Bf109F vs P-38F
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Hi Vincenzo,
>there is an other trouble why the luftwaffe data, in Rausch pages, are so different from your calculation?
Hm, in which regard? I used the same 635 km/h figure from the "Messerschmitt calculated" data sheet on his site. I didn't use the "Rechlin tested" 670 km/h figure because there is so little detail on the source.
Regards,
Henning (HoHun)
>there is an other trouble why the luftwaffe data, in Rausch pages, are so different from your calculation?
Hm, in which regard? I used the same 635 km/h figure from the "Messerschmitt calculated" data sheet on his site. I didn't use the "Rechlin tested" 670 km/h figure because there is so little detail on the source.
Regards,
Henning (HoHun)
Hi Vincenzo,
>So for you the data on page 12, of your link, aren't reliable?
On page 12, there is "Cw Schnellflug" ('Cd0 for high speed flight') listed on both pages as 0.023.
Power is listed as 1185 HP for 635 km/h, 1280 HP for 670 km/h.
As I pointed out above, this power difference is not going to yield the claimed speed difference due to the reduction in propeller efficiency.
So the data looks self-contradictory. Now the question is: Which speed is correct (if any)?
I spontaneously relied on the 635 km/h figure, but that's just a guess. By comparing to the (in my opinion) more trustworthy Me 109F-1/2 data sheet, I might be able to make it more than a guess, but I haven't found the time to make this kind of cross-check yet.
Regards,
Henning (HoHun)
>So for you the data on page 12, of your link, aren't reliable?
On page 12, there is "Cw Schnellflug" ('Cd0 for high speed flight') listed on both pages as 0.023.
Power is listed as 1185 HP for 635 km/h, 1280 HP for 670 km/h.
As I pointed out above, this power difference is not going to yield the claimed speed difference due to the reduction in propeller efficiency.
So the data looks self-contradictory. Now the question is: Which speed is correct (if any)?
I spontaneously relied on the 635 km/h figure, but that's just a guess. By comparing to the (in my opinion) more trustworthy Me 109F-1/2 data sheet, I might be able to make it more than a guess, but I haven't found the time to make this kind of cross-check yet.
Regards,
Henning (HoHun)
mkloby
Senior Master Sergeant
Hey HoHun,
I'm not greatly into the systems of much of the WWII planes, but why wouldn't the prop rpm stay constant and increase pitch to harness the additional power?
Hi Mkloby,
>I'm not greatly into the systems of much of the WWII planes, but why wouldn't the prop rpm stay constant and increase pitch to harness the additional power?
Hm, I think simply increasing the boost pressure from 1.30 ata to 1.42 ata without increasing rpm would probably induce detonation. Another reason might be that the internal forces would inevitably increase, perhaps beyond what the engine was stressed for.
I have a copy of the DB 601E power chart, so I know they increased the engine speed for Start- und Notleistung, but the engineering reason not to accept the higher boost pressure at constant boost is not obvious from that.
Your question is certainly well-justified as though it's typical to have emergency power ratings employ higher engine speeds than the typical "30 min" settings, many engines also were able to benefit from increased boost pressure at constant rpm (though this more typically was used to make a short-duration emergency rating more powerful).
I believe the general idea of exploiting the better propeller efficiency at lower rpm was used with the GM-1 injection systems, which at high altitude and great true air speeds would have really pushed the propeller tip speeds. GM-1 was typically used at the "30 min" rating engine speed - and the requirement to limit the internal forces in my opinion is one of the reasons GM-1 could only used at altitudes somewhat above the engine's full throttle height.
Regards,
Henning (HoHun)
>I'm not greatly into the systems of much of the WWII planes, but why wouldn't the prop rpm stay constant and increase pitch to harness the additional power?
Hm, I think simply increasing the boost pressure from 1.30 ata to 1.42 ata without increasing rpm would probably induce detonation. Another reason might be that the internal forces would inevitably increase, perhaps beyond what the engine was stressed for.
I have a copy of the DB 601E power chart, so I know they increased the engine speed for Start- und Notleistung, but the engineering reason not to accept the higher boost pressure at constant boost is not obvious from that.
Your question is certainly well-justified as though it's typical to have emergency power ratings employ higher engine speeds than the typical "30 min" settings, many engines also were able to benefit from increased boost pressure at constant rpm (though this more typically was used to make a short-duration emergency rating more powerful).
I believe the general idea of exploiting the better propeller efficiency at lower rpm was used with the GM-1 injection systems, which at high altitude and great true air speeds would have really pushed the propeller tip speeds. GM-1 was typically used at the "30 min" rating engine speed - and the requirement to limit the internal forces in my opinion is one of the reasons GM-1 could only used at altitudes somewhat above the engine's full throttle height.
Regards,
Henning (HoHun)
mkloby
Senior Master Sergeant
HoHun,
From what I read, the Bf 109F had a constant speed propeller. What I don't understand is if engine rpm increases, why would prop rpm increase? Why wouldn't the prop increase pitch in order to maintain a constant prop speed?
From what I read, the Bf 109F had a constant speed propeller. What I don't understand is if engine rpm increases, why would prop rpm increase? Why wouldn't the prop increase pitch in order to maintain a constant prop speed?
Hi Mkloby,
>From what I read, the Bf 109F had a constant speed propeller. What I don't understand is if engine rpm increases, why would prop rpm increase? Why wouldn't the prop increase pitch in order to maintain a constant prop speed?
It would, but there are just two ways to increase the power of a piston engine: Increasing the force of the power strokes, or increasing the frequency of power strokes.
If you increase the force of the power strokes, you'll increase the torque of the engine, and there is usually some kind of torque limit not close off the 30 min operating conditions. (Even worse and more complicated, bad things can happen in the combustion chamber. I'm not familiar with all the nasty details of that either, but "detonation" is a keyword there.)
Alternatively, you can increase the frequency of power strokes, which will keep you clear of the torque limits as the power is developed through an increase of engine speed at the same internal forces as before. That was historically the main route to emergency power for piston engines.
Below a diagram showing the operating conditions of the B-29's R-3350 engine. Note that there is a narrow band for each manifold pressure at which operation of the engine is both possible and sensible. You can always increase rpm without danger, that will needlessly increase fuel consumption. If you decrease rpm however, that will increase "engine pressures", internal forces or torque, which are more or less three different ways of looking at the same problem.
(While on the US B-29, it was the task of the flight engineer to keep the engine in the best operating range, many German aircraft were equipped with single-lever control systems that selected the optimum combination of rpm and boost and a couple of other variables such as ignition timing automatically, based on information from perhaps a half dozen of sensors. I'm not sure if the DB 601E in the Me 109F-4 was manually or automatically controlled, but the concept of staying in the small safe and sensible operating area as outlined in the B-29 graph was the same regardless.)
Regards,
Henning (HoHun)
>From what I read, the Bf 109F had a constant speed propeller. What I don't understand is if engine rpm increases, why would prop rpm increase? Why wouldn't the prop increase pitch in order to maintain a constant prop speed?
It would, but there are just two ways to increase the power of a piston engine: Increasing the force of the power strokes, or increasing the frequency of power strokes.
If you increase the force of the power strokes, you'll increase the torque of the engine, and there is usually some kind of torque limit not close off the 30 min operating conditions. (Even worse and more complicated, bad things can happen in the combustion chamber. I'm not familiar with all the nasty details of that either, but "detonation" is a keyword there.)
Alternatively, you can increase the frequency of power strokes, which will keep you clear of the torque limits as the power is developed through an increase of engine speed at the same internal forces as before. That was historically the main route to emergency power for piston engines.
Below a diagram showing the operating conditions of the B-29's R-3350 engine. Note that there is a narrow band for each manifold pressure at which operation of the engine is both possible and sensible. You can always increase rpm without danger, that will needlessly increase fuel consumption. If you decrease rpm however, that will increase "engine pressures", internal forces or torque, which are more or less three different ways of looking at the same problem.
(While on the US B-29, it was the task of the flight engineer to keep the engine in the best operating range, many German aircraft were equipped with single-lever control systems that selected the optimum combination of rpm and boost and a couple of other variables such as ignition timing automatically, based on information from perhaps a half dozen of sensors. I'm not sure if the DB 601E in the Me 109F-4 was manually or automatically controlled, but the concept of staying in the small safe and sensible operating area as outlined in the B-29 graph was the same regardless.)
Regards,
Henning (HoHun)
Attachments
drgondog
Major
There is an incredible disparity between the turn rates of the 38F and 109 in your plot - particularly at 1000m and 6500m where the velocities cross over.
Are you using the Clmax for the NACA 23016 airfoil 'bare' or with manuevering flaps? With manuevering flaps the USAAF had the P-38F turning with the P-51 at Eglin Field (between 12000 and 15000+ feet!
Your plots show the 51D to be approx 0ne degree per sec slower than the 109 on the deck but you show 5+ degrees difference between the 109 and the 38F on the deck and 4 1/2 degrees at 12,000 where the 38F had parity with a 51A.
Which Clmax are you using for the 38F and what value are you using for AoA at CLmax? What Cd0 are you using for the 38 and what values for trim drag (with or without maneuvering flaps)?
Where is the math?
mkloby
Senior Master Sergeant
HoHun,
Interesting - it just isn't make sense to me why increasing engine RPM would necessitate increasing prop RPM. On every aircraft I have flown with a constant speed prop, the blade angle is managed in order to maintain a specified RPM. If power is increased, blade angle then increases to maintain the same N2, if power decreases, it reduces blade angle.
I have read that the 109F had an electric operated constant speed prop. I am not sure how this one worked. I have read that it was automatic with a manual override option as well.
CharlesBronson
Senior Master Sergeant
P-38, better firepower and faster, with the correct tactic the messer is toasted. I think the both types meet each other over Tunisia in late 1942...right ? Many Me-109 in that teather of operation were the G type already.
By the way 635 mk/h sound a very good figure for the F-4 considering that is the same speed quoted for the G-2 variant wich had a little more powerful engine ( 1350 compared with 1475 hp).
By the way 635 mk/h sound a very good figure for the F-4 considering that is the same speed quoted for the G-2 variant wich had a little more powerful engine ( 1350 compared with 1475 hp).
Hi Mkloby,
>Interesting - it just isn't make sense to me why increasing engine RPM would necessitate increasing prop RPM. On every aircraft I have flown with a constant speed prop, the blade angle is managed in order to maintain a specified RPM. If power is increased, blade angle then increases to maintain the same N2, if power decreases, it reduces blade angle.
It works more or less like that on the Messerschmitt, too. If you set rpm to 2700 rpm to begin with, you could hypothetically control power through boost pressure alone, though that would not be the most efficient way to operate the engine.
Regards,
Henning (HoHun)
>Interesting - it just isn't make sense to me why increasing engine RPM would necessitate increasing prop RPM. On every aircraft I have flown with a constant speed prop, the blade angle is managed in order to maintain a specified RPM. If power is increased, blade angle then increases to maintain the same N2, if power decreases, it reduces blade angle.
It works more or less like that on the Messerschmitt, too. If you set rpm to 2700 rpm to begin with, you could hypothetically control power through boost pressure alone, though that would not be the most efficient way to operate the engine.
Regards,
Henning (HoHun)
mkloby
Senior Master Sergeant
You seem to be referring to engine RPM. I am referring to maintaining a specified prop RPM - there is no reason I can see that should cause prop RPM to increase turns, regardless of an increase in engine power at higher engine RPM.
Hi Mkloby,
>You seem to be referring to engine RPM. I am referring to maintaining a specified prop RPM
Hm, unlike as with some turbine types, on most aircraft piston powerplants engine rpm and prop rpm are directly proportional to each other as they're connected through a fixed-ratio gearing (or directly, in lower powered types). If I mixed the two terms, that would be because they change in unison so that I didn't think of them as separate
>there is no reason I can see that should cause prop RPM to increase turns, regardless of an increase in engine power at higher engine RPM.
Hm, I don't think that we already touched this topic, but increased thrust (regardless how it's achieved) does in fact improve an aircraft's specific excess power, improving its capability for sustained turns.
Instantaneous turns will be unaffected, of course. I guess that's what you were thinking of?
(Just so that the idealized term "instantaneous" won't cause unnecessary confusion: non-stabilized turns of finite duration would actually be influenced by power available
Regards,
Henning (HoHun)
>You seem to be referring to engine RPM. I am referring to maintaining a specified prop RPM
Hm, unlike as with some turbine types, on most aircraft piston powerplants engine rpm and prop rpm are directly proportional to each other as they're connected through a fixed-ratio gearing (or directly, in lower powered types). If I mixed the two terms, that would be because they change in unison so that I didn't think of them as separate
>there is no reason I can see that should cause prop RPM to increase turns, regardless of an increase in engine power at higher engine RPM.
Hm, I don't think that we already touched this topic, but increased thrust (regardless how it's achieved) does in fact improve an aircraft's specific excess power, improving its capability for sustained turns.
Instantaneous turns will be unaffected, of course. I guess that's what you were thinking of?
(Just so that the idealized term "instantaneous" won't cause unnecessary confusion: non-stabilized turns of finite duration would actually be influenced by power available
Regards,
Henning (HoHun)
Hi Charles,
>By the way 635 mk/h sound a very good figure for the F-4 considering that is the same speed quoted for the G-2 variant wich had a little more powerful engine ( 1350 compared with 1475 hp).
I don't know where you have the 635 km/h for the Me 109G-2 from, but the wartime summary report on the Me 109G-6 credits the Me 109G "Grundausführung" ('basic variant') with 630 km/h at 1.30 ata/2600 rpm. The 1475 HP you list is for the full 1.42 ata/2800 rpm emergency power setting.
Kurfrst - Leistungzusammenstellung Me 109 G.
As background: The 'basic variant' is essentially a G-2 with rigid tail wheel and all the little bumps of a G-6, but without the MG 131 blisters. It's a hypothetical plane serving as baseline for establishing the performance of G-6 series production aircraft with all the variations they were built with, but of course its performance figures were based on flight tests of multiple Gustav aircraft in different configurations.
The real G-2 was a bit cleaner still, and accordingly a bit faster as well.
Regards,
Henning (HoHun)
>By the way 635 mk/h sound a very good figure for the F-4 considering that is the same speed quoted for the G-2 variant wich had a little more powerful engine ( 1350 compared with 1475 hp).
I don't know where you have the 635 km/h for the Me 109G-2 from, but the wartime summary report on the Me 109G-6 credits the Me 109G "Grundausführung" ('basic variant') with 630 km/h at 1.30 ata/2600 rpm. The 1475 HP you list is for the full 1.42 ata/2800 rpm emergency power setting.
Kurfrst - Leistungzusammenstellung Me 109 G.
As background: The 'basic variant' is essentially a G-2 with rigid tail wheel and all the little bumps of a G-6, but without the MG 131 blisters. It's a hypothetical plane serving as baseline for establishing the performance of G-6 series production aircraft with all the variations they were built with, but of course its performance figures were based on flight tests of multiple Gustav aircraft in different configurations.
The real G-2 was a bit cleaner still, and accordingly a bit faster as well.
Regards,
Henning (HoHun)
CharlesBronson
Senior Master Sergeant
I saw that figure in the well know Squadron Signal and the Osprey collection ...I know, I know those are not technical editions.
