Cessna 140 Flaps, Newbie Question

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linoair
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Cessna 140 Flaps, Newbie Question

Post by linoair »

Hello All,

we recently bought a beautifully restored Cessna 140. After having received some basic taildragger instruction we are exploring all the little details of our new little airplane.

We have noticed, that the flaps extend fully on the ground (about 40 degrees) but under airloads in flight, they are pushed in about half the way. During landing rollout the flaps slowly deploy fully as airloads decrease.

If you push the extended flaps toward the retracted position by hand, you can feel a spring force pushing against you until flaps reach about half of the fully extended position.

Is this normal behaviour?

The attachement of the springs in the fuselage, as well as the cable from the flap lever seem to be correctly installed to the levers of the connecting rod in the fuselage.

Maybe some experienced Cessna 140 pilots can give us some advice?
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Re: Cessna 140 Flaps, Newbie Question

Post by 6643 »

I'm not sure I follow what you are saying, but, the springs should pull the flaps up and the cable and flap lever pull them down. It is not unusual to have some slop in the mechanism, allowing you to lift them somewhat when deployed, but, when the cable pulls them down and you push up to take the slop out, you should get the full 40 degrees. I have never heard anyone say they lower further as you slow down, but, I, for one, never looked.

When the flaps are up you can pull them down by hand against the springs.

It almost sounds like your springs may be pulling them down, but, then, what is pulling them up when you raise them?
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Re: Cessna 140 Flaps, Ne

Post by a64pilot »

Maule had a spring cartridge in their flap control cable allowing their flap speed to be higher. You would pull full flaps but not get the full 48 degrees until you slowed down. Continual flap deployment at high speed cracked the aft spar on a Maule, the spring cartridge stopped the cracking. With the cartridge the spring won’t allow full 48 degrees of flap until speeds less than 55 MPH.
I do not believe a 140 has such a device, unless someone added one.
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Re: Cessna 140 Flaps, Newbie Question

Post by 6643 »

6643 wrote: Sun Aug 22, 2021 7:10 am I have never heard anyone say they lower further as you slow down, but, I, for one, never looked.
Come to think of it, if you're looking at your flaps during roll out, you probably shouldn't be...
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Re: Cessna 140 Flaps, Newbie Question

Post by 6898 »

Must be something in the rigging or some give under a load . They should not flex at full flaps.
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Re: Cessna 140 Flaps, Newbie Question

Post by 6643 »

Any slop in the system, especially enlarged slots in the flap fittings, manifests itself by allowing the flaps to "droop" when there is no air load. You can push the flaps up to take this slop out while on the ground. In the air it doesn't matter, as long as it's the same on both sides. Rig the flaps for maximum deflection with the flaps held up manually against the mechanism.
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Re: Cessna 140 Flaps, Newbie Question

Post by 8475 »

I'm addressing this behavior on my 140 right now. I spent a full Saturday in the hangar trying to get both flaps to seat hard against the adjuster in the fully retracted position. Couldn't get them both to seat. One of them was always sprung off of the adjuster slightly.

Turns out the problem was that the slots in the flap drive brackets (there's a bracket on each flap, a little spigot about the size of your thumb) were worn out. This allows backlash in the mechanism that gets taken up when one of the flaps seats. The kinematics are counterintuitive, but have to do with the return spring imposing both a torque and a force to the spigots. When one of the flaps seats, this reacts the torque, leaving the other flap to react the force, which results in a downward deflection. (Or something like that.)

The solution is to minimize backlash in the coupling mechanism. You do that by properly sizing the slot for a close fit to the diameter of the coupling bolt. My spigots were worn out. Univair has replacements. I'll be riveting them into place this weekend.
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Re: Cessna 140 Flaps, Newbie Question

Post by 6643 »

If the wear is even on both sides it's less problematic, as long as it doesn't get to the point where it compromises the strength of the part.
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Re: Cessna 140 Flaps, Newbie Question

Post by 8475 »

6643 wrote: Sun Sep 05, 2021 12:53 pm If the wear is even on both sides it's less problematic, as long as it doesn't get to the point where it compromises the strength of the part.
John, I thought that too, but learned differently. ANY backlash will lead to one drooping flap. One of the flaps will be on its stop and the other will be drooping. Equal backlash left-to-right doesn't affect this. As I mentioned, it's counterintuitive.

As the flaps are being retracted, both flaps are being pulled upward by the mechanism. As soon as one of the flaps contacts its adjuster, the backlash in the other flap will be taken up in the opposite direction, causing it to droop. The bigger the backlash, the bigger the droop.

The only real solution is to minimize the clearance between the slot edges and the shank of the through-bolt.
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Re: Cessna 140 Flaps, Newbie Question

Post by 6643 »

I don't want to start an argument, but my plane feels otherwise. Both flaps are on the stops when retracted, and both have noticeable play when deployed. The play is in the width of the slots.
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