RE: How to Correct Measured Thrust for Density Altitude?

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RE: How to Correct Measured Thrust for Density Altitude? - 9/15/2012 7:37 PM

rctech2k7

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Tim, maybe you are referring to this one:

Explanation is located on:
http://www.grc.nasa.gov/WWW/k-12/airplane/thsum.html

John

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RE: How to Correct Measured Thrust for Density Altitude? - 9/15/2012 10:54 PM

TimD.

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quote:

ORIGINAL: rctech2k7

Tim, maybe you are referring to this one:

Explanation is located on:
http://www.grc.nasa.gov/WWW/k-12/airplane/thsum.html

John

Nice rctech2K7 !
What would need to be done is all parameters and DATA SETS need to be put into an Excel spreadsheet and write the program to have a quick logmarithic means for formulation. But again it is only accurate for a specific engine design.

It would be great if someone had the time to do it, but realistically with about 30 engine designs in the market place today this could be a full time job that could take a year or so. All conditions "variables" have to be tested twice and with one or two controlled conditions in order to collect the accurate data set needed.

So I guess to try and anwser the origional question is - How much time do you want to spend to get an exact anwser. I am in no way being a Jerk just trying to give you an anwser. The equations are not that difficult to build they just look scarry.

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RE: How to Correct Measured Thrust for Density Altitude? - 9/16/2012 1:53 PM

turnnburn

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Seems to me the obvious question is why do care ? Its going to effect all manufacturers engines of similar size and thrust approximately the same amount and there is little if anything you can do about it except wait for a cooler day to go flying .

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RE: How to Correct Measured Thrust for Density Altitude? - 9/16/2012 8:53 PM

highhorse

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quote:

ORIGINAL: turnnburn

Seems to me the obvious question is why do care ? Its going to effect all manufacturers engines of similar size and thrust approximately the same amount and there is little if anything you can do about it except wait for a cooler day to go flying .

The obvious answer is that some of us, as stated in the OP

, want to be able to make comparisons between test runs which happen on different days and under different conditions.

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RE: How to Correct Measured Thrust for Density Altitude? - 9/16/2012 10:30 PM

turnnburn

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I guess im still missing something. What are you going to with this info ? Use it to buy one turbine over others ? I mean Im sure its interesting and there is certainly no harm in it Im just not ultimatly sure what the point is ?

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RE: How to Correct Measured Thrust for Density Altitude? - 9/16/2012 11:55 PM

rctech2k7

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quote:

ORIGINAL: TimD.

quote:

ORIGINAL: rctech2k7

Tim, maybe you are referring to this one:

Explanation is located on:
http://www.grc.nasa.gov/WWW/k-12/airplane/thsum.html

John

Nice rctech2K7 !
What would need to be done is all parameters and DATA SETS need to be put into an Excel spreadsheet and write the program to have a quick logmarithic means for formulation. But again it is only accurate for a specific engine design.

It would be great if someone had the time to do it, but realistically with about 30 engine designs in the market place today this could be a full time job that could take a year or so. All conditions ''variables'' have to be tested twice and with one or two controlled conditions in order to collect the accurate data set needed.

So I guess to try and anwser the origional question is - How much time do you want to spend to get an exact anwser. I am in no way being a Jerk just trying to give you an anwser. The equations are not that difficult to build they just look scarry.

Yeah, it looks scary on a few glance and also becoming into engineering due to design aspect. If we analyze the formula for Ve, the only variable is the Temperature. Although efficiency and nozzle design are also the major factor that affects exhaust velocity, they are constant for an existing engine. Our turbine is being controlled by temperature and RPM so at maximum fuel supply I believe this exhaust velocity is being limited by these control parameters.

If we look at how the thrust is affected by the following parameters then we can make an assumption:

Thrust = density * A * V (deltaV)

The rate that the thrust change with respect to air density is constant while expo to velocity but velocity doesn't have a factor of density if I'm not mistaken. Velocity is in a second degree so if we try to look at how the velocity change with respect to the change of thrust, the result is smaller.

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RE: How to Correct Measured Thrust for Density Altitude? - 9/17/2012 1:32 AM

TimD.

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Would make a nice app. for the smart phones, but to much time involved for just a few hundred peolpe worldwide.

You are the one retooling the KJ and wren design correct ?

Tim

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RE: How to Correct Measured Thrust for Density Altitude? - 9/17/2012 3:10 AM

rctech2k7

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That sounds great, my little programming skills was out of date. Also looking forward for the solution of this thread. Negative about the second question.

John

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RE: How to Correct Measured Thrust for Density Altitude? - 9/17/2012 4:41 AM

highhorse

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quote:

ORIGINAL: turnnburn

I guess im still missing something. What are you going to with this info ? Use it to buy one turbine over others ? I mean Im sure its interesting and there is certainly no harm in it Im just not ultimatly sure what the point is ?

.

quote:

ORIGINAL: turnnburn

I guess im still missing something. What are you going to with this info ? Use it to buy one turbine over others ? I mean Im sure its interesting and there is certainly no harm in it Im just not ultimatly sure what the point is ?

There are many reasons to test output. General curiosity, pre or post maintenance, comparisons between installations, between brands, between fuels...

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RE: How to Correct Measured Thrust for Density Altitude? - 9/17/2012 11:42 PM

rctech2k7

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quote:

ORIGINAL: Dr Honda

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.

Actually, this is a good question... How do we measure air velocity? IMO, we can use pressure tube because dynamic pressure has a factor of air velocity. How about using pitch of a propeller, it's specified as inch per rpm or distance per unit time. Consider an ideal prop without power and load, when it starts to spin by wind, energy is transfer or a dynamic force is acting to accelerate prop but after acceleration a force becomes zero and the distance traveled per unit time by the prop's rpm turns to be equal to the air velocity...

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RE: How to Correct Measured Thrust for Density Altitude? - 9/17/2012 11:59 PM

rctech2k7

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If we look at how prop and turbine wheel is being spin I believe it's more about air velocity and mass is used to transfer energy then maintain. If I can find out that the differences between net velocity at almost the same temperature and rpm are negligible I can conclude that ratio of Thrust is equal to the ratio of density.

T1/T2 = density1/density2

or

T2 = T1 * (density2/density1)

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RE: How to Correct Measured Thrust for Density Altitude? - 9/18/2012 1:02 AM

marquisvns

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Salute to the OP, something we've been looking into, especially since the humid Southern Taiwan won't see industry standard 15C for engine testings.

What we have is 0.11kg of thrust difference per every 1C deviation from 15C, perhaps combine this with the above mentioned 2% loss per every 1,000 of elevation would be somewhat close? I'm sure there are other factors to consider, air pressure and humidity..., and perhaps some of these variables are exponentially curved

It really be awesome to have a good calculator or formula to generate a more accurate thrust for our miniature turbine engines.

Cheers,
Barry

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RE: How to Correct Measured Thrust for Density Altitude? - 9/18/2012 5:58 PM

rctech2k7

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It makes me to think how velocity is being measured and also to use the formula, please correct if I'm wrong.

I have no actual efficiency and nozzle pressure data but for someone who is determined I believe they can get this data from different ways...

Sample efficiency = 60%.
Based on approx 700 deg C EGT, Tt = 700 + 273K = 973K
NPR = 4

Value for following constant:

Specific heat of air at constant pressure, cp = 1.0 kJ/kg-K = 1kN-m/s^2-kg-K
1kN = 1000kg-m/s^2
cp = 1000 m^2/s^2-K

gama = 1.4

Ve^2 = 2 * 1000 m^2/s^2-K * 973 K * 0.6 * (1 - (1/4)^(1.4-1)/1.4)

Ve = 618 m/s or 1383 mi/hr

The higher the temperature, efficiency and NPR the higher the exhaust velocity

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RE: How to Correct Measured Thrust for Density Altitude? - 9/18/2012 7:16 PM

Eddie P

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I see some folks have been agitated by this thread. I for one think it's a really cool idea to formulate a "calculator" of sorts for turbojet engine performance in a variety of density altitudes. For one, it's an interesting set of data for those who care to know - secondly, it can help folks decide what engine might be a better choice for where they fly, not only what they fly. Not all of us (hardly any of us) fly at sea level on a 15 degree C/59 degree F day where most of the thrust numbers are adjusted to "sell" us at!

Twenty years ago I could have quoted a lot of industry calculations for various forms of power plants like pure turbojets and also turbofans. Those days are gone, I passed the course and dumped the data since I'm not designing jet engines! I think right now it's probably pretty easy to "generalize" that our engines, no matter what the manufacturer, are pretty similar. We all are using similar fuels with about 4-5% oil mix and are using centrifugal flow single stage turbojets. The biggest variable seems to be in the design of wheels where some engines put out a lot higher exhaust speed than others and I'm not sure how that will compare in terms of the graph for thrust vs. density altitude. I think a general data set will work closely for most any contemporary model jet engine out there - maybe two sets of data for similar engine types, etc.

Of course one of the biggest variables we deal with is "installed" thrust where the chosen pipe design will greatly add variables to the raw thrust an engine can deliver on a test stand. That will have to be considered on a separate basis and most everyone knows that a bifrucated pipe, like on an F-15 or 18, will be a worst case scenario in this consideration where as a short straight pipe may actually increase static thrust depending on design.

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RE: How to Correct Measured Thrust for Density Altitude? - 9/19/2012 10:47 PM

rctech2k7

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Good point… Our life is full of changes, we earned experience, spent hard work and accomplishment. We have so much we can do and enjoy the most out of it. I have respect for people who was born earlier and the glory I give to the Lord, because He allow everything to develop including knowledge of those people and they're the reason why I have the best technology today. Thanks for reminding the most important in life... Our Creator is offering gift of salvation from Christ because regardless of everything life is transient…

The subject matter I’m trying to help on this thread should be very simple without any agitation it won’t bring up those formula from NASA, this is one of the reason why in most cases I don’t show up specially to reveal my degree and experience. Just to know someone’s degree and experience or accomplishment in life is my happiness because I know how hard they strive to reach their dreams specially for those with barriers like being self-supporting or started in less fortunate situation...

My intention here is to stay behind but ended up pursuing my help. Now is to finalize what I started here. Thrust equation has only 2 factors that changes, density and velocity. Similar to what you've said velocity functions with regards to the design is already existing and most parameters are constant for the same engine. The only remaining is the density which has a constant rate and the slope varies with regards to thrust specification.

Thrust = density * k

Where k is consider to be constant for the same engine at full power (almost same temp and rpm).
k = Intake area * free stream velocity * (exhaust velocity - free stream velocity)
k = Thrust0 / density0

This is similar to:

T = T0 * (density/density0)

If we graph Thrust versus density the line is straight with intersection at (0,0) with slope “k” or Thrust0/density0. This is good for calculating or correcting Thrust using density variation…

Hope this help and thanks for your time…
Happy flying,
John

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