Advance Ratios and Gear Ratios

[Zipper730]

The problem is that, like I said, the torque is the result of the pressure in the cylinders (pounds per sq in acting on the piston tops) puhsing down through the connecting rods that turn the crank.

That makes sense

So we have a crap load of variables. If you are turning 2600rpm and using 4lbs of boost you get one pressure in the cylinders,if you are using 2600rpm and 6lbs boost you get a different pressure in the cylinders (roughly and that is the problem) 10% more pressure

Works out

if you are using 3000rpm and the same boosts the pressures will be slightly different.

How does torque vary with RPM for the same manifold pressure?

It gets even worse because this only works at one altitude or air density. Pressure in the manifold is related to the mass of air (weight of the air) but it is not a 100% correlation.
6lbs boost at sea level is not the same mass of air per minute as 6lb boost (or the same intake pressure ) at 10,000ft.

Wait, I'm confused -- I thought pressure was caused by the mass of air per given area? Why would these figures vary?

I still don't know what you are trying to do with this, The best props in WW II were only about 80% efficient which means 20% of the power going to them (no matter how you measure it) was wasted.

So would that mean if theoretical maximum thrust with 100% efficiency was 10,000 pounds and it was 80% efficient you'd get 8000 lbf? Or is it something else?

I'm curious if there's anyway to infer thrust by the following variables: Propeller diameter, RPM, manifold pressure, altitude, TAS, rate of climb? Even if the exact figures are not stated, sometimes it's possible to infer them. After all if you have 60x = 120, it doesn't say it outright, but x = 2, and that can be inferred by dividing both sides by 60.

 

[wuzak]

Wait, I'm confused -- I thought pressure was caused by the mass of air per given area? Why would these figures vary?

Temperature.

At 0ft the air needs less compression to reach the required MAP/boost than at 10,000ft. Even with aftercooling, the intake air temperature at 10,000ft will be higher.

I'm curious if there's anyway to infer thrust by the following variables: Propeller diameter, RPM, manifold pressure, altitude, TAS, rate of climb? Even if the exact figures are not stated, sometimes it's possible to infer them. After all if you have 60x = 120, it doesn't say it outright, but x = 2, and that can be inferred by dividing both sides by 60.

Not sure that you can. For one, you need to know more about the propeller than just its diameter to find its efficiency.

If you know the TAS and the drag at that speed and altitude, you can calculate the thrust.

 

[Zipper730]

Temperature. ... At 0ft the air needs less compression to reach the required MAP/boost than at 10,000ft. Even with aftercooling, the intake air temperature at 10,000ft will be higher.

Is there any rule of thumb to calculate that out?

If you know the TAS and the drag at that speed and altitude, you can calculate the thrust.

Where would you either find drag & speed and altitude, or how would you compute it out?

 

[Zipper730]

I just thought of something, well I thought of it last night while I was watching Forensic Files (not that it had anything to do with forensics): Wouldn't it be possible to approximate thrust by determining climb rate and climb speed? I figure if I know the speed, and I know how fast the plane diverges from the ground, that could give a climb angle, which would determine T/W ratio.

Of course, with the airspeed staying the same for awhile, the true airspeed would be going up and that would throw a wrench into the figures. I'm not sure if there's anyway around that.

 

[Shortround6]

You are trying to reinvent the wheel and take few shortcuts.

You have to figure out how much power (or thrust) is being used just to keep the plane in the air. Best climb speed is usually about the point where parasitic drag crosses over with induced drag

Contrary to many peoples opinion a low drag plane with high wing loading will often climb better than a high drag plane with low wing loading.

Once you know (or make educated guess) about how much power is left then you can calculate rate of cimb.

 

[pbehn]

The Spitfire and Hurricane with exactly the same engine and prop had completely different climb characteristics. The Spitfire climbed faster but at a shallower angle and higher forward speed.

 

[Simon Thomas]

How does torque vary with RPM for the same manifold pressure?.

FHP varies with RPM, which will impact brake torque. Track down the 'Power from fuel flow' pdf from Lycoming. That will provide some guidance from getting power from MP and RPM. The factors are going to vary, so you will need to back calculate them from known engine data.
For climb rates, try googling 'The Bootstrap approach to Aircraft Performance'. You will need to either test fly to obtain the parameters, or locate as much performance data that you can lay your hands on. The Benchmark software appears to match what you are trying to achieve, but I have never used it. (Only for Mac) Benchmark

 

[Zipper730]

You are trying to reinvent the wheel and take few shortcuts.

I was trying to get an approximation of thrust, as I was under the impression that if you know rate of climb and speed, you could determine thrust.

You have to figure out how much power (or thrust) is being used just to keep the plane in the air. Best climb speed is usually about the point where parasitic drag crosses over with induced drag

Parasitic drag is simply the drag produced by the amount of air flowing over the entire aircraft? Induced drag is drag caused by the production of lift, correct?

The key would be to determine the L/D ratio across the speed range of every single aircraft in the listing, I'm not even sure if such a specific database exists (though it'd probably be pretty cool). I would probably need help with the math part if integrals are involved.

FHP varies with RPM, which will impact brake torque.

What's FHP?

Track down the 'Power from fuel flow' pdf from Lycoming. That will provide some guidance from getting power from MP and RPM.

Fascinating...

For climb rates, try googling 'The Bootstrap approach to Aircraft Performance'. You will need to either test fly to obtain the parameters, or locate as much performance data that you can lay your hands on.

Well, I've been collecting a bunch of test-data from WWII Aircraft Performance, and some data I've found here.

The Benchmark software appears to match what you are trying to achieve, but I have never used it. (Only for Mac) Benchmark

I do have a Mac... unfortunately, it isn't suited for the later OS...

 

[Simon Thomas]

FHP = friction horsepower

 

[Zipper730]

FHP = friction horsepower

What is friction horsepower?

 

[Shortround6]

The amount of HP needed to turn the engine over at stated RPM. friction goes up with the square of the speed.

This is added to the power needed to drive the supercharger (if present, on most Lycoming light plane engines unless built in the 50s it won't be) and the power available at the prop shaft to figure the IHP (indicated HP ) which is the power being developed in the cylinders.

 

[Shortround6]

Try using your formula/theory on the MK 1 SPit and MK 1 Hurricane. Same engine and prop should mean same thrust. Difference of under 5 % in weight.
Spit climbs to 20,000ft in 8%less time?

 

[Zipper730]

The amount of HP needed to turn the engine over at stated RPM. friction goes up with the square of the speed.

I assume this applies to constant speed props? The ones that weren't constant speed went faster and faster as you went up to the critical altitude...

 

[Shortround6]

It applies to all engines.

any engine turning 2200rpm will have 21% more loss to the friction than the same engine turning 2000rpm. The same engine turning 3000 rpm will have 2.25 times the loss to friction than when running at 2000rpm. The Prop has nothing to do with the friction loss in the engine.



Some engines may do a little bit better, some do worse. While about 80% of the friction (rule of thumb) comes from the pistons and piston rings in the cylinder bore you do have friction loss in the bearings and the valve arrangements (stronger valve springs exert more pressure/friction on the cams through the valve gear). All major engines were dry sump and poor oil scavenging could lead to excess oil in the crankcase being a source of drag on the crank or connecting rods. This was part of the improvement of the early R-2800s going from 1850hp to 2000hp. Better scavenging/control of the oil in the crankcase.

The engines had to throttled back at the lower altitudes to keep from over driving the propeller. The engine was not unloaded as the plane climbed allowing it to reach higher rpm.
Much like a boat propeller when given too much throttle/power at stop or slow speed, the aircraft propeller would simply thrash the air, trying to push it "sideways" rather than give very efficient propulsion. I don't believe you can "cavitate" an airscrew but the effect is going to be somewhat similar. You are worried about the rate of advance. If you over rotate the prop ( it's rate of rotation exceeds the needed amount for the rate of advancement of the prop/aircraft) you will get high pressure on "back" of the prop blade while the "forward" side of the prop blade stalls. The Prop blades are wings/airfoils and if you use too high an angle of attack (too much rotation for the pitch angle) they are going to stall and your efficiency goes right in the crapper,

 

[Zipper730]

I totally misunderstood what you said, I thought you were talking about forward velocity of the aircraft, I was thinking about the rotation velocity going up on propellers that were basically fixed-pitch.

 

[Zipper730]

Is there any ways to calculate for throttling loss?

 

[Simon Thomas]

Yep. CFD should be pretty close, Crane technical paper 410 will get you in the ballpark.
What sort of accuracy are you hoping for?

 

[Zipper730]

Yep. CFD should be pretty close

CFD = Computational Fluid Dynamics?

Crane technical paper 410 will get you in the ballpark.

Where would I find that? As for accuracy, I was basically thinking a ballpark estimate.

 

[Simon Thomas]

CFD = Computational Fluid Dynamics?

Yes

Where would I find that? As for accuracy, I was basically thinking a ballpark estimate.

CRANE Technical Paper 410 US (2018) Google may find an old copy - the laws of physics don't change much. Treat the throttle plate like a butterfly valve, you will have to interpolate the ID factors as I doubt the actual ID of the carb will match a standard pipe ID.

 

[Deleted member 68059]

Is there any ways to calculate for throttling loss?

Explanatory Video

(You will need a compressor map for the engine in question)

Loss is comprised of compressor shaft power and increase in charge temperature lowering the density and hence power.

Loss will be about 10>15% in the case of a single-speed supercharger fixed to the crank, totally different
for multi speed SC, much lower with a swirl throttle, and almost non existant for a turbo.