
#1
Dec3109, 04:15 AM

P: 82

how is aspect ratio of a wing related to longitudinal static stability (LSS)?
like, if we increase the aspect ratio, is it going to increase or decrease the longitudinal static stability (LSS)? suppose we have an aircraft whose centre of gravity is fixed at a particular location, if the propeller is mounted on the nose of the fuselage instead at the wing, what's its effect on LSS ? 



#2
Dec3109, 04:24 AM

P: 4,780

For longitudinal stability you are primarily concerned with the pitch stiffness of the airplane. This is the slope of the Cm vs alpha curve, and should be negative. What you can do, is calculate the 3D pitching moment of the wing for the various aspect ratios you are considering. This curve will be added to the pitch moment curve of the rest of the airframe. Because the airframe does not change, the primary thing to see is: which wing creates a larger destabilizing pitching curve. This will tell you what you are after, and it is something you can roughly compute using theory, or references like Jan Roskam. Edit: Did you just say 'like'??? 



#3
Dec3109, 04:43 AM

P: 82

does it vary from aircraft to aircraft?
doesnt it has any generic variation? something like you know, aircraft with high wing config has more longitudinal static stability at higher angle of attack than compared with low wing config at higher AOA. 



#4
Dec3109, 04:50 AM

P: 4,780

longitudinal static stability (LSS)I hope your next post is typed properly, or I won't provide any more help. 



#5
Dec3109, 05:00 AM

P: 82

"I hope your next post is typed properly, or I won't provide any more help"
well when i post here, i try my best english with least mistakes. its bit difficult because m from non english speaking country, people here do speak english but barely with any grammar. m sorry. i'll try my best. 



#7
Dec3109, 05:07 AM

P: 82

i found out this from a textbook. i dont have it now but i'll post the details in a day or two. he mentions it clearly about this with a graph of Cm vs Cl for aircrafts with highmidlow wing configs. unfortunately he didnt mention the reason for this, but i dont think there's some obvious reason for this. m trying to find out more on this. 



#8
Dec3109, 07:38 AM

P: 4,780





#9
Dec3109, 10:35 PM

P: 261

I found an article which describes wind tunnel tests on wings with a NACA 0012 profile. They didn't calculate any stability derivatives, but they did tabulate the Cm variation with angle of attack for three different aspect ratios. It would be pretty straightforward to calculate the derivatives using a polynomial fit.
Here is the link: http://www.scipub.org/fulltext/ajas/ajas22545549.pdf 



#11
Jan210, 03:40 AM

P: 82

Thank you for that paper.
But the data didn't quite help me. When i calculate the slope of Cm vs AOA values for different Aspect Ratios, I don't see a definite pattern of increasing or decreasing slope. Can you guys help me further on this? 



#12
Jan210, 03:42 AM

P: 4,780





#13
Jan210, 03:49 AM

P: 82

Yes, the slopes that i found are negetive. so its stabilizing moment.
But the slope for AR = 1.9474 is greater than compared to AR = 2.761 and the slope of AR = 3.0198 is almost similar to AR = 1.9474, which is more than AR = 2.761. I just don't get it. 



#14
Jan210, 04:13 AM

P: 4,780





#15
Jan610, 12:07 AM

P: 594

It terms of longitudinal stability, this effect requires a larger tail surface for higher aspect ratio wings in order to provide greater longitudinal precision along the pitch axis. Wings of lower aspect ratio require less tail surfaces all the way down to delta wings, which require no tail surfaces at all, except in the form of elevons at the trailing edge of the delta planform. Thus, it's not just the aspect ratio which determines overall longitudinal stability, but the combination of wing and horizontal stabilizer planforms which work together as a system. 



#16
Jan610, 12:11 AM

P: 4,780





#17
Jan610, 04:53 PM

P: 594

Douglas noted this in the F4 Skyray. When they designed the Skyhawk (a nearly simultaneous design), they corrected for this by using a horizontal stab. By decoupling the moment arm, they dramatically reduced the negative lift aspect of the Skyray. As a result, the A4 was a remarkably agile 1950's era fighter that remains in service to this day. 


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