109 it fly wrong

[butch2k]

Gripen if you have the chart under your eyes you'll notice that Volldruckhöhe is given as 7000m in this chart. But indeed it's not easy to make calculation based on this chart based of such a few number of points.

[gripen]

Butch2k,
Well, the FTH given with the triangle is calculated, none of the planes actually reached it the average being 6700m. The reason that the one  Erla built almost reached it, is most likely the same why the MT-215 performed slightly below average at high altitude ie the engine did not run at specified rpm. In the case of the MT-215 the engine run at 2540rpm, in the case of the that Erla plane, the engine probably run at higher rpm than 2600. By looking the DB 605A chart you can see that at 1,3ata and 2800rpm, the high speed FTH would be around 7,1-7,2km.

Overall the MTT data should be taken with quite strict source critic; as an example you have probably seen a Datenblatt dated 12.5.42 which claims 7,5km FTH for steig-  und kampflleistung and speed 707km/h for that altitude, the same paper gives 732km/h at 7,6km at Start- und Nottleistung.

gripen

[HoHun]

Hi Seeker,

>Do you mean that at X feet; the oxygen content is such that one can open the throttle and not over rev the engine?

Engine speed was actually limited by the propeller, but you could open the throttle fully without overstressing the engine at full throttle height.

Regards,

Henning (HoHun)

[HoHun]

Hi GScholz,

>I thought "full throttle height" was the lowest altitude were you could run the engine at full power, while "critical altitude" was the highest altitude the engine could produce full power.

Ideally, these two would coincede :-) However, the definition of "critical altitude" depends on the definition of "full power", which might be an idealized term itself.

For example, turbo-supercharged engines in WW2 seem to have been rated at constant power over altitude while they actually gave slightly parabolic power over altitude. At critical altitue, they actually didn't yield full power any more.

Regards,

Henning (HoHun)

[HoHun]

Hi Butch,

>But indeed it's not easy to make calculation based on this chart based of such a few number of points.

Well, I only need a single point :-)

Here's a comparison graph of the Finish test data with the data I calculated from the engine graph (3023 kg, 2540 rpm, 1.3 ata).

http://www.x-plane.org/users/hohun/me109g-2.jpg

As you can see, I get a very good fit, so my model is sane. The difference at low altitude obviously is not due to modelling differences, but due to the Finnish engine performing differently from the reference engine. (Looks like the reference engine had a higher low-altitude supercharger speed.)

Above full throttle height, my graph actually looks more realistic than the FAF test data because the latter features a linear shape where it should realistically be slightly convex, like my calculated shape. I consider that a minor difference, though.

(Any small imperfections in the smoothness of my graphs are due to linear interpolations between tabulated values without any deeper physical meaning :-)

Regards,

Henning (HoHun)

[Seeker]

Originally posted by HoHun
Hi Seeker,

>Do you mean that at X feet; the oxygen content is such that one can open the throttle and not over rev the engine?

Engine speed was actually limited by the propeller, but you could open the throttle fully without overstressing the engine at full throttle height.

Regards,

Henning (HoHun)

Thx for trying; but I still don't get it! :-)

Or rather; I'm forming an idea; but you guys are so anal over definitions I want to be sure I've got this right (back ground: I'm trying to understand these unfamiliar concepts as a two stroke motor cycle tuner)....

of course RPM is goverend by load ; if the load is too great; the mill'll stop no matter what chemicals you're pouring into it. That's exactly why the terms you guys are using confuses me. When you say: "open the throttle fully without overstressing the engine at full throttle height." I read that to say:

Engine X has a safe operating limit of y inches of mercury (or lbs/sqft boost pressure). This I understand; as it's a measure of how hard the engine is allowed to suck (don't giggle, we're talking all kinds of motors; not just LW....).

I don't see where alt comes into it. (or rather; I understand how atmospheric pressure has an effect; just not in these definitions/terms)

I'd really like to get a grip on this. I understand 90% of what you're saying; but I'm seeing it from a sea level perspective; I'm not used to adding alt into the equation.

[Nashwan]

Engine X has a safe operating limit of y inches of mercury (or lbs/sqft boost pressure). This I understand; as it's a measure of how hard the engine is allowed to suck (don't giggle, we're talking all kinds of motors; not just LW....).

I don't see where alt comes into it. (or rather; I understand how atmospheric pressure has an effect; just not in these definitions/terms)

A geared supercharger is directly related to engine speed. That means it turns at the same rpm regardless of altitude.

An engine has a max allowable manifold pressure, say 60". If the supercharger is geared so that it can deliver 60" at 20,000ft, it can deliver a lot more at low altitude where the air is thinner.

As you can't adjust the supercharger speed, you close the throttle to restrict the manifold pressure getting into the engine. As altitude climbs, the supercharger is delivering less pressure, so the throttle can be opened gradually. When you reach full throttle height or critical altitude, the supercharger is able to deliver exactly the allowed manifold pressure, and no more, so the throttle no longer has to be closed.

Above that height, the supercharger can't deliver enough pressure, so the throttle remains fully open.

[gripen]

A new article serie just started in the "Suomen Ilmailuhistoriallinen Lehti" (a Finnish aviation history magazine). It's about the performance of the WWII piston engined fighter planes and the Bf 109G is used as a example in this serie. Author is Jukka Raunio who has previously written "lentäjän Näkökulma" book serie. The first part discuss about atmosphere and later parts will discuss propellers, super chargers, various performance aspects etc. It's written in Finnish.

gripen

[HoHun]

Hi Gripen,

>The set contains 15 points of which 11 are below 6700m the average being 646km/h at 6700m.The highest point is 656km/h at 6950m. The reference MTT point is just  626km/h at 6300m.

OK. I've now assumed that the 5.8 km full throttle height is given for climb speed (374 km/h true for the Me 109G-2), and used this to calibrate the ram efficiency which I get as 86.8%.

This leads to the following values for the four data sets that make the tighest group:

652 km/h @ 6,7 km: 51,6 kPa
646 km/h @ 6,7 km: 51,5 kPa
636 km/h @ 6,6 km: 51,9 kPa
608 km/h @ 6,5 km: 51,9 kPa

This gives an average of 51.72 kPa for full pressure for 2600 rpm/1.30 ata.

Judging from the 100 m difference in full throttle height to 2800 rpm/1.42 ata, this would require 52.4 kPa pressure for the higher power setting.

This value would make a Me 109G-2 (3032 kg, retractable tail wheel, 1.42 ata/2800 rpm) with the following data possible:

662 km/h @ 06.62 km
620 km/h @ 10.00 km

Since this is based on an improved foundation thanks to the data you provided, would you agree that this finally is a realistic estimate?

Regards,

Henning (HoHun)

[gripen]

HoHun,
Now your estimate for the FTH sounds to at right ball park ie about 15km/h (2,6%) increase for 100ps (8,4%) output increase (might be still at optimistic side).

But your estimate for 10000m seems to be still quite strange ie about 40km/h (6,9%) increase for about 75ps (9,6%) output increase. I have calculated output increase assuming about 450km/h climb speed RAM and standard atmosprehe.

Note that this approach does not count propeller efficiency, which certainly is a signifigant factor at 10000m. The article serie mentioned above will contain propeller efficiency charts for the Bf 109G (I'm not the author but I have collected some data for him and I have seen a work in progress version of the article). My original approach using simple estimate for rate of the speed change actually counts propeller efficiency; the change of the speed includes all the factors happening at given altitude.

Generally anything more than 600km/h at 10000m seems to be at optimistic side for the standard Bf 109G with DB 605A.

gripen

[HoHun]

Hi Gripen,

>Now your estimate for the FTH sounds to at right ball park ie about 15km/h (2,6%) increase for 100ps (8,4%) output increase (might be still at optimistic side).

Increase compared to what? :-) If you're comparing to the FAF Me 109G-2, remember that my aircraft also has less drag due to the retractable tailwheel.

>But your estimate for 10000m seems to be still quite strange ie about 40km/h (6,9%) increase for about 75ps (9,6%) output increase.

The speed difference is due to a number of different reasons:

1. The FAF Me 109G-2 10 km value is unrealistically slow. See http://www.x-plane.org/users/hohun/me109g-2.jpg

2. The FAF Me 109 is underperforming at 2540 rpm

3. The FAF Me 109 has a fixed tailwheel with higher drag.

4. The FAF Me 109 is running at lower power.

All these effects combined account for the complete speed difference.

>The article serie mentioned above will contain propeller efficiency charts for the Bf 109G (I'm not the author but I have collected some data for him and I have seen a work in progress version of the article).

Sound highly interesting :-) Will the author include English captions for the illustrations (or reproduce the charts with the original German captions)?

>My original approach using simple estimate for rate of the speed change actually counts propeller efficiency; the change of the speed includes all the factors happening at given altitude.

My approach does account for the drop of propeller efficiency with altitude (or rather with tip Mach number). You can see my calculated speed intersects the FAF speed at 10.3 km, so my propeller efficiency at altitude seems fine.

>Generally anything more than 600km/h at 10000m seems to be at optimistic side for the standard Bf 109G with DB 605A.

You're being pessimistic :-)

Regards,

Henning (HoHun)

[Straiga]

Originally posted by F4UDOA
Kweassa,

The P-51D had far worse stall characteristics.

The F4U stall was only bad for a carrier A/C. The P-40 and P-51 were both worse by comparison.

By whos comparison?

 A P-51 stalls quite nicely in accelerated, takeoff and departure, clean or configured, an F-4U on the other hand, thats stalled and ends up in spin and under 9,000 feet your dead! Not with a 51. The F-4U shows that you can make anything fly with a lot of power, a barn door with a prop!
 In todays airplane do you see GULL WINGs? No I didnt think so, its just a bad plane form design.

Straiga

[Straiga]

Hey everybody,

 I been reading a lot of post lately and the amount of info you guys come up with blows my mined, I must work to much or something to have time to research all of this different info, I know you guys arent married, are you?

 With all this time we need to teach AH how to make an airplane fly right. I probably going to hear different, but AH flight modeling is wrong. I flown airplanes from 1200 lbs to 550,000 lbs and if I had to work as hard flying planes like in AH in real life, retirement age for commercial pilots would be at 30 years old not at 60.
 When Pyro showed me picture of a helicopter, and we were talking about airplane aerodynamics and torque, I then knew the ship was sinking badly.

 With all the different airplanes I have flown for a living, the 262 in AH is the closest flying plane to real life, either a prop or jet plane. When you want to roll, loop or what every, just move the controlls and you go with out trimming aileron, rudder, elevator.
 Once a plane is trimed to fly at cruise you really dont need to trim anymore. Remember AW! simple.
 If your a real pilot like myself you will under stand this, if not you wont know until you been there, sorry.
 Dont take this wrong, Im just trying to make the way the planes should fly to improve on your perk points

 Fly the 262 in AH, just fly it, thats the closest thing in AH to the real thing, trim for cruise and have fun remember no need to trim anymore. Now isnt that easy. Thats a way a plane should fly!

 Hey Pyro, this isnt rocket sience, or boeing,keep it simple, its just a game remember. I could care less about the roll rates of a 109 or P-40, they probably wouldnt be close anyway.

 If you some flight experience like myself, lets here from you.


Later

[gripen]

HoHun,
May I remind you that measured speed of the MT-215 at 10100m CINA was 552km/h which I corrected to about 580km/h  at 10000m CINA 2600rpm with altitude correction, rpm correction, output correction and tailwheel. This value is in a very good agreement with other really tested data sets. Also the speed at 10000m is in very good agreement with the Bf 109G-5/AS data given the output, weight and drag difference.

Generally creating a chart does not prove anything if you don't want explain how did you reach such numbers.  I have explained quite thoroughly how I have reached my numbers and I have actually sent you real world data I use. Shortly we are back in the beginning; you just want to believe unrealisticly high performance numbers without real world tested proof.

You can contact directly The Suomen Ilmailuhistoriallinen Lehti and Jukka Raunio if you have something to ask about the data he is using:

Suomen Ilmailuhistoriallinen Lehti
Jakomäentie 8b C300
00770 Helsinki
FINLAND

I'm not involved with magazine, I'm just a reader who occasionally helps them to collect data. I pay normal price for the magazine.

gripen

[HoHun]

Hi Straiga,

>If you some flight experience like myself, lets here from you.

John Deakin, "What really counts":

http://www.avweb.com/news/columns/182039-1.html

Let me quote some bits:

"Sure, hours and years aloft can mean something, but other things need to be considered. Take a peek at the embarrassing bio at the end of this column; there are a few things there I'd like to dissect here. I've been getting a good deal of teasing about that from some, and a few have warned me that some may attach more credence to what I say than they should because of my apparent credentials. I hope that's not true, I hope they pay attention because what I say makes sense. If it doesn't make sense, then I want to hear about it."

Regards,

Henning (HoHun)