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Cirrus SR-20 Cruise Speed Error

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Cirrus SR-20 Cruise Speed Error

Postby angryminister » Thu Jul 06, 2017 4:56 am

Hi,

I'm currently trying to simulate a Cirrus SR-20. Everything generally works well, except that the cruise speed is approximately 110kts, importantly, around 40kts below published values of 150kts. I did some research and found on flight gear, the cirrus sr-20 model is modelled on the Piper PA-28, which has a cruise speed also of 110kts. I would like to increase this cruise speed to its required cruise speed of 155kts. Has anyone else had a similar problem, and if so, what have been some ways to solve this? I'm thinking of changing aerodynamic parameters, or the CG location, but am unsure whether this will have unrealistic consequences in the rest of the model. I'm using JSB-sim as an FDM.

Cheers.
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Re: Cirrus SR-20 Cruise Speed Error

Postby Thorsten » Thu Jul 06, 2017 5:05 am

Try to find out where the issue is - are you lacking thrust, are you having too much drag, are you having insufficient lift, forcing you to fly too high AoA and thus generate too much drag?

There's no single parameter that influences equilibrium cruise speed, it's a combination of many, and without more information it's impossible to say.

For instance, you can check climb rates to see whether your thrust/drag is okay, you can check attitude at cruise speed to see whether lift at AoA is okay,...
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Re: Cirrus SR-20 Cruise Speed Error

Postby erik » Thu Jul 06, 2017 12:40 pm

For some I had an SR-20 FDM model available:
http://adalin.com/ehtw.info/sr20.zip

This is fully an AeromatiC++ model but probably better than starting with the PA-28 I guess.

Erik
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Re: Cirrus SR-20 Cruise Speed Error

Postby angryminister » Thu Jul 27, 2017 5:24 am

Hi again,

I managed to get the model up to speed. Thanks Erik for that link! In the end it was the propeller that was causing the problem. As soon as I put in the data for the SR-20 prop instead of the Pipers, the cruise speed increased by 30 knots. However, this has come with the consequence of vastly decreasing the takeoff time, to a point where its way beyond realistic. The sim is going from 0 to rolling speed in around 7 seconds. Is this to be expected with the cirrus propeller that was linked? I wouldn't have thought that a propeller designed for such high cruise speeds could cause such a high acceleration in take off.

Thank, Owen
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