Curse of the three-bladed prop (Luftwaffe)

[skiswimcycle]

This topic has most certainly already been covered, but I wonder why the Me-109G-6 had a
very healthy 1475hp, yet had a top speed of 386mph (give or take).
The early Ki-61 had less horsepower and a bulky radiator fairing, yet if my memory serves correctly,
was only 15-20mph slower. Was the three-bladed prop of the 109, and the FW-190
not taking advantage of all the available torque and HP?

[Colin1]

Originally Posted by skiswimcycle

This topic has most certainly already been covered, but I wonder why the Bf109G-6 had a very healthy 1475hp, yet had a top speed of 386mph (give or take). The early Ki-61 had less horsepower and a bulky radiator fairing, yet if my memory serves correctly, was only 15-20mph slower. Was the three-bladed prop of the Bf109 and the Fw190 not taking advantage of all the available torque and HP?

The G-6 with the DB605A-1 had 1,455hp and was good for around 400mph at a loaded weight of around 3,150Kgs if I recall but which version of the G-6 are you talking about specifically?

The Ki61's powerplant sported 1,175hp and was good for about 360mph at a loaded weight of around 3,500Kgs.

I think there's a little more going on here than the choice of propeller, the Kawasaki was carrying an extra 350Kgs over the Messerschmitt and had roughly 300hp less with which to accomplish anything; it was 10% heavier and yet had nearly 20% less power.

I also don't think your reasoning is safe, you assume that the G-6 is only 20mph (closer to 40mph) faster than the Ki61 when it is just as reasonable to assume that the Kawasaki is a sound enough design to approach the Messerschmitt in terms of performance. Recall that the Bf109F could match the Spitfire Mk IX for speed with significantly less horsepower. This might suggest that the Ki61 is a clean design and that the ventral radiator isn't as bulky as you propose. I'm looking at the arrangement on the Ki61 in the banner as we speak and it seems quite tidy at a superficial, visual level.

Returning to propellers specifically, the airforce of each nation seemed, rule of thumb, to have their own remedies for harnessing the power of bigger engines, the Brits added blades, the USAAF added diameter and the Germans seemed to make their blades broader. German engineers were just as savvy as their Allied counterparts when it came to extruding more power from a system, if they felt they needed more blades or diameter, that's the way they would have gone.

[Shortround6]

There are (and were) entire text books devoted to propeller design. There is not only disk area but blade area to consider as well as the fact that no one propeller is the best at all altitudes and speeds. Different designers may have biased the design more towards one part of the performance spectrum than another. Without know what these biasis were or why they made the choices they did it seems a little difficult to second guess them now.

[vanir]

The Luftwaffe seemed to go for wider blades before they'd go to more blades. There could've been any reasoning behind this including simple asthetic preference.

Also worth considering is the altitude ratings of these engines and overall performance requirements. The DB-601E to the 601A-1 was much like the leap to the Merlin 45 from the Merlin II, whilst the DB-605A-1 effectively gave that same performance more lazily, efficiently and with a higher rated altitude (ca.6km versus ca.5km). The regular flight/combat performance of the 605A was still around 1200hp which is like a 601N working very hard or much the same as a 601E or a Merlin 45 (not at its overboost WEP rating).

In short most of the time these aircraft performed roughly similar for a few years, increasing in efficiency, serviceability and reliability, and though they did have much higher emergency ratings to be used for very limited period.
Propeller redesign was not a major consideration, new three-blade constant speed and aeromechanical types had just been evolved and fitted.

The new Fw190A had a very powerful engine, so was fitted with a wide paddle prop. But this a/c had problems getting the bench rating out of its airframe installation. The same prop would be good until 1945 as it turned out, with only a marginal increase in the output potential of the 801.

With the Messer the DB-605A had to be derated for a time owing to chronic overheating at the standard maximum power settings (1.42atm was no prob for the 601E but in the 605A it caused all the seals to rupture and flamed the engine, holed pistons, etc.). Some development continued not only to improve outputs at 1.55atm, 1.75atm and one day 1.8atm and 1.98atm, but simply to restore the 1.42atm start u-notleistung rating of the 601E.
It wasn't going to need a bigger prop for a while and was stuck with something like 1330PS between about mid42 and mid43 for maximum output (1.32atm max start u-notleistung derating). Roughly 1250PS at the rated altitude which actually isn't so bad comparatively for around 6km alt, not far behind a MkIX Spit and altogether way ahead of any MkV. It was always its altitude performance out of the box which made the Daimler so special, benefit of cubic inches.

With the MkIX the RAF intentionally concentrated on altitude performance. The four blade prop was to compensate for a much higher rating at medium and high combat altitudes, we're talking more than 300hp difference although the increase was only marginal at sea level. What this did was to vastly improve climb performance at those altitudes and of course alt speed performance and ceiling, but I do believe the LF MkV which entered service around the same time was quicker at low altitude, that still had the three blade prop didn't it?

For the Luftwaffe during 1944 its high altitude interceptors were advancing in development, and these were proposed with four-blade props although actually fitted with even wider three-blade ones. The Ta153 was always going to be four-bladed, so was the Me-109K-14 both of which never wound up in production (some K-4 are rumoured to have entered service with DB-605L fitted however, with three blade props and the Ta153 of course became the Ta152 projects based on modified Fw190 airframes to speed production).

A new very thick paddle three-blade prop was used on Fw-190 and Ta152 aircraft which exceeded 2000hp maximum boost performance, to increase climb and altitude performance. I think only Fw190F-9 and some A-9 got them, and all Ta152. Can't remember if the D-11/12/13 got them but probably.

Generally more/wider blades it seems were preferrable for altitude and climb performance, but required proportionately high engine outputs to compensate for parasitic horsepower drag on the crankshaft, not to mention I'm sure increased aerdynamic drag, whilst at sea level fewer/thinner blades were preferrable but still had to be matched so as not to overspeed the engine nor fail to make use of its available power for thrust.

The Germans probably preferred wider blades to thinner blades for better aerofoil control using the aeromechanical system, whilst the British and Allied nations preferred thinner blades for more consistent settings using the constant speed system. Overall blade area to reiterate, whether number of blades or width of the paddles, is governed by engine output and aircraft performance requirements regardless. I'm sure you'd find the blade area of a Ta152H is similar to that of a P-47D or Spit MkXIV.

And now I invite the forum for corrections ..........

[FLYBOYJ]

Also consider harmonic vibration, RPM match to the engine and fuel consumption.

[paradoxguy]

Originally Posted by skiswimcycle

This topic has most certainly already been covered, but I wonder why the Me-109G-6 had a
very healthy 1475hp, yet had a top speed of 386mph (give or take).
The early Ki-61 had less horsepower and a bulky radiator fairing, yet if my memory serves correctly,
was only 15-20mph slower. Was the three-bladed prop of the 109, and the FW-190
not taking advantage of all the available torque and HP?

Just a minor comment here--the 386 mph measurement was for the Bf 109G-6 with underwing 20mm cannon. The "clean" version of the Bf 109G-6 had a speed of 406 mph, from what I've read.

[Vincenzo]

little note DB 601E use C fuel, DB 605A B fuel this give almost a partially explanation that 601E run at higher pression w/o trouble

[Milosh]

Originally Posted by paradoxguy

Just a minor comment here--the 386 mph measurement was for the Bf 109G-6 with underwing 20mm cannon. The "clean" version of the Bf 109G-6 had a speed of 406 mph, from what I've read.

[drgondog]

if it is any use, both the RAF and NAA tested the p-51B with a 3 bladed prop and the XP-51F had a three bladed prop - no discernable difference in top speed... but as Joe noted the art of prop design is a lot more than speed.

The fastest Mustang was the XP-51G with a five blade Rotol prop. 495mph and celing of 46,000, limited to that ceiling only because no pressurization of cockpit.

[vanir]

Originally Posted by Vincenzo

little note DB 601E use C fuel, DB 605A B fuel this give almost a partially explanation that 601E run at higher pression w/o trouble

601E uses B4 fuel with 1.42atm, the 601N in the F-2 uses C3. The high pressures were attained by revisions of the piston crown, redesigned heads and improved valve timing (engine speed increased from 2600u/min 601N to 2700u/min 601E).
Half the trick about keeping detonation pressures is controlling temperatures in the cylinder, using things like exhaust excavation and fluid mechanics. Most of these changes were incorporated onto the 605A but the larger displacement provided new overheating issues for the same configuration which were the main cause of troubles in the larger engine. And this aside from oil temperatures which always plagued the 109 series.

I'd say you could call a 605A an overbored 601E and be pretty well on the money. But 601A and N motors are a completely different animal.

Ultimately about 1.5atm was possible on B4 fuel and 1.75atm on C3 without MW50 or other coolants, but this required very refined combustion chamber design, fluid dynamics and valve timing, being finally achieved late44. You might say Daimler-Benz finally had the DB-605 sorted in production about January 1945, which was a tad behind schedule for a motor due to enter mass production mid42.

[Vincenzo]

Originally Posted by vanir

601E uses B4 fuel with 1.42atm, the 601N in the F-2 uses C3. The high pressures were attained by revisions of the piston crown, redesigned heads and improved valve timing (engine speed increased from 2600u/min 601N to 2700u/min 601E).
.

true i've do confusion for invert alphabetic order, the new engine has low letter

[cherry blossom]

One other factor was that the Germans used synchronized guns and would have lost fire power if they had increased the number of blades.

[drgondog]

Originally Posted by cherry blossom

One other factor was that the Germans used synchronized guns and would have lost fire power if they had increased the number of blades.

very good point