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Inlet mass flow rate ratio

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Re: Inlet mass flow rate ratio

Postby jam007 » Sat Apr 15, 2017 3:22 pm

Maybe just scaling airflow with n1 rpm is the best approach. A turbine compressor is a propeller in way so it should work at a reasonable constant alpha for the blades.
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Re: Inlet mass flow rate ratio

Postby jam007 » Sat Apr 15, 2017 6:12 pm

Decided on these functions for the pitch effect:
Code: Select all
        <function name="aero/function/CA-1">
           <description>Quotient of inlet velocity and air velocity minus one</description>
           <min>
             <sum>
               <quotient>
                 <product>
                   <property>propulsion/engine/n1</property>
                   <value>4</value> <!-- Approx 400 ft/s at max RPM -->
                 </product>
                 <property>velocities/u-aero-fps</property>
               </quotient>
               <value>-1.0</value>
             </sum>
             <value>9</value> <!-- Avoids unrealistic effects at very low speeds  -->
           </min>
         </function>

     <function name="aero/moment/Pitch_intake">
       <description>Pitch moment due to airflow in intake</description>
       <product>
         <property>aero/qbarUW-psf</property>
         <property>metrics/Sw-sqft</property>
         <property>metrics/cbarw-ft</property>
         <property>aero/alpha-rad</property>         
         <property>aero/function/CA-1</property>
         <value>-0.05</value>
       </product>
    </function>


But testing and thinking about the effect I come to two conclusions:
* It is small for low flying speeds compared to other moments. And also it then scales proportional to speed rather than speed squared as CA decreases when speed increases. So it is a narrow band just near the stall speed at high alpha and high rpm that it might be significant in relation to other.
* The largest moment is actually in tight turns at high speed where the moment is opposing the turn. CA is then negative ~0.7. Still it is small compared to other aero moments ~1/10 or less.

For eg. Viggen with a more airsucking engine it might be a more important effect to include.
The high speed effect is not mentioned in the document. Is that a real effect?
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Re: Inlet mass flow rate ratio

Postby Johan G » Sun Apr 16, 2017 12:39 am

jam007 wrote in Sat Apr 15, 2017 2:55 pm:[...] I assume that airflow is non linear with thrust. Assuming that the engine is run close to optimum fuel/air ratio for different thrusts then makes the fuel flow a good start for calculating the intake flow of air.

Just a quick note: I would expect the mixture to be dependent on density altitude, or approximately on altitude.
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Re: Inlet mass flow rate ratio

Postby jam007 » Sun Apr 16, 2017 9:29 am

Johan G wrote in Sun Apr 16, 2017 12:39 am:I would expect the mixture to be dependent on density altitude, or approximately on altitude.

Maybe some dependence. According to Enginesim fuel/air goes from 0.021 to 0.024 when going from 0 to 50 000 ft. Not a huge difference. The mixture is close to the same but the fuel flow goes down. This is in agreement with stoichiometry+cooling.
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Re: Inlet mass flow rate ratio

Postby Necolatis » Sun Apr 16, 2017 9:53 pm

Thanks for all the input. Much appreciated.

I have been thinking about this. Must admit I think there is a good chance N1 does not scale with flow mass, as that would depend also on pressure in inlet, right. And pressure there probably depends on a number of factors, like inlet shape, free stream speed, inlet flow speed and such.

As for pitching I have decided to limit CA effect to 1-3 in the Viggen, there might be some reason, we don't know about, that is the reason they only talk about the effect within that limit. Just to be safe.

I think the reason high speeds is not mentioned is that the document only covers low speed.
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Re: Inlet mass flow rate ratio

Postby jam007 » Sun Apr 16, 2017 10:37 pm

Necolatis wrote in Sun Apr 16, 2017 9:53 pm:Must admit I think there is a good chance N1 does not scale with flow mass

Not with mass flow but with inlet velocity. The compressor blades must work at a limited angle of approach to the air flowing through it. So the rpm and the velocity along the duct should be proportional. The idea came after reading this answer on trust and rpm.
The mass flow is then dependent on temperature and density (eg. altitude).

That's my two cents anyway.
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Re: Inlet mass flow rate ratio

Postby Necolatis » Sun Apr 16, 2017 10:44 pm

Ah, okay.
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Re: Inlet mass flow rate ratio

Postby jam007 » Mon Apr 17, 2017 7:59 am

Necolatis wrote in Sun Apr 16, 2017 9:53 pm:As for pitching I have decided to limit CA effect to 1-3 in the Viggen, there might be some reason, we don't know about, that is the reason they only talk about the effect within that limit. Just to be safe.

I think the reason high speeds is not mentioned is that the document only covers low speed.

After a quick calculation I get a intake velocity of 160 m/s at full thrust and therefore CA=2.7 at landing speed for Viggen. So 1-3 should cover normal low speed range.
Yes, it might be just the focus of the document.
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