How good is the LA-7's Engine?

[niklas]

Originally posted by dtango
J_A_B:

On the contrary.  I have not seen conclusive proof that the La7 has greater endurance than historically possible.

365l/460l = 0.79 = 79%
Flight time in AH with 100% fuel 56min (Mil)
Flight time in AH with 79% fuel 44.3min
Flight time Real life (Mil) 35Min

Advantage in AH = 44.3/35 ~26.5%

So difficult to see?

niklas

[Puke]

- You got a problem with my posts? you do the test yourself instead of chipping in with a pointless insult.

You come across to me as though you are whining the poor little LA7 is being persecuted.  No matter who is right or wrong, it's an interesting discussion but your tone sounds like you take this personally and are near tears.  Maybe I'm wrong, and I'll apologize now.  Want me to edit out my comments?

but i hoped the La7 bashers would do their own tests

Just one of your quotes.  This isn't bashing of the LA7, but rather is the aircraft which happened to be used as an example.  This isn't a witch hunt and the Spit and 109 has been mentioned too.  So I just figured the discussion should utilize another small-fueled aircraft so that you don't take this personally.  I really do not want to go through your posts to pick out those statements that make it sound like you take this personally, but in my readings of your posts, that's how it sounds to me.

[dtango]

Niklas:

I'm assuming you're saying the real life data is from the data aforementioned way above- specifically:

2400 1020 575 608 1.020 620 355 0-35

I haven't seen anyone posting AH La7 endurance tests yet for 79% fuel, 1000m alt, the said rpm, manifold pressure, etc.

I assume you got your 44.3 min by using the simple ratio of:
55 min / 100% = x / 79%.  You can't use this ratio because it doesn't accurately reflect variables involved since fuel flow isn't linear.

Endurance = Specific Endurance (SE) * amount of fuel of interest
SE = 1/FF (units of time / unit of fuel)
FF = SFC * D * V / 325 (units of d=lbs, v=fps)

D is a pretty complex variable that varies with alt, velocity, and weight.

Tango, XO
412th FS Braunco Mustangs

[crowbaby]

Puke - no offense, but it seems odd that you use your perception of my tone to justify your insult? Besides, i thought i'd made it clear that what you quoted me on was in jest by starting the next paragraph with "Seriously, though..."

So far, I thought this thread was mostly doing pretty well. I mean, it's clear that we have some major disagreements here, and some very different ways of looking at the problem, but it hadn't degenerated into name calling.



Joeblogs - sorry didn't respond to your e-mail, only just saw it now, it's an occasional account.

[joeblogs]

I'd like to do this, but I've got to assemble some numbers on the exact models flown in AH.  Had hoped to finalize the numbers on the La-7 first...

-blogs

Originally posted by Puke
...

Just one of your quotes.  This isn't bashing of the LA7, but rather is the aircraft which happened to be used as an example.  This isn't a witch hunt and the Spit and 109 has been mentioned too.  So I just figured the discussion should utilize another small-fueled aircraft so that you don't take this personally.  I really do not want to go through your posts to pick out those statements that make it sound like you take this personally, but in my readings of your posts, that's how it sounds to me. [/B]

[joeblogs]

The reason why using the best efficiency of an engine may be an inappropriate proxy for the endurance of a given plane is that you can't be sure the engine is generating enough horsepower at that setting to actually keep the plane in the air.

The fact that you are carrying more fuel, or have more drag, etc on a plane influences the amount of miles you can fly.  It only influences the amount of time you can run an engine if you have to run it at higher settings to keep the plane in the air when the tanks are near full.  Otherwise there is no effect.

Assuming no bombload and a 1940s fighter, there is so much excess power on these planes I don't see this as an issue in the flight tests.  The fact that actual flight test data on US heavy iron gives you an SFC very close to the best SFC of the R2800 suggests the minimum horsepower required to keep these planes flying is not far above the horsepower associated with best efficiency of the engine.

-Blogs

Originally posted by dtango
Guys, crowbaby is on the mark when he says that the crux of the debate is comparing apples and oranges.

Firstly, there seems to be some confusion here as to the term "fuel consumption".  GPH, LPH are units of FuelFlow, not fuel consumption.

FuelFlow (FF) = SFC * Drag * Velocity / 550
[velocity in units of fps]

I hope it's clear as to the other variables that determine fuel flow.  It's more than just SFC values for the engine.

Hopefully it's also clear from this equation just how a/c weight factors into fuelflow as well since induced drag varies with weight.

2ndly, there seems to be some confusion regarding aircraft endurance as well.

Specific Endurance = 1 / FF.  SE is in units of time/unit of fuel.

To find flight endurance you multiply SE by amount of fuel you are interested in.

Hopefully it's clear that if you're going to compare endurance of aircraft you can't just compare SFC values.

Tango, XO
412th FS Braunco Mustangs

[dtango]

Joe Blogs:

Specific Endurance determines how long an a/c flies not how far and is a function of fuel flow:
SE = 1/FF

Fuel flow once again is:
FF = SFC * D * V / 325

This equation shows all the aerodynamic effects on fuel flow which includes total drag.  To say that drag has little impact assumes that drag (really power required = D*V) between aircraft in comparison is near the same.  For a/c like F6F-5, F4U-1 vs. the La7, drag is clearly a factor.  This is the same problem people run into when they look only at engine horsepower as a measure of linear acceleration / climb performance.

If we're interested in range then we use the Breguet Range Equation where we can the variables involved:

R = (V/SFC) * (L/D) * ln(Winitial/Wfinal)

Tango, XO
412th FS Braunco Mustangs

[joeblogs]

... they affect the minumum horsepower required to keep the plane flying.  Forget about speed, distance traveled, rate of climb, etc.  All that matters for endurance is the least no. of horses that keep the plane flying.  

So long as that requirement does not vastly exceed the horses supplied by an engine at its most efficient power rating, SFC is all you need to know.

But if you are thinking of comparing fuel consumption subject to some minimum speed at a given altitude, you are correct that drag, and other things matter.  Fix a required speed, back out the required horsepower, which depends on weight, drag and some other things to a power.  Then figure out the fuel economy of the engine at that required horsepower.

I've been working off flight test numbers where I can back out the required fuel from the data without having an equation that captures the drag and other components for a given plane...

-blogs

Originally posted by dtango
Joe Blogs:

Specific Endurance determines how long an a/c flies not how far and is a function of fuel flow:
SE = 1/FF

Fuel flow once again is:
FF = SFC * D * V / 325

This equation shows all the aerodynamic effects on fuel flow which includes total drag.  To say that drag has little impact assumes that drag (really power required = D*V) between aircraft in comparison is near the same.  For a/c like F6F-5, F4U-1 vs. the La7, drag is clearly a factor.  This is the same problem people run into when they look only at engine horsepower as a measure of linear acceleration / climb performance.

If we're interested in range then we use the Breguet Range Equation where we can the variables involved:

R = (V/SFC) * (L/D) * ln(Winitial/Wfinal)

Tango, XO
412th FS Braunco Mustangs

[dtango]

But if you are thinking of comparing fuel consumption subject to some minimum speed at a given altitude, you are correct that drag, and other things matter. Fix a required speed, back out the required horsepower, which depends on weight, drag and some other things to a power. Then figure out the fuel economy of the engine at that required horsepower.

Yes exactly.  Since all the tests results that have been hashed so far mainly deal with the aircraft at constant velocity at a fixed altitude then D*V becomes important.   In these cases certainly there is some portion of the flight that fuel flow is mainly a function of SFC*THP(avail) since there's excess power to accelerate the a/c but this is only true until the velocity stabilizes.  (Yes, there's also the case where it doesn't matter which you use since power avail = power required which occurs at max level speed.)

Off the top of my head the only time I can think of where you'd be looking at fuel flow = SFC*THP(avail) where you have constant excess power is in a constant speed climb.

The point is FF=SFC*Power(avail or required) which means you can only compare SFC's as indicators of fuel flow if power avail / power required for a given flight condition are equal between a/c which is again clearly not the case between the F6F-5 / F4U-1 vs. the La7.


Tango, XO
412th FS Braunco Mustangs

[niklas]

Originally posted by dtango
Joe Blogs:

Specific Endurance determines how long an a/c flies not how far and is a function of fuel flow:
SE = 1/FF

Fuel flow once again is:
FF = SFC * D * V / 325

This equation shows all the aerodynamic effects on fuel flow which includes total drag.  To say that drag has little impact assumes that drag (really power required = D*V) between aircraft in comparison is near the same.  For a/c like F6F-5, F4U-1 vs. the La7, drag is clearly a factor.  This is the same problem people run into when they look only at engine horsepower as a measure of linear acceleration / climb performance.

If we're interested in range then we use the Breguet Range Equation where we can the variables involved:

R = (V/SFC) * (L/D) * ln(Winitial/Wfinal)

Tango, XO
412th FS Braunco Mustangs

Ok Tango, thx for showing all of us that you successfully managed to read through the book "Flight Mechanics I". What you should learn now is to apply correctly what you read.

Youīre wrong in several aspects.

First: It doesnīt make sense at all to talk about specific endurance. How do you determine such values? You measure fuel consumption, time. Together with speed you can build up specific numbers. Then you want to use speed again to come back to values like time. Completly wasted effort, because we already know time. But it probably makes good impression on this board to throw around with forumlas.

Second: Your formula applies to jet engines. Actually you still have to distinguish single circuit jets and 2 circuit jets. You forgot for example the engine specific exponent Nv for speed, which is zero for single circuit jets (making v^Nv = v^0 = 1), or for props -1. A jet consumes more fuel when flying faster, because the airflow changes. A piston engine consumes at a given power setting and rpm setting a constant volume flow, thus fuel rate, independent of speed (We donīt change altitude so massflow is also almost constant).

Third: Because its for jets D is a Force, but for props the specific fuel cunsumption refer to a Power.

And again, range is completly unimportant for me. A piston engine doesnīt care about high or low cruise speeds (slow climb speeds with high power settings may be the exception), nor does it care about propeller settings and so on.

You did understand my example correctly, and my very simple calculations are correct in this case.

Personally i suppose that the La-5FN and La-7 are modelled with the unhistorical larger fuel tanks. This is the only explanation i have for the higher endurance. If so then I hope that the weight is modelled too.

niklas

[joeblogs]

First if you know gallons of fuel consumed over a period of time at a specific engine setting you can back out the fuel economy of the engine without knowing velocity or drag.  

If you don't know enough about the engine settings (RPM and manifold pressure compared to an engine chart) to infer the horsepower, you may need your equation to back it out from the speed and drag of the plane.  But that has to be an even rougher calculation than the ones made in this thread.

Second, at max power you should know the horsepower being used and again the equation is not necessary.

Third, there is no requirement that in order to compare two different planes we have to use conditions where they require the same amount of power.  We are normalizing by horsepower to get away from this.  But to do that you need to know the horsepower required for each plane to attain a certain speed at a certain altitude.  You could use an equation to figure that out, but I prefer using the flight test data.

-blogs

Originally posted by dtango
Yes exactly.  Since all the tests results that have been hashed so far mainly deal with the aircraft at constant velocity at a fixed altitude then D*V becomes important.   In these cases certainly there is some portion of the flight that fuel flow is mainly a function of SFC*THP(avail) since there's excess power to accelerate the a/c but this is only true until the velocity stabilizes.  (Yes, there's also the case where it doesn't matter which you use since power avail = power required which occurs at max level speed.)

Off the top of my head the only time I can think of where you'd be looking at fuel flow = SFC*THP(avail) where you have constant excess power is in a constant speed climb.

The point is FF=SFC*Power(avail or required) which means you can only compare SFC's as indicators of fuel flow if power avail / power required for a given flight condition are equal between a/c which is again clearly not the case between the F6F-5 / F4U-1 vs. the La7.


Tango, XO
412th FS Braunco Mustangs

[dtango]

Niklas:

REGARDING SPECIFIC ENDURANCE...
To find endurance of an a/c you multiply Specific Endurance (SE) (time/units of fuel) by the amount of fuel you are interested in.

To derive SE you need to know FF since SE = 1/FF.  Fuel flow is what is being discussed here (e.g. GPH...etc.).

To find fuel flow we need to know SFC and the power(avail or required) since FF = SFC*Power.

I'm trying to show the relationships that determine fuel flow in an attempt to shed some light on how the different things being hashed fit together with regards to a/c endurance.

Joeblogs- this is the equation for fuel flow rate.  At max level-speed yes we know what power roughly is but you can't just drop power from the equation above.  You still plug in the known power in the equation above.

The crux of this thread was based around the argument that SFC values at the most economic engine settings between the F6F-5, F4U-1, and La7 were about the same ~.4 lb/HP/hr.  I'm pointing out why comparing SFC values doesn't tell the whole story.


REGARDING EQUATIONS FOR JETS OR PROP A/C...
The above equations are for prop a/c.  They agree with you exactly when you say for "fuel consumption" for prop a/c refer to power.  That is exactly what I'm trying to point out.  D*V = power required.  At max level-speed power available = power required.

Yes there are multiple forms of the breguet range equation.  The reason I listed the equation was to point out that I have been talking about endurance of an a/c and not range.


CORRECTIONS ON  MY PART...
I did make an error in logic regarding your ratio.  You can use the ratio you stated there for specific parameters of flight you were comparing.  So the difference between 35 min vs. 44 min is an interesting anomaly but crowbaby points out some of the issues with this.

Do we know that the La7 in AH at MIL power is at the same engine RPM and manifold settings resulting in the same velocities listed at the stated alt?

2400 1020 575 608 1.020 620 355 0-35


Tango, XO
412th FS Braunco Mustangs

[niklas]

Originally posted by dtango
... since FF = SFC*Power.

...and SFC is constant for a constant RPM and throttle setting for a prop AC in a given altitude. So is Power. So is FF finally. And this way you can easily determine your flying time. Itīs so simple.

Originally posted by dtango
REGARDING EQUATIONS FOR JETS OR PROP A/C...
The above equations are for prop a/c.

No! First of all your Range formula is the solution of a "differential formula" (?) that is only valid for constant speed and constant cl. As a result rho has to change when the ac gets lighter. This formula describes the fact that you have to fly higher and higher when you get lighter to fly always at the most economic point - FOR JETS.

Because v has a exponent in this formula, this is not valid for props.
The exponent for the range solution is v^(Nv +1). Because Nv = -1 for props the exponent becomes zero, and v^0 = 1. V disappears.
For jets Nv = 0 so v^1 = v.

La-5FN and -7 have same high endurance, it really looks like La-7 enjoys a larger fueltank in AH than in reality (Just an assumption)

btw., german engine performance chart list SFC values for rpm/throttle settings quite often. Those are the g/Ps/h values...
FF for fuelflow is misleading btw. Thereīs either massflow or volumeflow. I assume itīs massflow. g/PS/h * PS = g/h is a massflow.

niklas

[dtango]

Power is constant for a given constant velocity.  Comparing different constant velocities power is different between the velocities.  Otherwise you would be able to fly at max level speed at cruise engine settings for instance.

EDIT: Just wanted to clarify- I think I'm discussing things in tangent with you .  As I mentioned earlier, your use of the ratio to get 43 min endurance for the La7 was correct.  I made an error in my logic (won't go into it).  You are right that fuel flow rate, fuel consumption rate, whatever we want to call is constant for a given constant power.  (Actually this thread was so long that I didn't glean the 35min vs. 43min. issue.  My bad.)

Regarding the range equations - I used that in reference to point out I was not talking about range.  Yes I know the limitations of the equation.

The FF, SE equations are for prop a/c.

Tango, XO
412th FS Braunco Mustangs

[Puke]

I know it's late, and maybe that's part of the problem, but now I'm starting to get lost.  I'm seeing terms such as velocity, range, weight and drag and stuff like that being introduced.   I didn't think any of that mattered because the comparison being made is that which how the typical AH pilot flies...100% throttle 100% of the time.  Velocity doesn't matter...range doesn't matter...drag doesn't matter, only time matters and is the benchmark.  If you could in real life, the comparison would be run by:  air-start an LA7 (or Spit) and an F6F at the same exact moment both with their 100% allotment of internal fuel and have them run at 100% throttle and compare how *long* they run.  Of course the LA7 will outrun the F6F-5, but we don't care about that (though that is probably due to drag/weight.)  At max power drag doesn't matter though.  In fact, give both aircraft 100% drag (put them up on chalks or something so they cannot move) and run the test.  Start them at the same time and slam the throttles to 100% and time how long they last.  They are both probably running at their most inefficient power setting when at 100%, just like how aircraft are flown in AH.  Do this at different altitudes if you can.  So now the point is to determine if fuel burn rates are as they should be.  I bet you an aircraft on chalks running 100% throttle burns the same exact fuel per minute as it does at 100% throttle when in the air.  So I'm lost with talk of drag, or range equations and the like.  With what little knowledge I have on the subject, I do think something smells fishy here.  This thread is a good one and it seems there are some really smart minds who have differing opinions.  I do find it very odd that those who create the flight tables for AH which strives and promotes "high fidelity" haven't made a single peep on the matter and only leads me to conclude that a few of you are on to something.  But I guess an LA7 at 100% throttle is running that much more efficiently than an F6F-5 at 100% throttle to come up with these numbers AH produces.  If drag and velocity and all that mattered for the 100% throttle test, then I'd expect a difference in the resultant time flown between "up on chalks" tests at various altitudes compared to actually flying tests.  

Anyway, so far this has been a great topic and I've actually learned a little bit...I think.