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F.F.V.S. J22 Fighter Aircraft
From a technical perspective
Service in the Swedish Airforce 1943-1952
From a technical perspective Service in the Swedish Airforce 1943-1952 |
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Fighter comparison, 1000-1150hp Fighter comparison, mid to late 1943 |
AerodynamicsPerformance projections When projected, the J22 was supposed to be as fast, or faster, than the available fighters of the time (1940). The major limitation was the available powerplants. The only engine that was available at the time was the Pratt & Whitney TWC3-G, or the Swedish made copy, the STWC3-G. The max power was 1050Hp for take off, which was similar to the Messerschmitt BF 109E and Spitfire I and II. Thus, in order to achieve the same performance, or better, the drag had to be very low. The wooden panels were polished to be very smooth and attention was given to minimize gaps in between the panels. It was concluded that the wooden panels would have 10% lower friction drag than a similar flush riveted aluminum design. The aircraft needed to be small in size in order to keep the wetted area as small as possible and the weight needed to be fairly low in order to keep the wing loading within reasonable numbers in order to make it maneuverable. Here is a list of some basic aerodynamic numbers: Wet area: 57.95m² 625ft² Drag coefficient, Cd_0 0.02202 Drag coefficient, wet area, Cds_wet 0.00608 Flat plate drag area, Cd_fp 0.352m² 3.8ft² As I will show in the comparison section, the overall drag of the J22 was about 20% less than the P51 Mustang which had a flat plate drag area of 4.65ft²! Early during pre-design it was concluded that the wing was going to have a single taper in order to minimize the induced drag, but also for ease of manufacturing. Compare this to the Spitfire with it's semi-elliptical planform. The wing tip airfoil was going to have a fairly large relative thickness as they wanted to minimize the twist in order to maintain low drag at high speeds, but also to maintain good handling at lower speeds. During pre-design, Bo Lundberg predicted to top speed of the J22 to be 570km/h (354mph). The top speed of the J22 was 575km/h (357mph) which was very close to prediction. The spinner added 5 km/h (3 mph) to the top speed. Airfoils I do not have any confirmation of the airfoils used, but my own research has concluded that the root airfoil was a NACA 23015 and the tip was a modified NACA 0012-64. The tip airfoil is somewhat of a mystery as it apparently was a modified symmetrical section. The modification was made to the leading edge which gave it more camber. The modification seems to be very similar to the NACA Tn 2228 (NACA Technical Note 2228 which was released in 1950. In this publication 6 different modifications are made to the leading edge camber of a NACA 63(sub 1)-012 airfoil section. The modification most closely resembling the leading edge of the J22 tip airfoil is the number 5 modification.
The purpose of this modification was to improve the stalling characteristics of the section by delaying the leading edge separation of the airflow. According to NACA Tn 2228 the stall angle of attack was increased 4-5°. It was concluded that the stalling characteristics remained almost the same but at a this higher angle of attack. Thus this was suitable for an airfoil at the tip to prevent tip-stall and to make it more maneuverable especially at high G-turns. This picture shows the tip airfoil and I have traced the contours in order to show the geometry.
The Focke Wulf 190 used the NACA 23015.5 as the root airfoil and the NACA 23009 as the tip airfoil. It had some tricky stalling characteristics due to the following facts:
I am sure Bo Lundberg and his aerodynamicists knew about the characteristics of the 23-series section airfoils and wanted something more benign for the wing tip. The 23-series sections are one of the most commonly used airfoils even today. The early Cessna Citation and the Piper Mirage/Meridian use the 23-series airfoils, just to mention a few. The NACA 0012-64 is an airfoil, as far as I know, that has not been used much, especially with a leading edge modification. As mentioned in earlier, the J22 wing also suffered from lack of torsional rigidity. All J22's were essentially individuals and they had different qualities. Most pilots seem to think that if you pulled hard in a turn it would tip stall and enter a spin. From my research I doubt that it was as bad as the FW190 though. Lift distribution I used a Vortex lattice software called "Nurflügel" to check the lift distribution of the wing. This is a software designed for flying wing analysis, but works fine for this application too. Later on we will compare this to the CFD analysis performed by my friend, Leonard Wish. This is the Spanwise loading in cruise flight, Cl x c.
The bold curve shows the lift distribution of the J22 wing at a cruise CL of 0.18 at 4000m and the thinner line the optimum elliptical lift distribution. The Oswald efficiency factor is 95.5% which is very close to the optimum elliptical lift distribution. This is the Spanwise distribution of the lift coefficient, Cl, at cruise CL=0.18:
The spanwise distribution of the lift coefficient during stall will predict where the stall begins:
The CL at which the wing will stall in the clean configuration is 1.35 at 14° angle of attack, as shown above. Looking at the graph, it shows that the stall begins at a point close to 55% semi span, just at the inner section of the aileron. A tip stall should/could be avoided if this was the case, since the ailerons should be able to function as they are not stalled. However, assuming that that this is the case during a straight and level 1G stall, then the stall characteristics would be benign, but during an accelerated stall, the wing would twist due to the higher bending and torsional load and the somewhat lack of torsional rigidity. For this purpose, I will assume that the tip will twist 2-3° during load and it will change the distribution as shown below:
As you can see, the point where the stall begins have now moved to about 60-65% semi span, in the middle of the aileron. This means that the ailerons are partially stalled and would not be able to prevent nor control the tip stall. This would prove that some J22's suffered from certain tip-stalling tendencies and others, which might have been more rigid in structure, would have had more benign characteristics. Flap contribution. The J22 use a single slotted flap, which can be deflected 40°. The spanwise lift distribution (Cl x c) of the wing with fully deflected flaps looks like this:
The maximum CL with the flaps extended is 2.0, which corresponds to a stall speed of 137km/h (85mph).
This is the distribution of Cl with fully deflected flaps. CL=2.0 and an angle of attack of 10°:
It can be concluded that the J22 had, for the most part, benign stall characteristics. In some situations, such as high G accelerated turns, it would most probably tip stall. The stall speed with flaps extended, was relatively low compared to aircraft such as the P-51 and the FW 190, which had stall speeds of about 100 and 110mph respectively. The lower stall speed also means that the instantaneous turn performance was better than the P51 and the FW 190. This will be shown in the Comparison section. Wing location The interference drag in between the wing and the fuselage depends, to a part, of the relative location of the wing. If the wing is located low or high in reference to the fuselage, a large wing fillet is required (Spitfire, DC-3) to decrease the interference drag, which in turn adds more wetted area. If a fillet is not used, the drag will be a lot higher due to the separation of the airflow in that region.
If the wing is located in the middle, or close to it, only a small fillet is necessary to smoothen out the flow. The wing location of the J22 is slightly below the center of the fuselage and the fillet is rather small. This contributed to the low interference drag of the wing-fuselage intersection of the J22. We will take a closer look at this in the CFD analysis.
Drag breakdown In a pie format, the drag breakdown looks like this:
As shown above, the drag contribution of the wing and the fuselage are almost identical. It is also interesting to see that the cooling drag is not abnormally high considering that it is a radial engine. Small general aviation aircraft, like Cessna's and Piper's, usually have a cooling drag contribution of 25-35%. I have included all the drag from gaps, surface protrusions and antennas in the various section. The improvement that could have been considered would have been to:
An overall decrease in drag might have been possible by 12-18%. This would have resulted in a Flat plate drag area Cd_fp of 3.12 - 3.34ft². The top speed would have increased to 372-380mph @ 11,500ft and the speed at SL with 1050Hp would have been 336-345mph, and with the 1200Hp version, the top speed at SL would have been 352-360mph. Of course, these are just speculations, but they probably could have been achieved.
Comparison with other WWII fightersThe following picture will show how the J22 compared to a number of WWII fighters. The graphs shows the wetted area, Cd_0, Cds_wet and flat plate area. Wet area comparison:
Cd_0 comparison
Cd_0 is a comparison of drag in reference to wing area. The Cd_0 of the J22 was very similar to the Spitfire. Cds_wet comparison
Cds_wet is in my opinion the best comparison of drag in reference to aircraft size. It measures the drag in reference to the wetted area. The Mustang is still the winner, but the J22, with a Cds_wet of 0.00608, is still very low. Flat plate comparison
The flat plate comparison gives a true drag comparison. The total drag is referenced as a flat plate. The J22 has about 20% less drag than the P-51, and about 30% less drag than the Spitfire. Conclusion The drag of the J22 was very low. Taken into account that it had a radial engine, with inherently higher cooling drag, the total drag was very low. The Cds_wet was 15% higher than the P-51, but compared to other radial designs, like the FW 190A, the Cds_wet was 14% lower! The Spitfire also used the Merlin engine, but being a liquid cooled design, it still had fairly high drag. The Cds_wet of the J22 was 30% lower, which is really interesting. It can be concluded that the aerodynamic design of the J22 was very clean, especially when taken into account that it had a radial engine!
Turn performance, instantaneous and sustained.
Instantaneous turn performance is in direct proportion to the wing loading. Lower wing loading equals better instantaneous turn. Sustained turn performance is a combination of Wing loading and power loading. Best sustained turn performance is with a low wing loading and a low power loading. I have not taken stick force gradients and stability into consideration here. The higher the number in the graph, the smaller the potential turn radius. The J22 compares fairly well. Compared to the P51D, it could out-turn it both instantaneously and sustained. The best comparison from a turn performance would be a Spitfire Mk IX. The chart is valid up to about 15,000ft.
Relative acceleration, level flight
In this chart I have made a very rough comparison of the relative acceleration in between a number of fighters. It is based upon power loading and drag. The chart would be valid for flights at 8-15,000ft and acceleration from 200-300mph true. Even though the J22 was underpowered in comparison, it could still beat most of the competition in a straight and level acceleration, up to 300mph. Beyond that it would most probably lag behind. If the opponent was to engage in a characteristic "Turn 'n Burn" dog fight, the J22 would have the upper hand, both turning and accelerating. If the opponent would engage with energy tactics, the acceleration would not be a factor. Needless to say, at higher altitudes the J22 would not have the upper hand. |
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