Shogo Hirahara asked 2 years ago
Hello Andre,
I am very impressed and excited by your introduced viscous loop in flow5.
My question is whether you plan to make the viscous loop option available in Type 8 analysis, or to enable to output non-dimensional stability derivatives from Type 6 analysis.
The background of my questions is as follows;
I am a modeller of small scale free flight paper gliders (wing span ~20cm) which are under non-linear aerodynamic characters of low Re (~10K) conditions as you know.
One of the typical style of our paper plane is to be launched vertically by rubber-band-catapult with about 40m/sec and automatically moves into gliding (〜5m/sec) subsequently.
The link is an example of vertical-rise-and-glide plane on Youtube, uploaded by my friend.
https://www.youtube.com/watch?v=Gwn_G04GecA
One of our forerunners revealed such a behaviour can be explained by non-linear characteristics of thin foil paper gliders. He pointed out such a plane has double Cm=0 points on Cm-AOA diagram. One of them are at AOA=0 which enable the plane rise vertically like a rocket. Since there is almost zero camber, there is no lift at AOA=0. Once it reaches to 20~30m height and zero speed, it flips into gliding with several degree AOA, where is the second Cm=0 point.
The shape of the Cm-AOA curve is like an upside-down hockey stick. An experienced researcher observed such a shape of curve with wind tunnel experiments. He published them but only in Japanese.
I tried and managed to reproduce (I should say mimic) his observations with flow5 T6 analysis with viscous loop.
I thought this could open up a way to use flow5 for optimising my plane designing. However, I realised viscous loop is not available in T7 stability analysis, non-dimensional stability derivatives are not exportable from T6 either, and I am stuck now.
Since I managed to set up an Excel sheet to run longitudinal and lateral stability analysis with using Etkin and Reid textbook, if I get stability derivatives output from T6 I may continue my project.
Thank you for your attention and I hope to have your comments.
Best regards,
Shogo
Hello Shogo,
Cm curves with two aoa such that Cm=0 are new to me, and I’m not sure that potential flow analysis can reproduce that, even with a viscous loop. The main reason is that potential flow is essentially linear, i.e. lift and other coefficients are proportional to the aoa. The viscous loop and the VPW can improve things a bit, but my feeling is that they should not be seen as anything more than first order corrections.
To answer more specifically your questions:
I don’t intend to implement the viscous loop in T8 analyses, because this type is essentially there to calculate stability derivatives at any aoa, and this calculation requires linearisation.
As for T6 analyses, it could be possible to linearise and calculate the derivatives, at least if the flat panel wake is selected and not the VPW. However this would raise code complexity to a level that I feel unable to manage.
I understand that this may be a bit disappointing from your perspective, but I don’t have easy solutions to offer.
Regards,
André