Topic: Flyer Chippy thread

[alvin sfc]

Look sharp chaps,bit of a flap on !  

[Dick Gower]

Let me clear up some things that have been said here...

First off a slip with full flap down to the roundout in a Chippie is magic. But then again I have nearly 400 hours in them. And I bet that I'm a "youngin'" here hour-wise.

Secondly if the guy who does maintenance on your airplane says that in a flap cable failure one flap will stay down while the other pops up I would almost be looking for another engineer. This couldn't be farther from the truth. First off the cables are pretty stout. Not as huge as the ones going to our rudders (!) but still adequate to get the job done. If one cable breaks both flaps are spring loaded and will pop up. It was designed to work this way. No worries there mate!

And just how do I know this? Well... One of the first things to break on my airplane when I bought her back in 1999 was the flap cable. I was on short final to my home airport, pulled full flap (yes I was below 70) and "clunk!" - no flaps. The flap handle was limp and both flaps were up. In fact I flew around for the next couple of weeks flapless while we sourced another cable.

So I hope this sheds some light on the subject.

Sorry, Chipmunkbob, that's not correct. We had a single flap failure on or chippy and got assymetric flap. And the cables are quite small diameter - about the same as bicyle brake cables. I think the cable that broke on yours was the one between the flap lever and the rotating drum from what you say. I hope the following explains.

The flaps are spring-biased to the 'up' position and each is attched by an actuating rod to a 'wheel', which when rotated acts like a crank to pull the flap down against this spring pressure. There is one of these in each wing. These 'crank wheels' are each rotated by a thin cable (one from each wheel) which is attched to the wheel's rim, so when the cable is pulled, the crank wheel rotates about half a turn, and the attached rod pushes the flap down. When the pull on the cable is released, the springs (and air loads, if airbourne) retract the flaps, turning the crank wheels, and reeling in the cable around the wheel's rim. The crank wheel actually comes to rest with the operating rod 'over centre', to lock the flap up. That's why you can stand on the walkway on the retracted flap and the flap doesn't go down under your weight.

These two thin flap actuating cables go to a 'drum' in the fuselage, around which they are wrapped. Roate the drum, and it pulls reels-in the cables, which rotae the crank wheels, thus extending the flaps. The drum is rotated by the action of the cockpit flap lever.

So you pull the lever, the drum is rotated, the cables wound in, which rotates the two 'carnk wheels', one in each wing. This pushes the actuating rod from its over-centre resting position, and on further to lower the flap, eithe fully or to half position, depending on the lever position.

The extended flaps are held extended by the cables. If either of the two thin cables to the 'crank wheels' snaps, you will get assymetric flap. If the cable between the lever and the drum snaps, both flaps will retract.

Hope this helps.

Vince Chadwick.

Can I add a bit here?

It looks a bit like a drum but is actually two AS013 pulleys mounted close together under the rear seat.  The cables from each flap (AS.1406/20.25 on LH AS1406/23 for RH) combine here into one cable (AS1387/16) and go via a torturous route over two more pulleys to the rear  flap lever.  This is the one that wears most and if it breaks both flaps retract (usually on the flare when everybody is watching!) .  If either the RH or LH cables go there will be asymmetric flap.

The LH and RH each join to another cable (C1-CF333 and 335) via a turnbuckle in the wing root before arriving at the actuating cam/lever.  These go 90 degrees around a pulley and this is where they wear.

There are two more critical cables that connect the operating cam in each wing root to the spring.  If this cable or the spring fails the flap concerned will go down over centre and will not retract with air loads.  There  is a mod. incorporating a check cable to prevent this and also a DH Australian mod (-Aust. No 3) which duplicates the springs.

[Vince Chadwick]

Thanks Dick. Didn't know about the possibility of a flap going down 'over centre' so it won't retract.

Cheers

Vince

[Dick Gower]

At the risk of digging up a three year old topic (should get on this site more often); whether one or both of the flaps retract in the case of the cable break depends on which cable.
There are three actuating cables and two return cables.  Ignoring the latter two, if the actuator cable(AS.1387/16") from the flap lever breaks, both flaps come up.  This cable bificates into two cables, one to each flap, from under the rear seat.  If one of these cables breaks one flap will fully retract but the other will stay down making for an interesting moment or two.

The cable from the flap lever winds a torturous path over several closely spaced pulleys and wears faster than the other two and has the highest loading and is the most likely to break.

The aero club in Melbourne operated twelve Chippys and side slipping with flaps was forbidden.  It was said that something dreadful would happen when the flaps blanketed the airflow over the rudder so naturally, being twenty something and therefore indestructable, I tried it.  Huge amounts of nothing happened!

Personally, I think the concern may be the stressing of flap centre hinge bracket with the sideways loading.

[Hugh Shields]

Perhaps a refreshing point of view on this topic. The Canadian Chipmunk civil and RCAF pilot operating handbooks are silent with respect to slipping with flaps. I have done it at times and suspect that loads on the system are moderate as airspeed is low.  However based on the fractured opinion on this matter I tend to limit the practice of slipping with flaps.

When talking to DHC Chipmunk prototype test pilots George Neal and Russ Bannock about this recently they said to their knowledge there was no restriction however George smiled and possed the question "why would anyone need to".  This to suggest if you need to slip with flaps then you've botched the approach to begin with!!

Cheers

H. Shields
Ontario, Canada