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nuckolls.bob(at)aeroelect
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 Posted: Mon Oct 07, 2013 4:15 pm Post subject: The dreaded downwind turn . . . |
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At 10:48 AM 10/7/2013, you wrote:
| Quote: | | When he started talking about the the wind accelerating to aircraft during the turn, he lost me. |
As soon as he says "a lighter aircraft will
accelerate faster than a heavier aircraft"
and bases the statement on an analysis of
"square feet of sail" . . . he blew it.
I sat next to a guy for several years who wrote
heavy duty software for autopilots that flew UAV's of all
stripe from 80Kts to 500 Kts. He tapped simple-ideas
to from my high school physics to describe how
the airplane flies.
I've been meaning to do an article on the
physics of this maneuver and have some drawings
done . . . somewhere on the hard drive. I'll
see if I can dig them up and perhaps finish
the article.
But while you read the words of folks wrestling with
the 'dreaded downwind turn' go to the POH data for
your airplane and get one number. Target IAS for
best glide angle. This is the speed at which your
distance over the ground versus altitude lost is
at a maximum.
When folks are talking about the physics of flight,
they're talking about airplanes that are being
'flown' . . . in other being controlled to conditions
that maximize performance. This generally calls for
a speed well above best rate of climb combined with
a 270 degree turn at 45 degrees of bank into the crosswind to
get pointed back toward the runway.
When the engine quits with a runway close behind
you, its easy for those performance numbers get obscured
by other things running around in your head. It takes
a Bob Hoover like attitude to first get the nose down
to achieve best glide angle whether you are turning
or not; stack in a 45-degree banked turn on top of
really adds pucker factor.
At speeds below best glide, lift/drag ratios can go
into the toilet in a hurry. On of my most cherished
flight instructors was checking me out in a Beech
Flying Club A36 one day. After three or four
by-the-book touch and goes he said "let me show you
something."
"Stay at pattern altitude until you're on final."
"Uh, okay . . ."
As I turned final I reached for the throttle . . .
"Nope, not yet . . ."
The runway disappeared under the nose and I reached
for the throttle . . .
"Nope, not yet . . . "
A few seconds later he said, "Okay. Close the throttle
and give me 75 MPH."
I set it up and was amazed. Sink rate went to something
around 1200 ft/min. A few seconds later I acquired a better
short-final view of the runway and he said, "Power up to
arrest your descent, push the nose down and give me 90
MPH over the numbers."
After that, the landing proceeded normally.
The point being that maneuvering around at speeds
below best glide is where the airplane sinks fast
even if you're not turning . . . faster still if
you turn. Best glide is well above those speeds
at which perturbations in IAS due to gusting can
begin to eat into your energy margins for maneuvering.
Best rate and particularly best angle of climb
speeds have the nose really high with a commensurate
boat-load of drag. Whether the airplane remains
controllable just before contact with the ground isn't
a matter of winds, it's a matter of altitude and
the pilot's willingness/ability to EXCHANGE energy
stored on that altitude for controllable airspeeds.
Airspeeds that bring you to the ground with energy
to flare and keep the wheels attached to the airplane.
The alternative is a 1000+ feet per minute descent rate,
no energy to flare and a probability of having to eat
your wheels.
There are two magic numbers that drive your decision
to turn around best glide speed and ground clearance
KNOWN to be sufficient to the airplane's demands as
determined by experiment and practice.
Barry Schiff tells us how in this article.
http://tinyurl.com/mo8wux4
Note that Barry mentions nothing about controllability
hazards for having made a downwind turn. That's because
the target approach speed for greatest probability of
success is well above that where perturbations in wind
velocity make any difference at all. See:
http://tinyurl.com/kzr95lk
http://tinyurl.com/m29yg5y
http://tinyurl.com/k9y3z4y
http://tinyurl.com/mmgmojr
It's all about lift/drag ratios and energy budgets.
If your choice of pitch angle is poor (IAS) or your
stored energy (altitude) is lacking then it's a bit
specious to drag 'hazards of downwind turns' into the
discussion . . . things were probably not going to
go well anyhow. You become a passenger in your airplane
doing experiments with the controls.
Bottom line is that EAA, of ALL organizations, should
have folks with talents on a par with the honorable
Mr. Schiff to vet their articles.
http://tinyurl.com/mtn32qf
Bob . . . [quote][b]
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email(at)jaredyates.com
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| Posted: Mon Oct 07, 2013 4:58 pm Post subject: The dreaded downwind turn . . . |
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I don't think Barry Schiff is eligible to write for Sport Aviation,
being that he has written mostly for AOPA Pilot and not Flying
Magazine. To get articles into SA, it seems that one must come from
writing about Bonanzas in Flying Magazine. I've been hoping that the
editorial staff shakeup was a Hightower legacy, and that the new staff
would bring us more like Brady Lane/Mike Busch and less like Mac
McClellan, but so far, no dice.
| Quote: | Bottom line is that EAA, of ALL organizations, should
have folks with talents on a par with the honorable
Mr. Schiff to vet their articles.
|
do not archive
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raymondj(at)frontiernet.n
Guest
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| Posted: Mon Oct 07, 2013 6:25 pm Post subject: The dreaded downwind turn . . . |
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I was thinking the same thing: If you can't trust an article from the EAA...
I'll be interested to see if/how they walk back the article in the next issue.
[quote]Raymond Julian
Kettle River, MN.
"And you know that I could have me a million more friends,
and all I'd have to lose is my point of view." - John Prine
Bottom line is that EAA, of ALL organizations, should
have folks with talents on a par with the honorable
Mr. Schiff to vet their articles.
http://tinyurl.com/mtn32qf
Bob . . . [b]
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uuccio(at)gmail.com
Guest
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| Posted: Wed Oct 09, 2013 2:03 am Post subject: The dreaded downwind turn . . . |
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When he started talking about the wind accelerating the aircraft during the
turn, he lost me.
As soon as he says "a lighter aircraft will
accelerate faster than a heavier aircraft"
and bases the statement on an analysis of
"square feet of sail" . . . he blew it.
Yes. I went back and tried to figure out what he was *actually* thinking
when he calculated the sail area necessary to accelerate the plane. From
the numbers he is using, I figured out that he is calculating the surface
that a flat 2000lb object would have to present to a 20kt wind to produce an
acceleration from 100kts to 140kts in 16 seconds:
A wind V will produce a force on the airplane equal to F = (1/2 p V^2).C.S.
Here the first term in brackets is the dynamic pressure (p is the density of
air), S is the “sail area” (the cross-sectional area presented by the
aircraft perpendicularly to the wind) and C is the drag coefficient. Since
this force is also equal to m.a (mass times acceleration), we can solve for
S:
S = 2.m.a / (p.V^2.C)
If we convert everything to compatible units (I’ve used metric here) we get
V=10.3m/s, p=1.225 kg/m^3, m = 2000 lb = 909kg and a = 40/16 kts/s = 1.275
m/s^2
Plugging everything and assuming C = 1 gives S = 17.83 m2 or 192 sq ft of
sail area.
Why is this nonsense? Well, first, C for the profile of an aircraft is
bound to be lower than 1… after all, the reason it flies is that it has a
somewhat aerodynamic shape!
Let’s look at the numbers for a normally loaded PA-28 whose weight is
approx. 2000lb. The wing surface is 160 sqft. But the surface that the
aircraft would present to the wind during the downwind turn would be less
than that, probably no more than 50-100 sqft, with a C that probably varies
from 0.1 to 0.5, depending on the angle the aircraft makes with the wind.
That already reduces the magnitude of the force applied by the wind by a
factor somewhere between 4 and 20…
Put it another way; if C really were 1 and the aircraft presented a
cross-sectional surface of only 50 sqft during normal S&L flight at max
speed (123 kts), then the power necessary to sustain S&L flight would be P =
F . V = ½ p. V^3.C.S, which comes out to about 1000 hp, about 8 times what
an 0-320 engine can deliver.
As Bob correctly states, what does in fact accelerate the plane from 100 kts
GS to 140 kts GS as the aircraft completes the downwind turn is mostly the
power produced by the engine (in a normal turn), or the tradeoff of
potential energy vs kinetic energy (in a descending gliding turn) and the
normal aerodynamic forces that the IAS produces on the airfoil...
Ø a 270 degree turn at 45 degrees of bank into the crosswind to get pointed
back toward the runway.
its’ actually 180+45=225 degrees back to intersect the runway and then a 45
degree turn to line up again… which means the total required turning is 270
degs.
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uuccio(at)gmail.com
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| Posted: Wed Oct 09, 2013 2:33 am Post subject: The dreaded downwind turn . . . |
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Put it another way; if C really were 1 and the aircraft presented a cross-sectional surface of only 50 sqft during normal S&L flight at max speed (123 kts), then the power necessary to sustain S&L flight would be P = F . V = ½ p. V^3.C.S, which comes out to about 1000 hp, about 8 times what an 0-320 engine can deliver.
Actually my estimate above is an underestimate because some that power has to go towards producing lift, not just overcoming drag…
Sacha
[quote][b]
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