14 Replies Latest reply on Jan 22, 2014 2:41 PM by Jared Conway

    RC plane force values not making sense

    Dwreck Basti

      Hi all,

       

      I'm new to CFD, and currently trying to analyze the lift and drag forces on an rc plane that I designed in SolidWorks 2013.

       

      The plane was designed as a single part with multiple bodies.

       

      I followed some youtube tutorials on how to setup the parameters. I'm aware that I need to include some more specific values

      in my setup, but just wanted an estimate at first.

       

      The plane is at a zero angle of attack and being simulated at a flying speed of 75 ft/s.

       

      The problem that I'm facing is that the output drag force value that I get is higher than the lift force value, which is incorrect.

       

      I've attached the model and the setup that I used.

       

      Can you guys please give me some pointers as to what I'm doing wrong and if my setup is incorrect?

       

      Thanks in advance!

        • Re: RC plane force values not making sense
          Jared Conway

          quick look at the setup it looks fine.

          the global goals should be ok for outputting what you want. many times people don't choose the appropriate faces for the output, global should elimiante that potential issue.

           

          here's my recommendation start much smaller, for example the wing profile in 2d to get an idea of how big the comp domain needs to be and how fine the mesh needs to be to get reaosnable results. there are several forum posts about this.

           

          then you can work your way up to the full model. maybe a half model to start.

            • Re: RC plane force values not making sense
              Dwreck Basti

              Thanks for your reply!

               

              I've tried doing it with the wing before and the values obtained were reasonable. I followed

              the same procedures for this one.

               

              I'll try increasing the comp domain and have a finer mesh. But I have a feeling that this will

              still give similar results as before, with the drag force being more than the lift. Maybe there's

              something that I'm missing in the setup?

                • Re: RC plane force values not making sense
                  Jared Conway

                  the problem is really straightforward, hard to error there and i don't see anything that can be a problem

                   

                  i think its going to come down to mesh level and having enough detail in the model

                   

                  for example we were working with a company doing a land speed record vehicle. they couldnt' get the drag to match their expectations. they made the mesh very fine, still no difference. we added wheels to the model which they hadn't included and things started to match quite well.

                   

                  the other issue could be limitations of the KE model but if you saw that a wing was working properly, that is less of a concern. I would focus on mesh and making sure the geometry is captured..etc before worrying about that.

              • Re: RC plane force values not making sense
                Dwreck Basti

                The force that I've been getting for the z-axis, which is for the drag,

                is around 5.6 lb-f.

                For the y-axis, which is for the lift, the force is about 0.35 lb-f.

                 

                Now, if I take the 5.6 lb-force as the lift and 0.35 lb-force as the drag,

                then it makes total sense. By estimating the respective coefficients

                (using the basic lift and drag equations), I'd say that this would be a

                reasonable approximation.

                 

                Could it be possible then that I didn't define my coordinate system properly?

                It doesn't look like that from my setup though

                  • Re: RC plane force values not making sense
                    Bill McEachern

                    Well if its any help. I did a Beachcraft for a mod shop once to estiamte appendage drag and pitot port effects of hte appendage for the flight test program. They sent over a model that I fixed and then analyzed it. My lift calc was pretty much the weight of the vehicle. The weight of course varies so my lift was calculated at, call it 6,000 lbf and the MTOW was about 6300 lbf. I did not have the propellors modeled, prop wash or thrust modeled or in anyway accounted for.

                    Other posters have use the global goals as opposed to surface goal and have gotten results that did not make sense - not too sure of the details now. You want to use surface gaols and pick all the external surfaces that make up the airplane for the lift and drag goals.