Napier-Heston belly scoop the first one?

[stepwilk]

The late-'30s Napier-Heston T.5 racer, designed to set the world absolute speed record (which of course it never did, after semi-crashing on its first flight) had a complex mid-fuselage belly scoop to suck in both cooling air for a radiator and boundary-layer air to remove drag. A roughly similar design was soon to become the bull's-scrotum characteristic of the P-51.

Question: can anybody think of an airplane earlier than the Napier-Heston that had a belly scoop--_not_ talking about a cooling scoop on or near the nose--or was the Heston's truly the first?

I'm always nervous about declaring in writing, for an article I'm doing for Aviation History magazine, that something was "the first" and then finding out that some obscure, one-off Finnish fighter actually preceded it...

[Lightning Mate]

Mmmm...

I'll look into that - sounds an interesting challenge. Methinks you may well be correct.

The clever aspect of the P51 duct was that, because of it's convergent profile, it contributed about 200 pounds of forward thrust.

(equation of continuity + Bernoulli etc + boring + blah blah...)

A friend of mine is qualified to fly it - wish I was....

LM

[barit1]

While I think it was always NAA's intent to generate thrust with the scoop/radiator/exit flap, I don't think they actually refined it to achieve a net thrust until the P-51H version.

[Mr BlueSky]

Interesting thought, would be nice to think so, but I'm sure there will be someone to prove it had been done before...;)

If she had not had that very bad first flight, would it have given the Germans ME209 a run for its money...

A little bit of history here: Napier-Heston air racer (http://www.airracinghistory.freeola.com/aircraft/Napier-Heston%20Racer.htm)

Here's a nice pic of the beast...

http://img.photobucket.com/albums/v187/Secudus/th_HestonType5Racer.jpg (http://img.photobucket.com/albums/v187/Secudus/HestonType5Racer.jpg)

[Brian Abraham]

The following is from "Spitfire - The History" by Morgan & Shacklady.

http://i101.photobucket.com/albums/m56/babraham227/l0007.jpg
http://i101.photobucket.com/albums/m56/babraham227/l0008.jpg

You can see that Meredith proposed in 1935 a belly radiator and the preliminary Spitfire drawing shows a belly mount. It is obvious that the design of the P-51 radiator draws on Meridith work.

P-51 Radiator
http://i101.photobucket.com/albums/m56/babraham227/51.gif

In the magazine 'Aeroplane' for May 1999, there is an article by the late Lee Atwood, vice-president of North American Aviation in 1940, entitled 'We can build you a better airplane than the P40'. The aircraft of course is the P-51.

Lee indicates that the propeller thrust at full power was about 1000lbs. However, the drag of the cooling radiator was of the order 400lbs! That is, nearly half the available thrust was required just for cooling the engine

By careful design of the radiator and its ducted cooling system, it was possible to use the heat released by the radiator to generate 350lb of thrust, thereby reducing the net drag of the cooling system to just 50lbs. This was a rather special achievement, possible due to the work F.W.Meredith, in 1935, at the Royal Aircraft Establishment at Farnborough. This reduction in cooling drag was mainly responsible for making the Mustang some 30 MPH faster then the Mk IX Spitfire, despite the higher critical Mach number of the Spitfire wing.

The question is, did the Napier-Heston T.5 racer draw upon Merediths work? I would presume it did.

Some excerpts of F.Meredith's notes

Summary.

(a) Introductory (Purpose of investigation). --The recent increase in the speed of aeroplanes has brought the question of cooling drag into prominence and forced the application of the principle of low velocity cooling. An analysis of the performance of a cooling system enclosed in a duct is required to guide further research and design.
(b) Range of investigation. -- The theory of the ducted radiator is developed and a basis of calculating the drag is provided.

The effects of compressibility are also investigated.
(c) Conclusions. -- It is shown that the power expended on cooling does not increase with speed for a properly designed ducted system but that, owing to recovery of waste heat, a thrust may be derived at speeds of the order of 300 m.p.h.

Attention is drawn to the importance of the momentum of the exhaust gases at high speeds of flight.

Introductory. -- Cooling of aero engines involves the exposure of a large heated surface to a stream of air, a process which involves the expenditure of power owing to the viscosity of air. Until recently, it appeared that this fact imposed an intractable limit to the speed of aircraft since, whereas the heat transfer only varies directly as the speed of the air over the surface, the power expenditure varies as the cube. Thus even though the exposed surface be adjusted until only the required heat transfer is effected, the power expenditure increases as the square of the speed.

The advent of wing surface cooling appeared, at one time, to offer a solution of this difficulty by effecting the cooling without any additional surface. There is, however, reason to believe that the heat transfer necessarily increases the drag of the wing. Apart from this, the Supermarine S 6 B utilised practically the entire exposed surface for cooling and additional surface inside the wing. Further advance in speed appeared to depend upon raising the temperature of the surface.

It is the purpose of the report to show that, by correct design of low velocity cooling systems, in which the surface (whether in the form of honeycomb radiator or of fins on the cylinder heads and barrels) is exposed in an internal duct, the power expended on cooling does not increase with the speed of flight, but that, on the contrary, it should diminish to vanishing point at a practicable speed beyond which the cooling system contributes to the propulsion.

Effects of compressibility of the air. -- These effects are four.

(1) The effective temperature of the air is raised by the kinetic energy of the main stream.
(2) The drop in pressure across the radiator is increased for the same mass flow by the reduction of density resulting from heating the stream.
(3) At altitude, the power necessarily expended in the radiator varies inversely as the square of the density and inversely as the cube of the available temperature difference.
(4) The available energy of the cooling stream is increased by the expansion after the addition of heat.

Effect of the momentum of the exhaust gases on the drag of an engine installation. - Various proposals have been made to utilise the energy of the exhaust gases to assist the induction of the cooling stream, although design to date has apparently been little affected by consideration of the momentum of the issuing gases.

Broadly it may be stated that the effect of the momentum is the same whether it be diffused with the cooling stream or not. It should be noted, however, that some of the benefit in thrust will be lost by a consequent increase of skin friction drag if the exhaust gases scrub an appreciable surface at high velocity. For this reason diffusion of momentum inside the duct my be desirable and this may be convenient method of diffusing the exhaust heat.

The thrust derivable from the rearward direction of the exhaust gases is given by the product of the mass flow and the velocity of exit and the latter quantity depends upon the internal design of the exhaust system. The thrust power is, however, also proportional to the speed of flight. Thus it becomes increasingly important to utilise this thrust as the speed of flight increases.

No attempt is here made to asses the power which may be available from this source. It is suggested, however, that if by the use of suitable deflectors for guiding the exhaust gases round the necessary bends and by the avoidable of excessive unguided expansions, an appreciable proportion of the original energy of the exhaust gases can be preserved, this will provide an appreciable increment to the thrust horse power of a high speed aeroplane.

Conclusions. -- The employment of the principle of low velocity cooling avoids the necessity for an increasing expenditure of power with increasing speed provided the exit conditions are adjusted to suit the speed.

Further the combined effects of compressibility and heat transfer from the radiator may reduce the power consumption to nothing if the size of the radiator is adequate. By the use of the heat of the exhaust, in addition, and appreciable thrust may be expected from the presence of the cooling stream.

Finally, attention is drawn to the importance of the momentum of the exhaust gases for a high speed aeroplane, although no attempt is made to deal with this point quantitatively.

[stepwilk]

thank you, Brian, that's wonderful; you've done a huge amount of work on my behalf

It is interesting that Arthur Haqg of Napier (ex-de Havilland) was said to be an expert in low-drag ducting of engine cooling and induction air, and he did much of the design work on the racer (along with George Cornwall of Heston). The airplane was Hagg's idea, originally--the thought of building something tiny and superlight around the Sabre. So whether he profited from Meredith's workor was proceeding in parallel we'll probably never know.

You've also reminded me that the P-51's magical ductwork didn't so much add speed to the airplane, at least at first, as it did reduce net cooling drag.

[drawbarz]

Stepwilk,

Still not back in circulation yet, but we're getting there. Just wanted to thank you for your email...

John

[Brian Abraham]

Stephan, not sure if you are aware of this article on the racer

1943 | 0955 | Flight Archive (http://www.flightglobal.com/pdfarchive/view/1943/1943%20-%200955.html)

It would seem from the drawing that the design did not draw upon Merediths work. I'm not sure exactly how you are defining the "first with a belly scoop". To me the Hurricane would seem to fit the bill of having a belly scoop (first or not), though I don't know if it embodied Merediths drag reduction ideas as did the Spitfire and P-51.

Edited to add I came across this article written by J. Leland Atwood of North American at An engineer's perspective on the Mustang | Flight Journal | Find Articles at BNET (http://findarticles.com/p/articles/mi_qa3897/is_199906/ai_n8870829/?tag=content;col1)

North American Aviation (NAA) Mustang fighter is generally credited with a 20- to 30mph speed advantage over most of its WW II contemporaries . This speed advantage also permitted a considerable increase in range that required more fuel, but not enough to significantly reduce speed. Records show that some 275 U.S. aces were "made" in P-51s. The reasons for the Mustang's significant performance capability have never been clearly explained, and I hope to clarify why its aerodynamic features enabled this capability.

To begin: in 1940, the British Purchasing Commission, which I dealt with, had a member-H.C.B. Thomas from Farnborough whom I found to be familiar with the Meredith Report. This report outlined a feature that could enhance the performance of any internal-combustion engine at high speeds by using a radiator form of heat dissipation. A low-velocity airflow through the radiator was one element of this, and it was apparent to me that the larger the radiator, the lower the speed of the air flowing through it; this approached one of the Meredith Report's objectives.

I therefore offered Mr. Thomas sketches and other descriptions of a Mustang design that had the main radiator in the rear of the fuselage. The alternatives were wing radiators such as those used on the Spitfire and the Bf 109, and under-engine radiators such the P-40's; both positions limited radiator size and the length and size of the ducting that could be used to handle and control the cooling air.

In addition to the radiator's rearward position, after the design contract had been awarded and at the recommendation of NAA's aerodynamics group, it was decided to use a new airfoil of a class generally designated as "laminar flow." This was being developed at NACA (later NASA) at Langley Field, Virginia. A 1939 report by Eastman Jacobs and others at Langley contained the results of the tests of some small laminar-flow airfoils. The drag on these small models was quite low, and there was some hope that laminar flow could be achieved much farther back on an airfoil than had been predicted by previous investigators. The publishers of the report, however, warned that they had not been able to obtain laminar flow on wings of anywhere near the size of those required for actual aircraft and that their tests were to be taken only as the results from laminar-flow models of not more than six inches in width.

In spite of this warning, however, both Ed Horkey (leading aerodynamicist at North American) and Bell Aircraft's chief engineer, Robert Woods, decided to try laminar-flow profiles on the P-51 and the Bell P-63, respectively. These airfoils were incorporated on the Mustang and the Bell airplane with the hope that laminar flow could be extended well back on their wings. Extensive efforts were made to polish and protect the P-63 wing's leading edge profile, but the results were equivocal. Those who advocated the laminar flow wing felt that the Mustang's outstanding performance resulted from laminar flow over most of the wing. Kingcobra designers felt they were getting a similar effect, although that aircraft's performance did not justify this conclusion.

With respect to the Mustang, many tests-including some in recent years-have shown that extensive laminar flow was not developed on the Mustang wing and that the drag of the wing was probably no less than that of conventional wings of the same thickness and taper ratio. On the other hand, the Mustang's cooling drag was much lower. This was the result of using a ducted radiator with a large area and a slow-speed airflow through it (Pr and P2); closing up the exit and creating a back pressure restored the momentum of the cooling of air (momentum lost in radiator transit). This was possible because of the radiator's cooling capability, which, to be adequate in a full-power climb, was much more than that required at high speed and high dynamic pressure. According to calculations given in a supporting paper, the drag created by momentum loss in passing through the radiator can be reduced from some 400 pounds to close to 30 to 40 pounds because of the offsetting momentum of the jet thrust from the radiator exit (V2).

Since these two effects, i.e., the wing drag and the radiator momentum recovery, have never been disentangled in the literature, a technical reason for the Mustang's performance has never been clearly identified.

NACA had taken the lead in airfoil development and had worked out a large series of airfoils that were used generally throughout the industry. For instance, the Spitfire wing was of the NACA 2200 series-13 percent thick at the root and 6 percent thick at the tip. This is the same airfoil series as is used on the DC-2 and the original North American BT-9 and AT-6 trainers. To improve the stall characteristics, I later changed the NACA 2200 series on the AT-6 trainer to the 4412 at the tip. It is quite probable that the Spitfire's wing, being only 6 percent thick at the tip, had a lower drag than the Mustang's wing as actually incorporated.

The point of all this is that nearly all WW II fighters operated at Mach numbers of .65 or less. The primary advantage presented by the so-called laminar-flow wing was therefore not in drag reduction but in high-speed dives, where temporary airspeed shock waves were created on the wing's upper surfaces and a loss of control and lift occurred as the critical Mach number was exceeded. This was a phenomenon we called "compressibility," and it became the subject of a huge amount of research. The Mustang pilot, with his laminarflow wing, had a higher critical Mach number, so he could point the nose down and know he could out-dive virtually any airplane and recover relatively easily. The P47 and P-38, however, with their older, fatter wings would hit compressibility and have to use their dive flaps to recover safely. So, besides being an overall clean design, the legendary Mustang's speed and range rest as much on carefully designed radiator airflow as on anything else. As is often the case in aircraft design, it was the seemingly small details that counted.

[Lightning Mate]

Brian,

Excellent stuff Mate.