Following the other thread about how much wing thickness is needed to conceal a retractable gear:
Not a lot.
Agreed, not a lot, neat.
Some may wonder why one main leg lags behind the other. This is the nature of hydraulics or fluid mechanics. The fluid will follow the path of least resistance. If one side is a tick more resistive then the other, then the other will move first and once moving is likely to continue to the 'stops'. Then the pressure will exert on the other side.
Indeed. One could make them go together but it would complicate things. What would be the simplest way of doing it? A 737 (etc) makes them move all together. Does it have 3 gear pumps?
The only issue with the current system is that when one leg moves before the other you get a significant yaw for the brief moment.
The other thing to watch out for is any start/stop behaviour i.e. judder. That could be due to poor lubing (which is a common problem in GA maintenance) or it could indicate the hydraulic pump on its way out (it can be overhauled, depending on which type you have).
The pump capability is 4000psi, apparently. The maximum available pressure is 1600psi, set by a little pressure switch which cuts the pump out at 1600psi which is reached as soon as the "up" gear stops (rubber bungs) are reached and squashed a bit. It's a very simple system which almost never gives trouble - so long as even the most basic maintenance is done.
Some may wonder why one main leg lags behind the other. This is the nature of hydraulics or fluid mechanics. The fluid will follow the path of least resistance. If one side is a tick more resistive then the other, then the other will move first and once moving is likely to continue to the 'stops'. Then the pressure will exert on the other side.
That is only true for short legs like in the TB20. In my 182 RG, the legs are very long and thus there is a much greater moment on the pivots. As the gear moves, the moment increases (as the vector becomes more vertical) and thus the resistance. As a result, the two legs move almost synchronously even though the system is one single fluid circuit as in the TB20. However, the nose gear shows the fluid effect, it moves completely into its position before the heavy main legs even start moving.
Does the TB20 have uplocks for the legs or are they held in position by hydraulic pressure? In the Cessna RG system, they're held by hydraulic pressure. Depending on how tight the system is, the hydraulic pump will engage every 20-30 minutes to keep the pressure above 1600psi. Without pressure, the legs will slowly fall down but in order to lock them into position, you need hydraulic pressure again because they have to turn another 45 degrees after passing the "gravity" position. There are people that have supposedly managed to pull them into position in flight using the tow bar. If it wasn't dangerous, I would give that a try one day just to find out.
That is only true for short legs like in the TB20. In my 182 RG, the legs are very long and thus there is a much greater moment on the pivots.
Yes, I had construed, with certain designs the gear (primarily the mains) might move more in synchronous on a single circuit system. On the 182RG the mains, as you say are 'long', and fold back.
Two things here:-
In flight the airflow will assist in a simultaneous moment (at least initially)
and:-
as you point out, from the point of view of the hydraulic cylinder, the gear gets 'heavier' as it retracts (leverage). Thus it is semi self leveling. If one side gets ahead of the other the hydraulic fluid will follow the path of least resistance and go to work on the other etc. etc. If the nose wheel mechanism offers the least resistance within the single hydraulic circuit system, then indeed that will raise first.
The only issue with the current system is that when one leg moves before the other you get a significant yaw for the brief moment.
I had considered this to be a side effect of non-simultaneous retracting, but not really 'felt' it on the Arrow.
I'm told to 'dab' the breaks before retraction (stops the wheels spinning in the bays or still spinning when the tires hit some rubber pads).
I don't seem to be able to do this 'dab' without a small L&R yaw input, which may be hiding the asymmetric yaw above (or is it the other way round!!!)
Who does this 'dab' on retraction and pros and cons?
I don't like doing it as it seems to produce a small 'upset' at a critical stage of flight.
Just occurred to me, instead of a 'dab' on the foot breaks, why not a brief pull on the hand break before 'gear-up'. On the Arrow the pilots right hand goes in that direction (towards the hand-break) for the gear lever.
Care would have to be taken NOT to press the lock button on the 'fly-off' habd-break system.
Who does this 'dab' on retraction and pros and cons?
I do it. Can't see any cons, has become second nature.
One thing you do need to be aware of, is the 2-4 knt reduction in airspeed during gear transition on a Cessna. The mains rotate to briefly stand 90deg to the airflow and thus - for a few moments - significantly increase drag. Not an issue in normal ops, but getting out of a short hot-and-high field with obstructions at the end, it can make a difference.
Who does this 'dab' on retraction and pros and cons?
I do it. Very gently as the tyre might slide on the rim.
I do it. Very gently as the tyre might slide on the rim.
I always do it. For several reasons. And never gently, because one of the reasons is that the wheels must shed water in form of rain, snow or slush before retracting in order to prevent them from freezing into the wheel wells. Applying the brakes gently will not achieve this goal! It is also important to do it before retracting the wheels (or in the early stages of retraction!) otherwise the moisture ends up in the wells where we don't want it. (BTW: In 25 years of flying retractables and always pressing the brakes hard before retraction - as I was taught and keep teaching myself - not one tyre has slid on it's rim!)
The next reason for stopping the wheels before retraction is the very limited clearance between the tyres (especially new unworn ones) and the wheel wells on some types. The centrifugal forces of a spinning wheel will slightly increase it's diameter so that it can come in touch with the wheel well (you can actually hear it doing "screech - bang" right under your bum when you forget to stop the spinning!). On some aircraft I have flown (e.g. the Cessna 421) this is described in the AFM.
And the third reason lies in the fact that spinning wheels - depending on the direction of retraction - will generate considerable precession forces (higher for larger and heavier wheels and fast spinning wheels - therefore maybe not really relevant for light types). In order to spare the retraction mechanism those forces (and the whole aeroplane in case of non-symmetrical retraction) a dab on the brakes is essential.
Some people also forget to dab/test the brakes after extending the gear. This is also essential as the brake lines could have been severed during retraction or extension. Also, retracting the wheels with hot brakes can lead to evaporation of the brake fluid. Finding out after landing that there is no brake pressure can be a little embarrassing.
