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How to Correct Measured Thrust for Density Altitude?

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Old 09-11-2012, 06:01 PM
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Default How to Correct Measured Thrust for Density Altitude?

Suppose I run my engine on a stand and get a number different from the mfr specs when it comes to max thrust...

The same engine in the same installation might put out more than specified on a cold day or less on a hot day because of the air density, making it impossible to make apples to apples comparisons between different engines or even when comparing the same engine from one day to the next.

Can someone provide me with a formula to correct measured thrust for air density, so as to derive a thrust value which has been corrected to a "standard day"?

Many thanks,
Don Ray
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Old 09-11-2012, 06:10 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

On a "Pure Jet" engine... there isn't a general formula for thrust and pressure/altitude density.

On a prop, or fan jet... you can calculate how the prop/fan pulls on the available air. In a pure jet... you loose power, and in turn, lose thrust. Since none of these engines have the same efficiency... and since our engines make thrust exponentially... the actual thrust lost equation would be specific to the engine you are working with.
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Old 09-11-2012, 06:16 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Density=(mass/volume)b also based on Bar.Mg pressure
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Old 09-11-2012, 07:23 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Dr Honda: I hope you are wrong. Otherwise, how can the folks at RCJI publish comparisons? Surely the weather changes would invalidate their data...

Tim: Thanks, but I'm NOT looking for definitions of air density, please re-read the OP ;-)
I'm looking for quantitative correlations between density and thrust for a given engine.
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Old 09-11-2012, 07:28 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Don, use these two tables:

http://wahiduddin.net/calc/calc_da.htm

http://wahiduddin.net/calc/calc_hp_dp.htm

They include useful examples.

Jack
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Old 09-11-2012, 07:53 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Thats just an ADR calculator, he is looking for a correction factor for weather conditions which would already be known.

I did a lot of dyno work on sprint engines and there are specific correction factors but those would not apply to jet engines.

We have run up the same turbines in different weather conditions and there is a huge difference in thrust. We were going to start testing and recording weather and make a chart from that. Not 100% accurate but would get us in the ball park.


From our testing most of them are touting #'s at sea level conditions, I think one mfg gives #'s at 1000' below- LOL.
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Old 09-12-2012, 03:35 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?


Quote:
ORIGINAL: highhorse

Dr Honda: I hope you are wrong. Otherwise, how can the folks at RCJI publish comparisons? Surely the weather changes would invalidate their data...

Tim: Thanks, but I'm NOT looking for definitions of air density, please re-read the OP ;-)
I'm looking for quantitative correlations between density and thrust for a given engine.

Well... I'm sure someone could come up with a VERY general formula. But at minimum... it would have to be adjusted for each class of engine. Also... it wouldn't be very accurate.

One thing to think about... when flying full scale, we can calculate performance based on Temp, and altitude. But all that is based off of lifting of the wings, and the available thrust from the prop. (another wing) Also... when working with a piston engine, we can make some assumptions on "Power" based on air density since the engine has a "Positive Displacement." So... putting some inefficiencys aside... we know a 4.0L engine will displace 4.0L within its normal cycle. With our RC jets... do you have any idea of how much air it draws? (I don't) Also... since we are dealing with a centrifugal compressor We don't have a volume reference based on "Engine Cycle." So... we would have to experimentally find the volume of the engine, in reference to RPM. That also means that EVERY engine would have a different correction factor.

You have to remember... our thrust is just the "Left Overs" from the engine running. If we had a prop or fan... we would have thrust based off of pressure density. (and the engien thrust would be negligible)

With all that said... I'm new to RC Jet's... and I don't fly full scale jets, so I could be off base.

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Old 09-12-2012, 03:47 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Ask John Wright or Colin Stause both on RCU John used to do the testing for RCJI and Colin is the current tester.

Mike
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Old 09-12-2012, 05:54 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Based on my testing here in Colorado at 5400', we lose about 3.5-4% per 1000' (desnity altitude). I've tested several different brands of turbines and for the most part came up with the same results. All my tests were done using the same equipment and the actual density altitude was calculated based on current conditions at the time of the test.

The 200 class engines push about 38-39 pounds here and the 160-170 class turbines push about 28 pounds. Those are averages for 6000-7000 density altitudes we see here on cool days. In the summer when we regularily see 9000', it's much worse.

There's a noticeable difference in thrust when I travel to FL to fly.
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Old 09-12-2012, 06:23 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?


[quote]ORIGINAL: highhorse

Dr Honda: I hope you are wrong. Otherwise, how can the folks at RCJI publish comparisons? Surely the weather changes would invalidate their data...

The weather never changes in England, always cold and wet!!!!!S
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Old 09-12-2012, 06:39 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Yes of course the RCJI results have always been adjusted for temperature and pressure differences so they are converted to standard ISA conditions. The UK does not deviate much from ISA conditions making the allowances small on most days. It became more obvious when Olivier did testing in the heat of the Middle East. Don if you want the formulas then I suggest you PM Colin Straus.

John
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Old 09-12-2012, 07:34 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Hello,
once I made a excel sheet to calculate this. I translated it to english. But I can not upload a excel file here, sorry. Maybe someone has webspace for download?

Torsten
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Old 09-12-2012, 09:48 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Let me try if I can help…

Considering Thrust (T)= mass flow rate * exhaust velocity

T = density * volume flow rate * velocity
T = density * A * (D/t) * velocity
T =density * A * velocity ^2

Consider test stand thrust T1 = density1 * A1 * velocity1 ^2
and test 2 thrust, T2 = density2 * A2 * velocity2 ^2

Same engine, Area A1 = A2 therefore

T2/T1 = density2 * velocity2^2/( density1 * velocity1^2)

T2 = T1 * [ density2 * velocity2^2/( density1 * velocity1^2)]

T2 = T1 * (density2/density1) * (velocity2/velocity1)^2

If T1 = T2 then

density1 * velocity1^2 = density2 * velocity2^2
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Old 09-12-2012, 10:03 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Gah, my head hurts - this thread is beginning to parallel the nonsense discussed in the (in)famous "Downwind Turn" discussion thread posted elsewhere . . . []
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Old 09-12-2012, 06:05 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Hopefully I got the formula that he’s looking for… If you are confused try this simple step by step.

Thrust if a force and it’s also a product of mass flow rate and velocity of a moving gas from exhaust nozzle considering engine on a test stand. This formula can be derived from Newton's Law of Motion or Force = mass x acceleration. Acceleration (a) is a rate of change in velocity, at constant rate, a = (final minus initial velocity)/time, when started from rest initial velocity is zero. Therefore a = velocity/time. Subs to "a" in Force T = m * velocity/time. This can written as T = (mass/time) * velocity. Same thing when using change in momentum per unit time which is [(mass * velocity)final - (mass * velocity)initial]/time, initial velocity = 0

To make it short we can use Thrust Force T = (mass flow per unit time) * velocity
Mass = density * Volume, while Volume = Area * Distance of displacement,
Now using rate: mass flow rate = density * Volume flow rate
where Volume flow rate = Area * Distance/time, distance per unit time = velocity

Substitute those above expression to get Force (T)
T =density * Area * velocity * velocity
can be written simply as:

T = density * A * velocity ^2

This means that density has constant effect to Thrust while expo on velocity.

Using at least two data from test reading you may get T1 and T2 with functions as stated above, let's try another same maximum velocity where V2/V1 = 1, we can get

T2 = T1 *(density2/density1)

It can be used to determine new Thrust given with old Thrust T1, old and new density at same exhaust velocity.
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Old 09-12-2012, 07:49 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Remember on Gilligan's island when Gilligan would hear and argument and agree with both sides?

I think that Dr. Honda is saying that it's hard to compute thrust for a specific turbine design just from basic parameters like inlet diameter, rpm, etc. That seems reasonable, they are complex beasts and there is so much art in the combustion chamber and NGV to name two that seem a little spooky .. you would not expect to take the mechanical drawings of a model turbine and make an accurate prediction of thrust with hobbyist-level math and computations.. if you had access to supercomputers and hydrodynamics codes as P+W does .. well different story .. but sadly we don't :-(

But rctech2k7 (resistor code username? geek! :-) ) is saying that for a given specific engine, you can pretty accurately scale its own performance (not compare to any other) by ratio of density altitude for the two test sessions that are to be compared, e.g. one at done at standard atmosphere, plus any other. Again, pretty sensible to me. I'll bet its close enough for any needs we have. I believe it's what most of the testers actually do but perhaps they will comment.

Regarding people's natural fascination with specs .. I have learned over the last several years of climbing the turbine learning curve that small performance differences are insignificant in determining actual utility and enjoyment in our hobby, and what really matters is reliability, predictability of operation, and great service when you need it.

Dave
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Old 09-13-2012, 04:49 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Quote:
ORIGINAL: BaldEagel

Ask John Wright or Colin Stause both on RCU John used to do the testing for RCJI and Colin is the current tester.

Mike
Yup, agree. Any idea how to contact them?



Rctech2k7:

Got it (I think), thanks. To test my understanding:
Given that the std lapse rate of atmospheric pressure is 1"/1000ft and std seal level pressure is 29.92".......

If I measure 30lb T @ 5000ft density altitude, then the corrected thrust for std conditions would be.....(taking off my shoes and socks so that I can count on my toes as well).....

(29.92/24.92) * 30 = 36lbs

I wouldn't mind confirmation from some more folks with a background in this science (vs a background in guesses ;-) but the above formulation makes sense to me and doesn't give me too bad a headache.




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Old 09-13-2012, 05:08 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Quote:
ORIGINAL: ww2birds

Remember on Gilligan's island when Gilligan would hear and argument and agree with both sides?

I think that Dr. Honda is saying that it's hard to compute thrust for a specific turbine design just from basic parameters like inlet diameter, rpm, etc. That seems reasonable, they are complex beasts and there is so much art in the combustion chamber and NGV to name two that seem a little spooky .. you would not expect to take the mechanical drawings of a model turbine and make an accurate prediction of thrust with hobbyist-level math and computations.. if you had access to supercomputers and hydrodynamics codes as P+W does .. well different story .. but sadly we don't :-(

..................

Dave

Exactly.

The thing is... if someone has done some EXPERIMENTS on power with different temps, humidity, altitude.... etc. Then we can build some "Curve fitting" equations. With experimental data... we don't need the "True Physics"... we just need a generalization. And... for hobby related info... it's close enough. I was un-aware of anyone doing good, back-to-back/comparison data, with altitude, on our engines. And to be accurate/usable data... it has to be THE SAME ENGINE AND TEST RIG used in all the tests. Saying... "my K80 made 18lbs of thrust ast sea level... and Joe-Bob's K80 made 17lbs at 4000' MSL" insn't usable data. That thrust change is still within the error of the engine, and the people operating it... and in the test rig.


BUT......... to have an accurate, generalized equation for thrust lost in a pure jet.... it's impossible.



Oh... did I mention that I have a Degree in Physics... worked in an R&D lab for 8 years.... and now own my own shop, and I build engines??? (lol)
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Old 09-13-2012, 05:20 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

A quick calculation that I use is :
2% loss of thrust per 1000 feet of corrected altitude

Of course, this is a generic formula, but certainly close. Often times thrust is lost from the turbines due to improper installations...ie: pipe to bellmouth length, poor inlet designs ect.
Not opening a can of worms here-please.....just saying that not to many of us can tell if the motor is putting out 31 pounds ...or 32.2 pounds of thrust, unless it is on the stand. There would be this much deviation from a 7 am flight compared to a 3 pm flight on the same day in the summertime. Like Dave says...I feel also that this issue has way to much thought put in to it! I know that the higher elevation pilots automatically figure these performance figures in to their choice of motor size when choosing a motor.

On the other hand...I have seen jet installs that have 6 additional do-hickies, a couple of reduction reducer thing-a ma-bobs, ten feet of additional airlines (spaghetti) and an additional half pound of epoxy!!!! And these guys are worried about a pound of thrust!!!

Todays turbine market has become very competitive, and this has resulted in much better customer support, dependability and pricing.

Chose what you are comfortable with and enjoy the hobby!!!
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Old 09-13-2012, 07:36 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Thanks for the estimate, Ron. Thanks also to Fuloughed ual. It's helpful but I'm still hopeful that We can nail down confirmation of a solid formulation.


Please note, all, that the original question was focused, and I'd rather not see the thread derailed into various derivative discussions. I am specifically asking about testing engines on a stand and wanting to compare apples to apples. That means I need to compensate mathematically for density altitude.

Thanks,
Don.
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Old 09-13-2012, 04:15 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?



edit.

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Old 09-14-2012, 06:14 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?


Quote:
ORIGINAL: Dr Honda

Quote:
ORIGINAL: ww2birds

Remember on Gilligan's island when Gilligan would hear and argument and agree with both sides?

I think that Dr. Honda is saying that it's hard to compute thrust for a specific turbine design just from basic parameters like inlet diameter, rpm, etc. That seems reasonable, they are complex beasts and there is so much art in the combustion chamber and NGV to name two that seem a little spooky .. you would not expect to take the mechanical drawings of a model turbine and make an accurate prediction of thrust with hobbyist-level math and computations.. if you had access to supercomputers and hydrodynamics codes as P+W does .. well different story .. but sadly we don't :-(

..................

Dave

Exactly.

The thing is... if someone has done some EXPERIMENTS on power with different temps, humidity, altitude.... etc. Then we can build some ''Curve fitting'' equations. With experimental data... we don't need the ''True Physics''... we just need a generalization. And... for hobby related info... it's close enough. I was un-aware of anyone doing good, back-to-back/comparison data, with altitude, on our engines. And to be accurate/usable data... it has to be THE SAME ENGINE AND TEST RIG used in all the tests. Saying... ''my K80 made 18lbs of thrust ast sea level... and Joe-Bob's K80 made 17lbs at 4000' MSL'' insn't usable data. That thrust change is still within the error of the engine, and the people operating it... and in the test rig.


BUT......... to have an accurate, generalized equation for thrust lost in a pure jet.... it's impossible.



Oh... did I mention that I have a Degree in Physics... worked in an R&D lab for 8 years.... and now own my own shop, and I build engines??? (lol)
I agree, it’s really complicated to determine parameters of an engine, because it takes time and some guts to analyze thermodynamics process. To be practical I just used value on the output side of an engine and get comparison between the test result. It also eliminate other variable including constant... BTW, I've got mistake here, I should consider intake airflow because in reality air accelerates around the turbine and becomes free stream. So even the engine is on a test stand it has airspeed when it's running.

Here's the formula for Thrust from http://www.grc.nasa.gov/WWW/k-12/airplane/turbth.html so we can solve it again.




BTW, rctech2k7 is a short for RCTech2007. Rctech was taken when I signed up but since I started in RC on year 2007 I used 2k7, where "k" is derived from a technical prefix kilo.

Good to know that we have someone here with degree of Physics, very interesting... You might help us about adiabatic expansion. BTW, my background is electrical engineering. What I've also learned makes me to appreciate the network and technical part of this hobby specially the high energy performance of our jets, flight control system, etc...

Let's go back to the topic, I tried to approximate using this formula and it's almost the same although the fuel consumption might change it slightly when running at low air density where mass flow to the exhaust will increase...
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Old 09-14-2012, 06:39 PM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Quote:
ORIGINAL: rctech2k7


Quote:
ORIGINAL: Dr Honda

Quote:
ORIGINAL: ww2birds

Remember on Gilligan's island when Gilligan would hear and argument and agree with both sides?

I think that Dr. Honda is saying that it's hard to compute thrust for a specific turbine design just from basic parameters like inlet diameter, rpm, etc. That seems reasonable, they are complex beasts and there is so much art in the combustion chamber and NGV to name two that seem a little spooky .. you would not expect to take the mechanical drawings of a model turbine and make an accurate prediction of thrust with hobbyist-level math and computations.. if you had access to supercomputers and hydrodynamics codes as P+W does .. well different story .. but sadly we don't :-(

..................

Dave

Exactly.

The thing is... if someone has done some EXPERIMENTS on power with different temps, humidity, altitude.... etc. Then we can build some ''Curve fitting'' equations. With experimental data... we don't need the ''True Physics''... we just need a generalization. And... for hobby related info... it's close enough. I was un-aware of anyone doing good, back-to-back/comparison data, with altitude, on our engines. And to be accurate/usable data... it has to be THE SAME ENGINE AND TEST RIG used in all the tests. Saying... ''my K80 made 18lbs of thrust ast sea level... and Joe-Bob's K80 made 17lbs at 4000' MSL'' insn't usable data. That thrust change is still within the error of the engine, and the people operating it... and in the test rig.


BUT......... to have an accurate, generalized equation for thrust lost in a pure jet.... it's impossible.



Oh... did I mention that I have a Degree in Physics... worked in an R&D lab for 8 years.... and now own my own shop, and I build engines??? (lol)
I agree, it’s really complicated to determine parameters of an engine, because it takes time and some guts to analyze thermodynamics process. To be practical I just used value on the output side of an engine and get comparison between the test result. It also eliminate other variable including constant... BTW, I've got mistake here, I should consider intake airflow because in reality air accelerates around the turbine and becomes free stream. So even the engine is on a test stand it has airspeed when it's running.

Here's the formula for Thrust from http://www.grc.nasa.gov/WWW/k-12/airplane/turbth.html so we can solve it again.




BTW, rctech2k7 is a short for RCTech2007. Rctech was taken when I signed up but since I started in RC on year 2007 I used 2k7, where ''k'' is derived from a technical prefix kilo.

Good to know that we have someone here with degree of Physics, very interesting... You might help us about adiabatic expansion. BTW, my background is electrical engineering. What I've also learned makes me to appreciate the network and technical part of this hobby specially the high energy performance of our jets, flight control system, etc...

Let's go back to the topic, I tried to approximate using this formula and it's almost the same although the fuel consumption might change it slightly when running at low air density where mass flow to the exhaust will increase...


There are to many inconsestant variables to give a defined theory used, to give a fixed equasion for constant IE- mmHg, ambient C, fuel density, internal pressure ratios per engine so on.
If you were to do your testing it would take LARGE samples with all of the given probalaties differentially mixed to get a defined equasion for a specific engine but it would not translate to any other engine.

The applied math I see being used here is quite accurate m2,v2-m1,v1 but all of the other factors have to be put into the equation as well. It could become a job for a couple of weeks to identify one engines parameters itself.

I think the values given by WREN are probally the quickest and easiest to use.

Tim
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Old 09-15-2012, 04:40 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Quote:
ORIGINAL: rctech2k7




I agree, it’s really complicated to determine parameters of an engine, because it takes time and some guts to analyze thermodynamics process. To be practical I just used value on the output side of an engine and get comparison between the test result. It also eliminate other variable including constant... BTW, I've got mistake here, I should consider intake airflow because in reality air accelerates around the turbine and becomes free stream. So even the engine is on a test stand it has airspeed when it's running.

Here's the formula for Thrust from http://www.grc.nasa.gov/WWW/k-12/airplane/turbth.html so we can solve it again.




BTW, rctech2k7 is a short for RCTech2007. Rctech was taken when I signed up but since I started in RC on year 2007 I used 2k7, where ''k'' is derived from a technical prefix kilo.

Good to know that we have someone here with degree of Physics, very interesting... You might help us about adiabatic expansion. BTW, my background is electrical engineering. What I've also learned makes me to appreciate the network and technical part of this hobby specially the high energy performance of our jets, flight control system, etc...

Let's go back to the topic, I tried to approximate using this formula and it's almost the same although the fuel consumption might change it slightly when running at low air density where mass flow to the exhaust will increase...

I agree with Tim on this.


The above pic is great example of "eye candy." Sure... the formula may be sound... but there is no way to measure any of that without expensive, high-end test equipment. Let's just look at the thrust end. How are we supposed to measure the velocity? It's very hot, so we would burn ourselves in a "Garage" situation. Also... most of our engines are making thrust in the supersonic range. (+1000 MPH) So... I don't know about you... but my little kestrel wind gauge wont' go that high. (lol)


To try to calculate things is just too hard. If you guys want a generalized equation... then build a test rig, and test as many engines as you can at sea level.... then re-test them at 2000 MSL, and 4000 MSL. Also... we will need the data on LL and HH conditions at those altitudes. At that point, we can take the data, and build a equation that fits the curve.

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Old 09-15-2012, 10:34 AM
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Default RE: How to Correct Measured Thrust for Density Altitude?

Actually on the above formula, you don't need to measure velocity manually, if you have Thrust data you can get information about the velocity. The only difference is the mass flow rate but you can assume they have constant flow rate or you may consider fuel flow rate if you would like.
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