From: 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 From: David Lednicer <dave@amiwest.com> Newsgroups: rec.aviation.military Subject: Re: Mustang, was it's wing really laminar flow? Very long Date: Wed, 05 Feb 1997 09:10:29 -0800 Charles K. Scott wrote: > 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. Recent NASA tests have shown that laminar flow can exist on a wing in the slipstream of a prop. However, it exists on an intermittent basis, which leads to higher drag. > 2. Vibration: Maybe > 3. Airfoil surface condition: Yes - and don't forget all the paint lines - NACA wartime data, taken in flight on a P-51 showed that sanding the paint down improved the laminar run. > 4. Manufacturing tolerances: NACA found surface waviness to be the biggest factor in killing the laminar flow. By reducing the waviness to less than approximately .004 inches over a 2 inch span, they were able to get the laminar flow > 5. Wing Surface Distortion in Flight: This couples with 4. ------------------------------------------------------------------- David Lednicer | "Applied Computational Fluid Dynamics" Analytical Methods, Inc. | email: dave@amiwest.com 2133 152nd Ave NE | tel: (206) 643-9090 Redmond, WA 98052 USA | fax: (206) 746-1299 |
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