Hi Maik, > the "incidence data I have for the bo" I mentioned is not the angle of > attack, but pitch or "the mechanical incidence angle".
Ah, ok. That explains a lot :) > But with the > negative pitch you cited for the ch53e, the rotor would be able to > produce much "negative" force (as far as I know inly very few > helicopters are able to produce "negative forces", e.g. nh90 for carrier > operations, but not in flight). You would get much overspeed at descent. The minimum pitch angle at the blade root (or pitch bearing) for most helicopters is somewhere around 0 degrees, sometimes a bit more, sometimes a bit less. This is probably dictated by airworthiness requirements related to autorotational flight. If the pitch angle, and hence angle of attack would be too big during autorotation, drag on the blades would stop the blade in a matter of seconds, with not-too-nice consequences. > you mean 1 degree flap angle reduce the incidence by 0.45 degree? I > would to have to update the bo105.xml file then. Not exactly. See pp. 14-15 of [1]. 45 degrees pitch-flap coupling introduced through the control mechanism geometry reduces the blade pitch angle by 1 degree for every degree of blade flapping. So for normal helicopters, the blade flapping (coning angle) is somewhere around 5 degrees, resulting in a blade pitch reduction of 5 degrees. Adding pitch-flap coupling to the pitch equation gave a better match for the trim results, so it just seemed logical. Thesis, literature survey and a a3 page containing the source code structure can be found here (for now): http://users.pandora.be/tuinbels/afstuderen/ Note that the model described in my thesis has 9 structural degrees of freedom (6 for fuselage + coning angle, lateral and longitudinal tilt) and 4 (only inflow) or 8 (inflow and wake distortion dynamics) aerodynamic degrees of freedom, resulting in a total of 22 (or 26) states that have to be integrated each time step. Greetings, Wim PS My superviser used an analytical approach in calculating the rotor forces, her PhD thesis can be found here [2]. You might want to read that on too. [1] http://users.pandora.be/tuinbels/afstuderen/final_thesis.pdf [2] http://www.fmp.lr.tudelft.nl/content/phdthesis_mdpavel.pdf On 12/19/06, Maik Justus <[EMAIL PROTECTED]> wrote: > Hi Wim, > wim van hoydonck schrieb am 19.12.2006 11:31: > > Hi Maik, > > > > > >> yes, but it is still the double value than the bo. But the incidence > >> data I have for the bo is measured at 0.7r and therefore much smaller. > >> > > [...] > > > >> I am not sure if that matches the data. With 8° twist and -1.4° pitch at > >> minimum you will get -9.4° at the tip and something about -7° mean > >> (weightened with r^2). In autorotation you will get much overspeed. >The > >> data I have for the bo is measured at 0.7r. The effect is, that a pitch > >> of 0 degree gives approx. in the same lift as a blade without twist. > >> > > > > I think you are mixing incidence angle (angle of attack) with pitch angle > > here. > > > > > the "incidence data I have for the bo" I mentioned is not the angle of > attack, but pitch or "the mechanical incidence angle". But with the > negative pitch you cited for the ch53e, the rotor would be able to > produce much "negative" force (as far as I know inly very few > helicopters are able to produce "negative forces", e.g. nh90 for carrier > operations, but not in flight). You would get much overspeed at descent. > > [1] is appendix A of my MSc thesis (about helicopter flight dynamics > > modelling & simulation) > oh, can you mail me an copy of this thesis? > > in which the mathematical model of the main > > rotor is derived/explained. Figure A.2 contains a sideview of an > > airfoil along the span of a rotor blade, which should explain the > > difference between the two. > > > > The main contribution to the vertical component at a blade section > > (U_p in [1]) comes from the induced inflow. If the induced inflow is > > not calculated accurately (or not at all), the pitch angles at the > > blade sections will be much smaller than in reality, since inflow > > reduces the angles of attack. > > > > How is inflow calculated in Yasim? I only can find some reference to > > 'downwash', but I don't see any familiar equations overthere. > > > > > It follows an old theory of Prantl with an "empirical" correction > factor. (in function calculateAlpha in rotorpart.cpp) > >> One important point for the simulation: most rotors have a delta3 effect > >> (this means: reducing the pitch with increased flapping angle). Do you > >> know this parameter for the ch53e (or for other helicopter?) > >> > > > > At our faculty, we normally set delta_3 equal to -45 degrees for the > > bo105 (i.e. positive blade flapping reduces the blade pitch angle, so > > stabilizing). > you mean 1 degree flap angle reduce the incidence by 0.45 degree? I > would to have to update the bo105.xml file then. > > Pitch-flap coupling for the AS 330 Puma is zero. I am > > not sure about other helicopters. > > > > > > Greetings, > > > > Wim > > > > > > > Maik > > ------------------------------------------------------------------------- > Take Surveys. Earn Cash. Influence the Future of IT > Join SourceForge.net's Techsay panel and you'll get the chance to share your > opinions on IT & business topics through brief surveys - and earn cash > http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV > _______________________________________________ > Flightgear-devel mailing list > Flightgear-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/flightgear-devel > -- Avoid hangovers - stay drunk! ------------------------------------------------------------------------- Take Surveys. Earn Cash. 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