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eskflyer(at)lvcisp.com
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 Posted: Thu Dec 21, 2006 5:46 am Post subject: Ski installation |
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Michel and list
I sent Michel a copy of ac43.13-2a chapter 5 ski installation.
This is what is to be followed in any ski installation in a certified
aircraft but should slso be followed in experimental in my humble opinion .
Page 38 of chapter 5 says for a skis rating of 1500-3000 lbs you should have
a downword force of 20-40 lbs applied at the fore end of ski to start to
slacken the aft safety cable . This is approximate . In my opinion again I
use about 35-40 lbs of force applied and find that in landing it helps to
smooth out the bumps and act as a bumper but does not throw me back into the
air either .
Take care fly safe fly slow fly snow
John Perry
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Michel
Joined: 10 Jan 2006
Posts: 966
Location: Norway
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| Posted: Thu Dec 21, 2006 12:35 pm Post subject: Ski installation |
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On Dec 21, 2006, at 2:43 PM, john perry wrote:
| Quote: | I sent Michel a copy of ac43.13-2a chapter 5 ski installation.
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Thank you, John. My skis are copies of the Avid Flyer wheel penetration
skis and I rigged them in accordance to the documentation I got at the
time. I have, in the front and as instructed, two separate bungees and
an extra safety wire.
But, as I was installing these, I was told of something that happened
at my airfield before I started flying. An ultralight aircraft with
homemade skis was about to land when someone on the ground noticed that
one ski was pointing downward. He called the pilot from a handheld VHF
radio and told him about the situation. The pilot tried to no avail to
stall the aircraft and shake it in the hope of getting the ski up
again. After a while, and running short of fuel, he did an emergency
landing off the runway, in a place with deep snow. The plane tipped
around but the pilot could walk away without a scratch. He had only one
bungee and, of course, no extra safety wire to that ski.
That's when I started wondering how a ski can tilt downward in flight.
My opinion is that the shape of the ski tip would create an
aero-dynamic pressure upward and landing with a ski pointing upward is
certainly not the same hazard as when pointing down.
Cheers,
Michel
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Kitfox 3 - Jabiru 2200 |
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smokey_bear_40220(at)yaho
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| Posted: Fri Dec 22, 2006 8:24 am Post subject: Ski installation |
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I Michel,
You need to remember that the ski is flying in the air
and not on it like on the snow. With the curve up, it
acts like the top of a wing upside down and lifts down
in front. Once it goes down a little, it will pull
down hard because the axis is near the middle and not
forward of the center of pressure, which is around 25%
of the length.
Merry Christmas,
Kurt S.
--- Michel Verheughe <michel(at)online.no> wrote:
| Quote: | That's when I started wondering how a ski can tilt
downward in flight.
My opinion is that the shape of the ski tip would
create an
aero-dynamic pressure upward and landing with a ski
pointing upward is
certainly not the same hazard as when pointing down.
Cheers,
Michel
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Lynn Matteson
Joined: 10 Jan 2006
Posts: 2778
Location: Grass Lake, Michigan
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| Posted: Fri Dec 22, 2006 9:10 am Post subject: Ski installation |
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Kurt-
Does this mean that once the ski-equipped plane slows to landing
speed...and the ski stops producing lift...that the ski will then
return to the normal (bungees relaxed) position...about 15° up, as I
read here?
Lynn
On Dec 22, 2006, at 11:24 AM, kurt schrader wrote:
| Quote: |
<smokey_bear_40220(at)yahoo.com>
I Michel,
You need to remember that the ski is flying in the air
and not on it like on the snow. With the curve up, it
acts like the top of a wing upside down and lifts down
in front. Once it goes down a little, it will pull
down hard because the axis is near the middle and not
forward of the center of pressure, which is around 25%
of the length.
Merry Christmas,
Kurt S.
--- Michel Verheughe <michel(at)online.no> wrote:
> That's when I started wondering how a ski can tilt
> downward in flight.
> My opinion is that the shape of the ski tip would
> create an
> aero-dynamic pressure upward and landing with a ski
> pointing upward is
> certainly not the same hazard as when pointing down.
>
> Cheers,
> Michel
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Lynn
Kitfox IV-Jabiru 2200
N369LM |
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smokey_bear_40220(at)yaho
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| Posted: Fri Dec 22, 2006 7:41 pm Post subject: Ski installation |
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Hi Lynn,
Well, you should ask someone with more ski experience
than me. I can only address the aero theory here to
say what "should" happen.
In theory, there is no difference between theory and
reality. In reality, there is often a big difference.
Remember, a theory is just an opinion by someone who
thinks they are important. You get that in schools.
Experience rules. You get that in life. In neither
case do I claim to be important, just still trying to
do my best to help.
In theory, I expect that the bungy would pull up to
the relaxed position unless unbalanced ski weight or
aerodymanic force pulls it into tension. If it is
aerodynamically pulled tense, then some force with
resulting drag is being applied. If I am right, the
force on the bungy is a vector equal to the lift and
drag applied. Threated just like a wind vector, the
aft part of the bungy pull is = to drag. The verticle
part is = to lift (down) which the wing must overcome.
At landing speed and angle of attack you still have
some lift and drag on the ski.
What do you ski flyers actually see happen?
Kurt S. S-5
--- Lynn Matteson <lynnmatt(at)jps.net> wrote:
| Quote: | Kurt-
Does this mean that once the ski-equipped plane
slows to landing
speed...and the ski stops producing lift...that the
ski will then return to the normal (bungees relaxed)
position...about 15° up, as I read here?
Lynn
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Michel
Joined: 10 Jan 2006
Posts: 966
Location: Norway
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| Posted: Sat Dec 23, 2006 4:27 am Post subject: Ski installation |
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On Dec 22, 2006, at 5:24 PM, kurt schrader wrote:
| Quote: | You need to remember that the ski is flying in the air
and not on it like on the snow. With the curve up, it
acts like the top of a wing upside down and lifts down
in front.
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Okay, I understand, Kurt. Hum, this was one of my concern when I first
installed the skis. Was the bungee tight enough to keep the skis
horizontal in flight? So, I opened the door, on the first test flight,
and saw that the aft retaining cable was tight, then I thought it was
okay and that the shape of the tip was aerodynamically holding the ski
nose-up.
Next time I install the skis (snow has first to come to Norway!) I will
use a dynamometer to see how much the bungees pull.
Cheers,
Michel
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Kitfox 3 - Jabiru 2200 |
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av8rps(at)tznet.com
Guest
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| Posted: Sat Dec 23, 2006 5:28 am Post subject: Ski installation |
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If skis are rigged properly, with proper tension and proper springs or
bungees, the tips will ALWAYS stay up in flight. Kitfoxes aren't capable of
such high speeds that they can pull the ski tips down. If they do, they
need heavier springs or bungees on the front cable.
Paul Seehafer
300 hour ski pilot using Avid wheel penetrations skis
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