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

TimD.

My Feedback: (207)

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.

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 .

highhorse

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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.

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 ?

rctech2k7

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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.

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

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

highhorse

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

rctech2k7

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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...

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)

marquisvns

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 [X(]

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

Cheers,
Barry

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

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.

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

757Driver

My Feedback: (90)

I am not siding with or against any of you.
Now I will present some information on the matter from a professional pilots perspective.


Thrust:
For a given throttle/thrust lever setting the amount of thrust developed by a jet engine or piston/propeller engine is directly proportional to the air density. A low air density decreases the thrust available when compared with that for a normal atmosphere. This fact is particularly important for take-off from a high elevation aerodrome, especially if the surface ambient air temperature is high, because the reduced thrust available decreases the rate of acceleration. This increases the length of the ground roll to attain the lift-off speed (VLOF), even at the maximum take-off thrust/power setting.


As altitude increases the thrust reduction caused by reduced air density becomes more significant. At 40 000 ft a jet engine only develops 31% of the thrust it would at mean sea level for the same thrust lever setting at the same gross mass.






Engine Performance:
The essential forward thrust required by all aeroplanes is provided by one of three types of engine fitted to those aircraft. They are either a piston/propeller type, sometimes referred to as a reciprocating engine, a turbo-propeller engine or a jet engine. (Ramjet engines and Scramjet engines are not considered in this manual.) No matter what type of engine is considered the thrust produced by each of them is directly affected by air density.




Density altitude:
The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere is density altitude— the altitude in the standard atmosphere corresponding to a particular value of air density.


Density altitude is pressure altitude corrected for nonstandard temperature. As the density of the air increases (lower density altitude), aircraft performance increases and conversely as air density decreases (higher density altitude), aircraft performance decreases. A decrease in air density means a high density altitude; and an increase in air density means a lower density altitude.


Density altitude is used in calculating aircraft performance. Under standard atmospheric condition, air at each level in the atmosphere has a specific density, and under standard conditions, pressure altitude and density altitude identify the same level. Density altitude, then, is the vertical distance above sea level in the standard atmosphere at which a given density is to be found.


The computation of density altitude must involve consideration of pressure (pressure altitude) and temperature. Since aircraft performance data at any level is based upon air density under standard day conditions, such performance data apply to air density levels that may not be identical with altimeter indications. Under conditions higher or lower than standard, these levels cannot be determined directly from the altimeter.


Density altitude is determined by first finding pressure altitude, and then correcting this altitude for nonstandard temperature variations. Since density varies directly with pressure, and inversely with temperature, a given pressure altitude may exist for a wide range of temperature by allowing the density to vary. However, a known density occurs for any one temperature and pressure altitude. The density of the air, of course, has a pronounced effect on airplane and engine performance.


Regardless of the actual altitude at which the airplane is operating, it will perform as though it were operating at an altitude equal to the existing density altitude.


For example, when set at 29.92, the altimeter may indicate a pressure altitude of 5,000 feet. According to the AFM/POH, the ground run on takeoff may require a distance of 790 feet under standard temperature conditions.


However, if the temperature is 20 °C above standard, the expansion of air raises the density level. Using temperature correction data from tables or graphs, or by deriving the density altitude with a computer, it may be found that the density level is above 7,000 feet, and the ground run may be closer to 1,000 feet.


Air density is affected by changes in altitude, temperature, and humidity. High density altitude refers to thin air while low density altitude refers to dense air.


The conditions that result in a high density altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some combination of these factors. Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude.


Performance:
The effect of pressure altitude and ambient temperature is to define primarily the density altitude and its effect on takeoff performance. While subsequent corrections are appropriate for the effect of temperature on certain items of powerplant performance, density altitude defines specific effects on takeoff performance. An increase in density altitude can produce a twofold effect on takeoff performance:


1. greater takeoff speed and


2. decreased thrust and reduced net accelerating force.


If an airplane of given weight and configuration is operated at greater heights above standard sea level, the airplane will still require the same dynamic pressure to become airborne at the takeoff lift coefficient.


Thus, the airplane at altitude will take off at the same indicated airspeed as at sea level, but because of the reduced air density, the true airspeed will be greater.




And most importantly this NACA document clearly defines why there can be no ONE correction factor for every engine. I think it best to make several runs at different density altitudes and plot the performance, graph and extrapolate for an estimated standard day thrust figure.
http://naca.central.cranfield.ac.uk/...-rm-e51j15.pdf


Looks like we got some fuel to burn fellas...

dbsonic

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Actually for electric motor driven aircraft the engine is unaffected by density altitude unlike internal combustion type. However the propellor efficiency component is still lost.