Free Flight, Free Speech

Over the years I've flip flopped between the axis systems of LIFT SIDE DRAG and X Y Z, learning the pro's and con's of both systems....

And I'm now coming to the conclusion that a flight model using both systems at the same time is the way to go... This isn't an easy option but using the pro's of each and discarding the con's could make for an interesting flight model.

Only time will tell

Simon

does nt JSBsim documentation advise somewhere not to use both simultaneously? is there any true to that?

IAHM-COL wrote:does nt JSBsim documentation advise somewhere not to use both simultaneously? is there any true to that?

Actually JSBsim manual says you can't mix them....

Using JSBsim internal axis system this is true, but using external forces you can use body, wind and local axis all at the same time.

bomber wrote:Over the years I've flip flopped between the axis systems of LIFT SIDE DRAG and X Y Z, learning the pro's and con's of both systems...

Can you summarise what you found to be the the pros and cons of each axis system?

I'll try however I've a feeling of standing over a trap door and it definitely going to take a few posts as they come to mind..

LSD pro: And I know this is going to sound obvious but using Lift and Drag makes it easier to bench mark the planes performance using LIFT to DRAG ratio which is commonly known about a plane.
LSD pro: It's easier to get your head around as the DRAG is allways opposite to the direction of travel in a plane that's flying and LIFT is allways perpendicular under the same conditions.
LSD pro: If you spit the wing up into sections and apply washout to each section such that they're all effectively seeing different angles of attack the produced LIFT and DRAG are ALL pointing in the same direction allowing you to build them up into a single LIFT DRAG value.

LSD con: Although having split the wing up and applied differing AoA to each wing the resulting LIFT DRAG can only be applied to a single AoA that being being for the fuselage. In a clockwise spin condition where the left outer wing is seeing -10degs the fuselage 60degs and the right outer wing 30 degs the values from the co-efficients don't translate easy to the plane as a whole.

Interesting stuff; please add to the list as you think of more, even though most of those don't apply unless using your approach it's still interesting. Also I think the LD ratio will still be valid for comparison between axis systems.

After some thought I realised that because moments are always in body axes (never in stability axes) - so for your approach I think only body axes makes sense. I think stability axes are usually used due to the way data is measured in wind tunnels - where it is easy to move the aircraft but not the wind. The important thing is to be consistent and to ensure that transformations are applied where they should be.

You can mix axis systems you just have to do the correct transformation; which used to be very tedious set of calculations to do by hand but as it's a relatively simple matrix multiply which is easy within a program.

so Lets look at the other two axis systems... Normal, Side, Axial & X, Y, Z.. These both use body axis.

NSA con: Normal and Axial coefficients are values derived for an AoA along the cord line between the leading and trailing edge... If your rigging angle is zero ie aligned with the body axis then all's great, but if you have a rigging angle or in my case washout along the wing resulting in 8 different rigging angles then you have to translate the normal and axial values into body axis Z and X.
NSA con: is a harder system to get your head around, its hard to accept that at negative AoA a wing can not only be generating LIFT but also THRUST.
NSA con: You're still going to have to translate into LIFT and DRAG for Lift to Drag comparisions, so you've effectively had to work out 3 different systems, Normal, Axial... translated to both body X Z and Lift Drag... it's a lot of work setting it up.
NSA pro: As you're calculating the Z and X values for each section of the wing based on their own AoA's and translating them all to a common body X Z frame modelling of spin is possible.
NSA pro: As you're working in a common body frame for both wings and h-stab it's easier to calculate their moments and gain an understanding of longtitudinal stability.

One more thing to consider... for a reflected ground effect force the use of LOCAL as an external force ensures that this is always perpendicular to the earth..