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Cannot properly increase propeller pitch and engine throttle  Topic is solved

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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Tue Nov 30, 2021 4:01 pm

Also you created your engine, and tested it proving it couldn't go above 310hp. Which tells me you're going too fast too low yet again pointing to your drag being too low
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 4:55 pm

Hmm. I think it is a bit of both - the drag curve is too low at high speeds, but the Cp is too low at low speeds where drag has little effect.
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Tue Nov 30, 2021 5:01 pm

I wouldn't advise changing the prop but instead look at the drags of skin, form, induced.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 5:37 pm

But again, drag has zero influence when I'm standing still on the runway … so increasing the drag won't help there. We need to change both - the drag needs to be increased at high speeds, and at low speeds the power required needs to be increased. Actually we might even have to decrease drag at low speeds - takeoff distance is twice as much as in the POH.
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Tue Nov 30, 2021 6:26 pm

You wanted help in getting an accurate engine and prop combination.

It's not a panacea.

All it's doing now is showing you where more errors are, ie in your flight model.

Go to page 130 of the POH, a very important section. I'd expect the flight model to use the same datum point but it doesn't.

There are 3 parts to a good flight model

1) engine/prop combination
2) weight & ballance
3) aerodynamics

Anyway if anyone else needs any help with an engine prop combo, just open a new topic.

Good luck with your flight model.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 7:45 pm

Even if there are errors in the flight model, changing it won't change that I'm getting too much HP when I am standing still on the runway because drag has no effect at that point - it is simply 0. So can you please change the Cp values for advance ratio 0 to 0.5 so that is fixed ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Tue Nov 30, 2021 8:45 pm

Can I suggest then that the quality of your work in establishing the bsfc numbers that you're using has led to the engine producing more power than it should be able too

2600rpm
35.5inhg
285hp
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 9:09 pm

That would be another option, but with the PronyBrake on I got the right numbers. But maybe I will just use the tables you posted and adjust the BSFC numbers so that it is right again.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 9:25 pm

Well, let's ignore the HP mismatches for now and hope that increased drag will fix it. But there is one thing left we didn't do yet - the 5 minutes boost ! <takeoffboost>6.58</takeoffboost> gives me a MP of forty-something instead of 36.5 ? :shock: :?
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Tue Nov 30, 2021 10:01 pm

Sorry - solved - I overlooked the takeoffboost is in PSI, not inHg ! :oops:
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Re: Cannot properly increase propeller pitch and engine thro  

Postby TheEagle » Thu Dec 02, 2021 12:50 am

I have to say sorry, S&J and Ysop - the plane's drag is all right, so is the propeller and engine. The true reason that the plane was going at 250kts instead of 160 kts was this table in the basic drag section of the aerodynamics:
Code: Select all
          <table>
              <independentVar lookup="row">damage/left-wing-ripped-off</independentVar>
              <independentVar lookup="column">damage/right-wing-ripped-off</independentVar>
              <tableData>
                        0       1
                  0    1       0.6
                  1    0.6    0.2
              </tableData>
          </table>

I must have messed something up when I created the functions that set these properties … Well, this chapter is solved now, and I am sorry to have bothered you with this. Since I commented out that table, the thing is going at 162 kts - almost spot on. The climb speed and RPM seem good too, so I think I can close this topic now.

Well, I have just one question left - what does air-intake-impedance-factor do ? When I played around with it I observed that it reduces HP, RPM and fuel flow. Can anyone explain to me what exactly this property does ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby LesterBoffo » Thu Dec 02, 2021 4:09 am

It's about the engine's volumetric efficiency, How easily the engine draws in it's intake fuel air and blows out it's exhaust. An engine with small engine valves but otherwise the same compression and bore-stroke would make less power and use less fuel because the intake and exhaust valves or their manifolds would be acting as choke points or restrictions on the flow of intake and exhaust gasses.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Thu Dec 02, 2021 5:09 am

Hmmm … the bigger the property's value, the smaller the intakes ? Unlogic …
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Re: Cannot properly increase propeller pitch and engine thro

Postby benih » Thu Dec 02, 2021 7:21 am

TheEagle wrote in Thu Dec 02, 2021 5:09 am:Hmmm … the bigger the property's value, the smaller the intakes ? Unlogic …

The bigger the factor, the bigger the resistance ;)
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Re: Cannot properly increase propeller pitch and engine thro

Postby MariuszXC » Thu Dec 02, 2021 9:11 am

Simplifying a bit, impedance is another word for resistance.
The larger the value, the more difficult it is for engine to breathe new air in.

For turbocharged engines there could probably be another similar factor, as turbine presents some restriction to exhaust gases, but, as the end effect is the same, it can probably be skipped and any effect included in input impedance.
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