FSXA Table 1548 Turboprop/helicopter torque scalar

[mgh]

P = rho*(T*R). Unless math no longer works??

The maths shows that if pressure and temparature are both changed in proportion by, say 10%, there is no change in density.

Are you claiming that such a change in ambient pressure and temperature would not affect the engine thrust, and that ambient pressure and temperature in the engine equations can be replaced by densiity?

If so, can you provide some independent support for that?

 

[WarpD]

The maths shows that if pressure and temparature are both changed in proportion by, say 10%, there is no change in density.

Are you claiming that such a change in ambient pressure and temperature would not affect the engine thrust, and that ambient pressure and temperature in the engine equations can be replaced by densiity?

If so, can you provide some independent support for that?

I believe I stated that pressure is directly related to temperature and rho. I don't believe I stated one thing regarding thrust. I also stated the relationship with regards to the discussion of torque, which is the thread's primary topic.

 

[mgh]

I believe I stated that pressure is directly related to temperature and rho. I don't believe I stated one thing regarding thrust. I also stated the relationship with regards to the discussion of torque, which is the thread's primary topic.

I'd already given the same relationship between density, pressure and temperature in Post #20.

What is the relationship with torque and do you have any independent support for it?

 

[jx_]

what assertions? I gave commonly published equations, and explained to you their meaning.

You continue to insist, so I will play along...

It then continues to non-dimensionalise the variables to show that:

Fn/(Pa * A)= f1(N * sqrt(A/Ta), Vt/sqrt(Ta))

and then reduced that to the form

Fn/delta = f(M/sqrt(theta), M)

where delta and theta are pressure and temperature ratios.

Just curious if you realize what non-dimensional means?

Also, do you know the difference between Delta and Pa or Theta and Ta?

Do you understand the difference between Delta and Total Delta?


I'm not trying to be funny here or win an argument or be demeaning, confrontational, or rude in anyway... but you are clearly not understanding the differences.

 

[jx_]

The maths shows that if pressure and temparature are both changed in proportion by, say 10%, there is no change in density.

Are you claiming that such a change in ambient pressure and temperature would not affect the engine thrust, and that ambient pressure and temperature in the engine equations can be replaced by densiity?

If so, can you provide some independent support for that?

I think you misheard. The statement was a change in density RATIO affects torque.

A 10% increase in ambient, changes all your ratios because ISA (or your P0 & T0) is a constant.

 

[jx_]

Maybe this will help.... directly from my engineering manual.

Please note that the only thing in this equation that deals with temp or pressure is MASS FLOW (m·) :

Fnet = [( m· core air + m· fuel ) × Vcore exhaust ] + ( m· fan air × V fan exhaust ) – ( m· air × V airplane )

And here is the section that discusses temperature and pressure:

air density effect on thrust

You’ll recall from an earlier chapter that air density is a function of temperature and pressure. The
density of the air passing through an engine will have a very significant effect on thrust, because
density has a direct effect on the mass flow rate that we call in the thrust equation. Thus any
change in temperature or pressure will affect the engine’s thrust.

As for torque, torque is simply Torque = (HP * 5252) / Prop RPM and has no direct reliance on anything atmospheric. It only changes as density affects turbine thrust, horsepower, and prop resistance.



If this doesn't explain it and you still believe otherwise, never mind.

 

[mgh]

Your anonymous quotation makes my point for me.

You’ll recall from an earlier chapter that air density is a function of temperature and pressure. The density of the air passing through an engine will have a very significant effect on thrust, because density has a direct effect on the mass flow rate that we call in the thrust equation. Thus any change in temperature or pressure will affect the engine’s thrust.

The emphasised sentence means a 10% increase in pressure and a 10% increase in temperature will affect the engine's thrust even though the density is unchanged.

 

[Roy Holmes]

Are you claiming that such a change in ambient pressure and temperature would not affect the engine thrust, and that ambient pressure and temperature in the engine equations can be replaced by densiity?

If so, can you provide some independent support for that?

http://www.dept.aoe.vt.edu/~lutze/AOE3104/thrustmodels.pdf

Have a look at equation 18 in this paper written by Virginia Tech.

Actually the whole course is interesting

http://www.dept.aoe.vt.edu/~lutze/AOE3104/

Roy

 

[Roy Holmes]

Although this thread started about table1548, we now seem to be in a discussion about what affects jet thrust.

No one has stated what the effect of pressure ratio alone does to thrust, no one has stated what temperature temperature ratio alone does to thrust.

In the attached jpg I plotted Mach 0 thrust drop off with altitude using EngineSim results. I used Mach 0 so as to only consider pressure ratio delta, temperature ratio theta and density ratio sigma. It can be seen that of the three, sigma is closest.

Now, if I were to plot delta/theta I get a good match because it overlays the sigma line. So IMHO the reason why one needs to consider temperature and pressure is simply because they affect density.

Roy

 

[jx_]

Mgh,

That's not what it says. And this entire thread has been about ratios, but,

What it actually says is: density has a direct effect on the mass flow rate that we call in the thrust equation. "THUS" any change in temperature or pressure....


Thus means therefore as a result of the previous statement!



mass flow = density * area * velocity. The mass of air is only a result of density. 10% hotter & higher pressure air nullifies itself and will not change thrust.

If density does not change, Mass flow does not change, nor does thrust regardless of temperature and pressure. Temperature nor pressure are ever taken into account in the thrust equation.

Your 15% increase applies to the corrected equation results.


The source is not anonymous it's from my Boeing binder.

If you don't believe me nor the facts I have cited, google 'thrust equation'. You'll see they start with F = mass x acceleration and go from there to account for airplane specific losses.

here's the first result I found:

http://www.grc.nasa.gov/WWW/k-12/airplane/thrsteq.html


Good luck. But please stop misinforming people in the various forums you post in.

 

[Roy Holmes]

In my post#50 above I referred to equation 18 in course notes from Virginia Tech. The equation shows that thrust varies with density ratio raised to some power.

In post#51 it is apparant from the chart that the thrust drop off with altitude and density ratio are slightly mismatched. However, if I raise density ratio by the power of 1.06 they match to 4 places of decimals up to the tropopause.

Based on that I can say that EngineSim thrust at Zero Mach in the standard atmosphere varies as (density ratio)^1.06 with increase in altitude.

This is real world data. It has nothing to do with corrected/referred performance calculations.

Aircraft sensors can measure pressure and temperature, but density has to be calculated. Non-standard pressure or temperature affect thrust, because they change air density.

Roy

 

[mgh]

Mgh,

mass flow = density * area * velocity. The mass of air is only a result of density. 10% hotter & higher pressure air nullifies itself and will not change thrust.

If density does not change, Mass flow does not change, nor does thrust regardless of temperature and pressure. Temperature nor pressure are ever taken into account in the thrust equation. .

I suggest you work through the following stages of a jet engine. The input are ambient pressure and remperature, and Mach number. The outputs are jet velocity and mass flow.

Inlet - http://www.grc.nasa.gov/WWW/k-12/airplane/inleth.html

Compressor - http://www.grc.nasa.gov/WWW/k-12/airplane/compth.html

Burner - http://www.grc.nasa.gov/WWW/k-12/airplane/burnth.html

Turbine - http://www.grc.nasa.gov/WWW/k-12/airplane/powtrbth.html

Nozzle (jet velocity) - http://www.grc.nasa.gov/WWW/k-12/airplane/powtrbth.html

Nozzle (mass flow) - http://www.grc.nasa.gov/WWW/k-12/airplane/mflchk.html

The last two are interesting. They result in

Jet Velocity = sqrt((2 2 Cp * Tt * eff * {1 - {Pa / Pt}^((gamma-1)/gamma)])

Mass flow = area * Pt / sqrt(Tt)) * sqrt(gamma/R) * M (1 + ((gamma-1)/2) *)^((gamma +1)/2*(gamma -1))

where Pt and Tt are the total temperature and pressure in the nozzle, and Pa is the ambient pressure. The tota values don't vary within the nozzle and equal to those are the turbine exit.

If you do work thriough them you'll find that jet velocity mass flow do vary with changes in ambient pressure and temperature. Their effects don't cancel out.

http://www.grc.nasa.gov/WWW/k-12/airplane/EngineTheory.pdf describes their application to the initial NASA EngSim model for a single-spool turbojet.

 

[Roy Holmes]

If you do work thriough them you'll find that jet velocity mass flow do vary with changes in ambient pressure and temperature. Their effects don't cancel out.

I agree because each change in ambient pressure and temperature changes ambient density. They would only cancel out if they resulted in no change in ambient density.

Roy

 

[mgh]

I agree because each change in ambient pressure and temperature changes ambient density. They would only cancel out if they resulted in no change in ambient density.

Roy

Not so. I've worked through them with my spreadsheet based on the NASA Eng Sim.

The first attachment shows the variation with altitude. The second shows the variation when ambient temperature and temperature are changed so as to maintain a constant density. In both f is the thrust ratio.

There's nothing in the thermodynamic equations to cancel them out.

 

[Roy Holmes]

The second chart addresses constant density not ambient density which varies with altitude. If you made a change in ambient pressure and did not change ambient temperature the ambient density would change and thrust would change. If you then made a change in ambient temperature such that the ambient density was returned to its original value, thrust would be the same as the original value.

Roy

 

[mgh]

The second curve shows how thrust varies when both ambient pressure and temperature are varied so as to keep density constant.

It does show that thrust can vary even though density is constant.

 

[taguilo]

The second curve shows how thrust varies when both ambient pressure and temperature are varied so as to keep density constant.

It does show that thrust can vary even though density is constant.

Just curious,how do you vary pressure and temperature to keep density constant?
I mean, do you simply increment/decrement , for example, pressure 10 % and temperature 10 % keeping a fixed ratio on both variables? Are you using ºC for temperature calcs?

Tom

 

[jx_]

Mgh,

I now see where your confusions lie. Let's clarify everything! Please read all the way through for clarity. It's not too long.

Density affects thrust production (Thrust), while density ratio Sigma scales the amount of that Thrust due to changes in altitude or temperature. Those changes to Thrust are applied to the original measured Force (F) pounds of thrust production. The original measurement is usually static at SSL, corrected to ISA. You are confusing the differences between Sigma scaling Thrust, and actual density (not the ratio) affecting the engine's propulsive efficiency and mass flow.


Thrust production is the equation I posted that I said relies on density for Mass Flow. Ambient temperature, ambient pressure, Delta, and Theta have no independent affect on Mass Flow.


You've also jumped around a bit from torque to ESHP to thrust...


So...



From here on I am only talking about actual values of power, torque, or thrust; and what the difference would be between sea level ISA and any other altitude or temperature. I will also clearly identify the difference between ratios and ambient values or use the Greek names to identify RATIOS. I will not be addressing the conversion of heat energy and compressed air into momentum (thrust production).


First I will make some assertions:


Ambient Torque / Sea level torque = sigma
Ambient Power / Sea level Power = sigma^Mach
Ambient Gross Thrust at altitude (Fg amb) = Sea Level Gross Thrust (Fg sl) * sigma


These are commonly used equations that you can find online. I will now continue by ONLY addressing thrust.




I believe you are confused by this:



Fg ambient = Fg sl * Delta * sqrt(1/Theta)

This is identical to the first equation, Fg amb = (Fg sl) * sigma.


Delta * sqrt(1/Theta) = Sigma





Now, go here:

http://www.grc.nasa.gov/WWW/k-12/Missions/Jim/Project1ans.htm

Scroll down to #17 and review the published formula and examples.


Then open this PDF and read the last sentence on the first page and the first sentence on the second:

http://onlinelibrary.wiley.com/doi/10.1002/9780470117859.app4/pdf


Then do the math problem for both.

As you can see, thrust falls WITH density ratio Sigma. If Density RATIO does not change, neither does thrust.





To address post #54:

Those are all internal engine pressures and temps. They are part of the production of total and useable thrust. Yes, those are different values! The temps and pressures here are mechanically altered and will differ from ambient air. In the end you get Gross thrust which is what we would need to figure changes with altitude.



To address post #58:

You keep repeating this. If Sigma RATIO doesn't change thrust scaling won't either. If density rho does not change, neither will thrust production. And the two are not interchangeable.




NOTE TO ROY: You are correct about Sigma^n, but as far as I know it only varies from 1 in very few circumstances or altitudes up around 50,000. The 1.06 you are getting is actually a 1.0. The discrepancy is something minor I was trying to explain to you before about FS uncorrecting the fans speeds to n1. Your fan speeds are offset by 6% at static SL.

 

[mgh]

Just curious,how do you vary pressure and temperature to keep density constant?
I mean, do you simply increment/decrement , for example, pressure 10 % and temperature 10 % keeping a fixed ratio on both variables? Are you using ºC for temperature calcs?

Tom

The equation of state is

density = pressure / (temperature * R)

where

pressure is in Pascals
temperature is in degrees K
R is the specific gas constant for air = 287.1 J/kg/deg K

Changing pressure and temperature in the same proportions obviously results in no change in density. I changed the prssure in Pascals and the temperature in degrees K to keep the density connstant.

Consider an aircraft developing a thrust F at any combination of ambient pressure and temperature. If the ambient pressure and temperature then both change by x% the density will not change.

The thermodynamics of jet engines indicate that the thust will change even though the density doesn't. If that were not the case why do analyses of jet engine performance always begin with ambient pressure, temperature, and Mach number when they could be simplified by using only density and Mach number?

 

[taguilo]

Consider an aircraft developing a thrust F at an y combination of ambient pressure and temperature.. If the ambient pressure and temperature then both increase by x% the density will not change.

Yes, but I guess temperature for calculation of theta and thrust/torque related is measured in ºC (ISA derived)? So, if you increase pressure in mbars, for example, and increase temperature in ºC there will be a change in density indeed. For example, a 10 % increment from 15 ºC equals 0.5 % of increment in ºK.

Tom