Retractable Undercarriage
 

Chaps,

Just looking for a bit of insight from those of us that fly smaller aircraft with retractable undercarriage, such as the A109.

Do the nose wheels simply castor? If so is the castoring degree of freedom generally lockable for start-up and shut down, or the machines rely on the rear wheel brakes to prevent fuselage yaw on the ground during start-up?

Many thanks,
CRAN

Yes, generally castoring, self centering when no weight on wheels, and lockable for start/stop and rolling T/O & landings.

Many thanks.

CRAN

AS365 is self centring and lockable for running landings / take offs but isn't locked for start up/shutdown.

The self centring uses two cams which engage as the oleo extends after take off - if the nosewheel isn't centred, the gear won't come up due to a microswitch.

The 109 has strict limits on nosewheel angle, towing can be tricky in a tight hangar when you hit the limits. But the S76 has a fully castoring nosewheel, and it is possible to have it 180 out before retraction - so pilots need to be careful to taxy forward a bit to ensure it is pointing the right way before retraction.

Pumas in the early days had cases when the nosewheel would stick at 90 degrees before it retracted. You soon knew about it when a towing lug punched a hole in the nosewheel bay roof.

Getting to the OP, the wheel brakes should also be set to hold the helicopter against torque during start and stop even with the nose wheel lock engaged. The static friction of the nosewheel tyre is not always enough to hold against a spirited application of torque.

:-) I love your choice of words there John

Reminded me of a time in Bombay when one of our now departed pilots used a slightly less diplomatic form when the other pilot was starting the 61 sans rotor brake and she started rockin an rollin..... "Pour the Pi$$ to her ole pal" LOL

Let me guess Rocky R.?

Thanks for all the posts guys; really useful as always.

Other than the obvious reduction in roll-over stability (tripod versus quad), the extra weight and the lack of ability to land on soft or unprepared surfaces are there any other undesirable characteristics of retractable wheels?

Thanks in advance?
CRAN

It sounds like you are aware of landing on soft surfaces, where the wheels sink down and the hydraulic lines to the brakes get torn off.

Worse still the as365 which sunk one side to the stabaliser... Pilot carried on and later in flight this tore off causing the aircraft to tear apart mid air

http://9m-igb.mot.gov.my/download/Ai...dentReport.pdf

Like when doing slam acceleration maintenance checks John. ;)

"are there any other undesirable characteristics of retractable wheels"

Well there's the cost of purchase and maintenance.

Retractable wheels are going to work normally off the hydraulic system (although may be electric). During landings at ad-hoc sites especially, they are the most vulnerable part of the system to damage, and therefore can compromise the rest of your hydraulic system unless fail-safe protections are built in.
The possibility of the undercarriage getting stuck (or partially deployed) is obviously another undesirable characteristic)

The most obvious may be that people forget to lower them for landing!

Maintenance costs
Increased complexity of hydraulic system.

Good guess......but I suppose the ole pal part gave it way ;-)

Retractable undercarriages are fitted to helicopters for the same reason that birds fold their legs; it reduces drag. As a example when the S76A first came out there were occasions were there would be and undemanded undercarriage lowering. When it happened to me the speed dropped from 145 to 135 knots immediately. The 330 and the 332, when required to operate with the gear down would also suffer from speed and range penalties.

Skids are a waste of time when moving large passenger loads as they found out with the 214ST. There was pandemonium when it was positioning on the apron and it was soon fitted with a wheeled undercarriage, albeit, not retractable.

The problems arising with retractable undercarriages do not manifest themselves very often. Certainly not enough to worry anybody.

There was an occasion on the snowy slopes of Norway where a Gazelle fitted with skids landed an shut down. The pilot had just left the aircraft when it slid down the slope and impaled itself on a snow bank. The calls went out and the pilot's superior came out, landed on, shut down and proceeded to lambast the first pilot. At this the second Gazelle slid down the slope and mated with the first one.

We did not have the same trouble with the Pumas; slam it in hard, the wheel would break through the crust and it would rest on its belly. An the end of the day when the weather was closing in and the squaddies were retreating down the hill to be picked up the German 205s had to give up.

We, however, could nail our Pumas to the side of the hill.

What was the result - quintuplets?Never heard of it happening, what was the cause? Electrical presumably, gear handle, relay etc? Ex many hours on A.