flap position

[GeorgH]

Hi folks,
I was playing around with new camera mounting positions/ Back home when I checked the footage I was surprised to see how much the flaps move without doing anything. If you watch the video, the flaps are set for take off to the second notch. Watch the distance from the tip of the right flap to the glare shield and see how far the flaps are being pushed up with increasing speed. I have a similar video during a landing when the flaps are on the third notch. After touch down with decreasing speed, the flaps come down a huge amount.
I guess thats normal since you can't actually bring more tension into the cables.
Georg

[S Lathrop]

Interesting. There is a lot of flex going on somewhere. Maybe it is the cumulative flex of the entire system. It is something that one might explore.

As best I can tell, it appears that the flap rises evenly, making me think it is in the actuating mechanism. This might be just the torque tube flexing under loads, in which case all is normal. Might be fun to calculate the loads on that tube.

[kestrel]

My 4 place has electric flaps with no cables. The entire system is overhead and there should be very little flex up to the long torque tubes. I believe it is the torque tubes in the wing that are primarily responsible.


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Bearhawk

[Battson]

In the Maule series, they have springs in the flap cable system, so if the pilot lowers the flaps at too high an airspeed, they are retracted by the aerodynamic force to prevent damage.

If you're flying much slower than the limits, they stay pretty well in the same place. If you get near Vfe for each notch then they really start to fold up. But yes, when the fan stops, they sit much lower still...

I figure 52 degrees of flap on the ground is really more like 40 or 45 when you're flying at 40kts.

[jaredyates]

I have noticed the same effects Georg, particularly when editing video. I don't refer to my flap settings by degrees just for this reason.

[GeorgH]

I don't do either, Jared, I was just surprised how the setting changes. If you try to push the flaps back that far by hand, you pretty much fail! ;-)

[Helidesigner]

I noticed the same thing in my plane. I sited on some window markings in flight. On the third notch of flaps I lost 10 degrees of flap travel at about 60 kias. I investigated what was going on by conducting strain measurements on the 1/8 flap cables in my basement. I loaded them up with known weights and measured the stretch. I estimated how much flap handle pull I needed to get to the third notch at 60 kias which gave me an estimated load on the cables. I found that the cables are responsible for about ~ 90% of the deflection. Two thirds is on the single run and one third is on the cable pair running up to the flaps. This makes sense since the single cable takes all the load and the pair splits it. A the next annual I am going to look at replacing the 1/8 single cable with a 3/16 cable which has 2.25 more cross sectional area. The math says that will give me 55% less cable stretch which will be about 40% less flap deflection. The net result should be a knot or two lower speed. Not a lot but I hate losing performance.

[jaredyates]

Russ Erb's solution was to shorten the cables so that the flaps would be deflected down on the ground, but flush in flight, essentially using airflow to tension the cables. I suppose this incurs a drag penalty, though it is probably minor.

[GeorgH]

Thanks for the tip Jared!

[Mark Goldberg]

I asked Bob a few minutes ago about any problems changing out the 1/8" cable to 3/16". First, he thought the calculation that 90% of the cable stretching was in the single run was perhaps not correct. He thought that might be too high a number. I won't get into the reasons he gave me for feeling that way.

If you were to substitute 3/16" cable - you would most likely need to change to a bigger diameter pulley. Also, the designed in failure point now is in the single run. If you went up to 3/16" cable for the single run it would probably shift the failure point to one side or the other.

Right now, the 1/8" cable does help a little by stretching if you over speed your airspeed with flaps down. With the 3/16" cable it would not help that.

Just some things to take into consideration. Mark

[Battson]

I asked Bob a few minutes ago about any problems changing out the 1/8" cable to 3/16". First, he thought the calculation that 90% of the cable stretching was in the single run was perhaps not correct. He thought that might be too high a number. I won't get into the reasons he gave me for feeling that way.

Mark - I think he meant that 90% of the deflection overall was due to cable stretch, and 10% due to the other linkages / torque tubes etc.
Of that 90%, 2/3rds was attributed to the single run.

In other words 60% of the total deflection was due to the single run, 30% due to the side cables, and 10% due to the torque tubes and other places.

I find that's one of the hard things talking about building a plane - so many parts and tasks to discuss, it's hard to avoid confusion at times.

[jaredyates]

If you went up to 3/16" cable for the single run it would probably shift the failure point to one side or the other.

This is a really good point. You definitely want the flaps to operate and fail in pairs! Having both flaps fail up or having both flaps get stuck down is an inconvenience. Having one flap up and one flap down is a life-threatening emergency.

I'm using the as-designed setup with no pre-loaded tension, and it has been working fine for me. In theory flaps are intended to increase both lift and drag, but I find that in this airplane the drag element is more pronounced than the lift element. The first notch mostly takes up slack, with an inch or two of deflection in flight. The second notch is what I use for a normal approach and landing. The third notch is what I use for a steep approach when I'm concerned about runway length. If that setting doesn't provide enough drag because I'm high on the final, then I can forward slip and have plenty of drag available. The last notch I have not yet used in flight, in part because of the force required to select that notch, and in part because I haven't needed any more than what the third notch can provide.

This is coming from someone with less experience in the airplane than many of the other contributors here, but the flap flex issue doesn't seem to me like one that is worth fixing, especially if the fix introduces any increase in risk.

[Battson]

This is a really good point. You definitely want the flaps to operate and fail in pairs! Having both flaps fail up or having both flaps get stuck down is an inconvenience. Having one flap up and one flap down is a life-threatening emergency.

My father in law had a flap failure in a Cessna 172, one flap extended to 40 degrees and the other never extended. He noticed that he needed more aileron than usual and then noticed one flap wasn't down, but otherwise he continued the approach as normal and landed with one flap.

I was surprised by that, but it's hard to argue with that kind of experience.

[kestrel]

My father in law had a flap failure in a Cessna 172, one flap extended to 40 degrees and the other never extended. He noticed that he needed more aileron than usual and then noticed one flap wasn't down, but otherwise he continued the approach as normal and landed with one flap.

I was surprised by that, but it's hard to argue with that kind of experience.

I had a C172 flap cable fail while at 40 degrees. There was a loud bang, combined with sudden yaw and roll. Air speed was high enough for good control. (No, not above the white arc). After turning my head to confirm the problem, retracted the other flap and came around for an other approach. The crimp from Cessna had failed, not the cable itself.


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Bearhawk

[Helidesigner]

Battson has the interpretation correct. I do not know how well the 3/16 cable will go around the pulley. At annual with the floor removed I should be able to try things out and see what works. Right now I have the flap trailing edges aligned with the ailerons with the flap handle at the lowest notch. In flight the cables stretch and the flaps ride up into a reflex position. At low to mid gross weight I found between 2 and 3 kts of additional speed with the flaps reflexed. I made a new intermediate notch where the flaps are deflected about 5 degrees on the ground. On takeoff they ride up and align with the ailerons. I find this better for climb out and medium flight speeds. Regarding cable strength, the 1/8 cables are PLENTY strong to take the flap loads. If a cable goes it will be due to a poor crimp. They have not popped yet so I do not think they ever will.