4
$\begingroup$

I am trying to draw the V-n Diagram for a small aircraft I built, but I am stuck in the gust envelope part. I know how it is supposed to look. However, I am having a hard time calculating it. I would like to know (1) why it's composed of four lines; and (2) how to find the slope of each line. So far, I have these three equations:

enter image description here

enter image description here

enter image description here

Also, (3) why do these equations describe the gust envelope, and (4) what each variable in the equations stands for?

$\endgroup$
3
  • $\begingroup$ Where did you get the equations? $\endgroup$
    – fooot
    Commented Jul 12, 2018 at 17:03
  • $\begingroup$ They are from a study guide from an aero design competition. Fairly reputable source. I've also seen them in other places, but if they are wrong, please let me know. $\endgroup$ Commented Jul 12, 2018 at 17:06
  • 1
    $\begingroup$ @fooot Those are straight from the now deprecated FAR 23.341 and may be used in the absence of a more rational method to calculate gust loads. $\endgroup$ Commented Jul 13, 2018 at 3:26

1 Answer 1

2
$\begingroup$

The gust envelope is four lines overlaid on the flight envelope, usually the plus/minus of maximum gust at design cruising speed (Vc) and half maximum gust at maximum speed (Vd) as required by the certifying authority. This is usually +-50ft/sec (15.2m/sec) and +-25ft/sec (7.6m/sec).

enter image description here

  • n is the normal acceleration in a sharp edged gust
  • K is the tailplane gust alleviation factor. In a canard this is an aggravation factor.
  • u is the airplane mass ratio

The source contains examples of how the lines are calculated. The following information (defined) is required:

enter image description here

$\endgroup$
2
  • $\begingroup$ Thanks a lot for this. The only thing I didn't understand was the 2-D aerofoil lift-curve slope. Is it a tabled value different for each airfoil? If not, how do I calculate it? $\endgroup$ Commented Jul 12, 2018 at 23:42
  • 1
    $\begingroup$ @UnicornLord It is different for each airfoil and derived from the airfoil's lift/AOA table. The reference's reference 1 shows how to calculate it. You can also google "lift curve slope" to see alternate descriptions. It usually turns out to be around 5-6/rad or about 0.1/degree. The example had 5.7/rad, oddly similar to but completely unrelated to the wing aspect ratio. $\endgroup$
    – Pilothead
    Commented Jul 13, 2018 at 0:24

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .