# xxxxx

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#### Jean - Claude

##### Senior Member
On sait que accélération = Vitesse / tempsou en abrégé : a = V/t
La distance x parcourue, quand l’accélération a est constante, est donnée par la formule x = ½ a . t[SUP]2[/SUP]
D’où on tire que t = √(2x /a)
En remplaçant cette valeur dans l’équation a = V/t, on trouve : a = V / (√(2x /a)
De là, on obtient a = V[SUP]2 [/SUP]/2x

Ainsi, pour amener la composante verticale de descente planée de 4,5 m/s à 0 en 0,4m de course verticale, il faudra que l'’atterrisseur impose une décélération constante de : a = 4,5[SUP]2[/SUP] / (2 *0.4) = 25 m/s[SUP]2[/SUP]soit 2,5 g, puisque g vaut environ 10 m/s[SUP]2[/SUP] . Attention, ne pas oublier d'ajouter 1g pour tenir compte de la gravitation de notre monde réel, soit 3,5 g
Et pour la chute d'une autorotation verticale (environ -8 m/s ) stoppée en 0,4 m/s de course il faudra une décélération verticale de a = 8[SUP]2[/SUP] / (2 *0.4) = 80 m/s[SUP]2[/SUP]soit 8 g (+1g)

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#### Doug Riley

##### Platinum Member
I fly a friend's G-force equipped Butterfly now and then. I don't care for the particular model of this gear on his machine. It presents the possibility of a forward-then-sideways capsize, as the nosewheel does not have a long-stroke leg as the mains do. I tie the gear off in the retracted position.

There's no reason why a properly-designed long-stroke gear that allows vertical descents from any height won't work. There are films on YouTube of Cierva-derived machines with this ability. The necessary gear travel to reduce the G's to about 2 is in the vicinity of 2.5-3.0 feet. The nose gear will need to be energy-dissipating as well, unless you revert to taildragger gear.

People insist on using swing axles, double swing axles, inward-inclined struts, springy compression gizmos that store energy instead of converting it to heat, and other unhelpful elements.. Fixed outriggers with purely vertical travel would be much better. Just copy Cierva and Pitcairn.

#### C. Beaty

##### Gold Supporter
Cessna type spring gear, including Eurotubs, rely on tire scrub to dissipate spring energy but that type of gear has serious shortcomings; alight one wheel first and get pole vaulted sideways or become a jack rabbit on an iced over runway.

Long stroke landing gear with vertical wheel travel and hydraulic damping is hard to beat. And that doesn’t require the sort of monkey motion incorporated in “"G-force"” landing gear.

#### DANNAL

##### Newbie
I had the wide rough terrain g force on mine with the mechanism to hold it up. I never once let it down, 5000' paved runway. I often thought I would use it in an emergency but probably should have gotten familiar with how it landed extended. Dan

#### Heron

##### Platinum Member
You may never need it, but its there when time comes!
Practice and have it ready to use!

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