The windmill, constantly spinning due to the unbearable heat, inspired me with the thought that it’s time to write something about aviation.
The last of my visits to the military museum in Brussels resulted in a reminder that it is worth continuing the story about the “Spitfire” plane. Here you can read the first story about the plane: https://www.salon24.pl/u/alpejski/1084147,maly-rozowy-bohater But one by one.
While visiting the exhibition, I finally stood in front of a machine that seemed to be calling – look at my propeller!
It is a type of “Spitfire” MK XIV, not so often found in museum collections, equipped with an impressive five-blade propeller with a diameter of 3.18 meters. I stood almost nose to nose in front of him and smiled at him. Such a propeller is not a parade, but also a big problem for the machine and the pilot. You weren’t easy to fly – I whispered in my nose, and the plane seemed to smile under its breath and did not deny it.
The first versions of the “Spitfires” had two-blade fixed pitch propellers.
This meant that the angle of attack of the blades could not be changed. Such a propeller in a fighter had to be constructed in such a way that its optimal operating parameters occurred at high air speeds. This can be seen at first glance in the photo below. Look at the blades of this propeller – their constant angle of attack is very large.
The “Spitfire” prototype with a wooden two-blade fixed pitch propeller. Visible straight exhaust outlets from the engine. Photo: www.worldwarphotos.info
“Spitfire” Mk I early series. Visible exhaust manifolds – one per two cylinders – facing rear. This solution gave a few kilograms of thrust from the exhaust gases ejected from the engine. In the later version of the manifold, the thrust was even up to 18 kilograms! Photo: www.worldwarphotos.info
However, at low speeds, such a propeller does not work effectively, and this is in the critical phase of the flight, which is the take-off of the machine. These problems result from the calculus of the air flow velocity vectors on the blades, which must take into account the translational motion of the machine and the rotational motion of the propeller.
In the next version of the “Spitfire”, this problem was corrected by introducing a three-blade propeller with an adjustable pitch, enabling effective operation in a wide range of advancing speeds of the machine. The propeller works efficiently when it is able to use as much of the engine power as possible, producing the highest possible thrust in the given aerodynamic conditions. The speed of the aircraft, its climb and acceleration parameters in the air and on the runway depend on it. The proof of this was the improvement of the machine’s performance, only by replacing the propeller.
The Mk I, with a two-bladed propeller, reached 577 km / h and needed 382 meters to lift off the ground. When this version received a modern three-blade propeller with staggered pitch, the take-off distance was shortened to 291 meters and the maximum speed increased to 584 km / h.
“Spitfire” Mk I in the London museum with a three-blade propeller. Visible later version of the exhaust manifold. Photo: Alpejska
The first Rolls Royce “Merlin” Mk II engine powering the “Spitfire” Mk I had a power of 1030 hp. The last versions of the “Spitfire” Mk XXIV were powered by Rolls Royce “Griffon”, monsters that developed a power of 2050 hp. For this reason, from time to time the “Spitfire” received another additional blade in the propeller. And so, from the Mk IX version, a four-bladed propeller was used, and from the Mk XII version, a five-bladed propeller.
Of course, the speed of the Spitfire grew with each modification.
When a four-bladed propeller was used after the introduction of the 1720hp “Merlin” Series 60 engine, the “Spitfire” Mk IX reached a top speed of 664 km / h.
This version was, according to the opinion of most pilots, the best in the “Spitfire” family. She climbed bravely and quickly to large ceilings and, at an altitude of 11382 meters, was still perfectly maneuverable, making tight turns. Its flight properties did not pose a threat to the pilot, it maintained good directional stability in the entire range of ceilings, and the surfaces of the ballasts and rudders satisfactorily compensated for the torque of the large propeller during take-off and flight.
The “Spitfire” Mk IXc from the Polish squadron proudly presents its four-bladed propeller. A conformal auxiliary fuel tank is visible under the belly of the machine. Photo: www.worldwarphotos.info
This version reached the limit of the development of the “Merlin” engine, from which it was impossible to extract more power with the use of acceptable operating conditions of the modification.
Therefore, the designers decided to reach for the real beast, which was the successor of “Merlin”, Rolls Royce “Griffon”, which was developed for a long time. It reached the power of over 2,000 HP and despite the same number of cylinders as in the Merlin, it was slightly larger.
“Spitfire” Mk XIV. Three major changes to the structure of the airframe are clearly visible. To accommodate the larger than Merlin engine, the “Grffon” was extended to the front of the fuselage and elegant moldings of the timing key covers were used. A five-blade propeller with a slim cap and a drop-shaped cab canopy providing excellent visibility for the pilot complete the set of changes. Photo: Alpejski
Source: Salon24.pl: Strona główna by www.salon24.pl.
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