Also, the following is directly to the point of what I first wrote: "Even laminar-flow wings on the likes of the P-51 probably didn't produce laminar flow once you factor in design variation, surface roughness, dirt, etc."
From
http://yarchive.net/mil/laminar_flow.htmlFrom: Charles.K.Scott@dartmouth.edu (Charles K. Scott)
Newsgroups: rec.aviation.military
Subject: Mustang, was it's wing really laminar flow? Very long
Date: 5 Feb 1997 14:13:04 GMT
Was the Mustang's laminar flow wing laminar or not?
That is a question asked often in several groups and recently, after
finishing "Pursue and Destroy" by Leonard "Kit" Carson, I believe I
have found a definitive answer.
Mr. Carson's credentials are that 1. He flew the Mustang in combat. 2.
He was an engineer who understood aerodynamics. 3. He was a test pilot
for a while after W.W.II. He goes into extreme technical detail while
telling about the Mustang and his career flying it.
Carson begins his analysis of the Mustang and it's laminar flow wing
back in the late 20's and early 30's when NACA, the National Advisory
Committee for Aerodynamics began it's research on airfoils, airflow,
and other aspects of flying. The airplane companies were in no
position to do this research because they did not have the money to
develop and build wind tunnels. He described airfoil research prior to
NACA as piecemeal, with many airfoils being developed by the OTLAR
method (Oh That Looks About Right, my words, not his)
It was during the thirties that NACA established the relationship
between turbulent flow and drag. Their measurements indicated that the
3/32 inch rivets heads and lap joints on the typical metal airliner
"dissipated" 182 horsepower. On one airplane they were measuring, they
found that a coat of paint cost the airplane 91 horsepower over the
same airplane with bare aluminum. They learned that mere dust, fine
sand or a piece of scotch tape "would cause the smooth laminar layer
next to the wing surface to jump over To a turbulent, high drag
condition."
Then, in 1938, in a wind tunnel designed to smooth out the airflow
through the tunnel (designed by Jacobs and Dryden, prior to this wind
tunnel, flow through the tunnel was too turbulent to test laminar
theories) a new type of airfoil was tested that set new and incredible
drag coefficients compared to any airfoil previously tested. It
recorded a drag coefficient of .003 "which was about half of the lowest
ever recorded before for an airfoil of similar thickness."
Further tests conducted in England "demonstrated that laminar flow and
a reduction of drag could be obtained for a considerable distance over
a smooth full scale wing."
This was in the wind tunnel, however, and it turned out that an
enormous gulf existed between test aircraft and the wind tunnel and
combat aircraft.
The following reasons were given by Carson explaining why in real life
laminar flow simply did not occur on the P-51's wing.
1. The effects of propeller Slipstream. Airflow within the arc of the
prop is very turbulent, "the whole fuselage and inboard section of the
wing next to the fuselage operate in that turbulent stream. Tests in
the Langley wind tunnel revealed that airflow within the arc of the
prop (the prop was 11 feet in diameter which meant that turbulent air
was encountered all the way out to within 13 inches of the inner gun
position) was "90 to 95 percent turbulent" (in other words non laminar)
2. Vibration: "Engine and propeller vibrations transmitted through the
structure will induce transition to turbulence." Tests indicated that
laminar flow on twin engine aircraft was greater with one engine
feathered than with both running. Engineers surmised that the lack of
engine/prop vibration on the dead engine side promoted laminar flow.
Honest, that's what the book said. Of course with both props turning,
more of the wing would be bathed in the prop slipstream which as has
been mentioned above, trips laminar flow to turbulent.
3. Airfoil surface condition: "Mud, dirt, ice and frost will induce the
transition to turbulent conditions." "Fuel truck hoses, ammo belts,
tools, guns and large feet in GI. shoes found the way to the tops of
wings" the scrapes and dents this servicing caused had negative effects
on laminar flow.
4. Manufacturing tolerances: "The Mustang was the smoothest airplane
around in 1940, but there is a practical limit in construction. We're
talking about surface roughness or waviness of .01 inches which will
cause transition to turbulence." (remember the afore mentioned dust
and scotch tape which was observed to trip airflow to turbulent). Some
aerodynamicists have stated that true laminar flow did not occur
outside the wind tunnel until the advent of Burt Rutan's Vary E-Z in
the early 70's with it's incredibly smooth fiberglass over carved foam
wing and aft mounted engine which of course kept the wing ahead of the
prop slipstream.
5. Wing Surface Distortion in Flight: Flight brings flight loads which
can and did distort the wing and cause ripples in the wing surface
which were fully capable of tripping the laminar flow to turbulent.
Carson went on to state: "The Mustang wing was a high lift
configuration, as well as low drag. . . the Mustang in squadron service
was not laminar to the same extent as the wind tunnel development
models. Not one day in the past 34 years (the book was written in 74)
has it performed in that manner for any or all of the reasons just
given."
So if it wasn't the laminar flow wing that gave it it's high speed and
extensive range, what was it?
The most prominent speed secret was the dramatic reduction of cooling
drag. Placing the airscoop on the belly just in front of the rear edge
of the wing removed it as far as was practicable from the turbulence of
the prop and placed it in a high pressure zone which augmented air
inflow. Tests in the wind tunnel with the initial flush mounted scoop
were disappointing. There was so much turbulence that cooling was
inadequate and some doubted that the belly scoop would work. The
breakthrough was to space the scoop away from the surface of the belly
out of the turbulent boundary layer of the fuselage. Further testing
showed that spacing it further out would increase cooling but at a cost
to overall drag. Various wind tunnel tests established the spacing at
the current distance which represents the best compromise between
spacing out from the turbulent flow of the fuselage, drag and airflow.
With the flow into the scoop now smooth and relatively nonturbulent,
the duct leading to the radiator/oil cooler/intercooler was carefully
shaped to slow the air down (the duct shape moves from narrow to wide,
in other words a plenum chamber) enough from the high external speeds
to speeds through the heat exchangers that allowed the flow to extract
maximum heat from the coolant. As the air passed through the radiators
and became heated, it expanded. The duct shape aft of the radiator
forced this heated and expanded air into a narrow passage which gave it
considerable thrust as it exited the exhaust port. The exhaust port
incorporated a movable hinged door that opened automatically depending
on engine temperature to augment the airflow. The thrust realised from
this "jet" of heated air was first postulated by a British
aerodynamicist in 1935. The realization of thrust from suitably
shaped air coolant passages is named after him and called the "Meredith
Effect". Some have said that at certain altitudes and at a particular
power setting the Meredith effect was strong enough to actually
overcome all cooling drag; this is not regarded as being accurate by
most aerodynamicists. It greatly contributed to overall efficiency of
the cooling system but never equaled or overcame cooling drag.
Combine the low overall drag of the Mustang with significantly greater
internal fuel tankage than either the Spitfire, Messerschmitt or
Focke-Wulf 190 and you can easily see how it could fly so far. Add the
two 105 gal external wing tanks and the Mustang was fully capable of
flying to any target the heavy bombers could attack in the ETO. Kit
Carson mentioned that he flew more than 35 missions during which he was
in the cockpit for more than 5 hours.
Finally, Carson was interested to find, while reading flight test
reports in research for his book, that the quoted top speed for the
P-51B was less than what was attained during test flying. The
information is as follows:
Report: NA-5798
Title: "Flight Test Performance for the P-51B-1
Date: January, 1944
Test Weight: 8,460 lbs
High Speed: 453 mph true airspeed at 28,800 feet at 67" HG and 1298 HP,
war emergency power, high blower, critical altitude.
The quoted top speed for the B model Mustang is 440 mph.
I can only speculate that it is likely the test airplane used in the
above mentioned flight was a well maintained and unblemished Mustang.
It's probable that the actual combat aircraft would not be able to
quite equal that performance. Never the less, Carson notes this
information and concludes with the following:
"It's easy to see why many pilots preferred the P-51B, including
myself, even if it did have only 4 guns and the "birdcage" canopy. If
you can't hit'em with 4 guns, two more aren't' going to make your aim
any better."
Corky Scott
