain that is an antique that I'm flying strictly for nostalgia’s sake.
  • Chuck Burgoon
    by Chuck Burgoon 5 years ago
    They also used fiberglass rods for battens back then.
  • John Olson
    by John Olson 5 years ago
    Pulling the Mylar inserts and.swapping sides with them.works great for removing folds and getting the curve out of them. Only takes a few minutes but it takes lots of hands. 4 guys makes short work of it.
  • John Olson
    by John Olson 5 years ago
    I don't remember that Monty. Huh. I'll be checking Bessie in the midnight hour...
  • Chuck Burgoon
    by Chuck Burgoon 5 years ago
    It’s in the sail cut. Sort of like a bat wing compared to a bird wing. You don’t really have to have battens to make one fly fairly well. They originally only were used to reduce trailing edge flutter. You could use a trailing edge tensioner line to do this. Only after many years did “pre-formed” battens come into the picture. Despite their performance benefits I prefer straight batten wings for my 103 “road trikes”.

    I want to have either a radial batten flex-wing or a batten-less flex-wing. First I need to buy a sewing machine and learn how to use it, because none of the sail makers I’ve talked to want my money.

    I’ll post a picture of one of my old straight batten wings with my 103 trike.
  • Chuck Burgoon
    by Chuck Burgoon 5 years ago
    I flew the Chronos for a couple of seasons on my Cosmos 532 trike. It was pretty advanced for its time. Like the Bullet, Bandit, Tiburon, and Hazard. All pretty hot for their day. My friend has been trying to sell his Bandit (10M) for years…can’t get rid of it. Shame, it’s a pretty nice wing.

    Regarding flutter…I can’t live it…it makes me crazy.

    There are a few tricks to get rid of it…if you have it.

    I prefer sail makers who use multiple layers of heavy cloth in flutter prone sections…3 and 4 layers are used…with a small price in handling characteristics.
  • reb wallace
    by reb wallace 5 years ago
    john Fetz in lake havasu built one for less than $20 bucks. he test all our fabric and threads
  • Chuck Burgoon
    by Chuck Burgoon 5 years ago
    Sail tester I use:

    http://www.youtube.com/watch?v=GzmNWblFI0o

    You can buy it from:

    http://www.air-techinc.com/

    I think it was around $35
  • Todd Asher
    by Todd Asher 5 years ago
    Well, if my life and my passenger are in the balance, for $70 some odd I'm gonna use what the manufacturer used. Small price to pay for the comfort of knowing, for me anay. Take care!
    Todd
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