Engineering Change Notice for Max Flap Speeds: 4-Place Model B, Companion, and Patrol

[Nev]

Flap force and deployment angles on NSB

I measured the flap deflection on my own aircraft yesterday and found it to be 44° without air loads, and only 30° with air loads (at F4 at 55 KCAS and close to MLW). I guess I could adjust for greater deployment angle. Obviously this will vary significantly between our aircraft depending on how the flap torsion bar is set up, and the cable tension or pre-loading.

F4 required 85lbs force to deploy at 55 KCAS. (It's a very difficult thing to measure so I'll call it approximate). The force required at F4 was higher than at other flap settings (followed closely by F2).

A few additional thoughts:

Increasing the deployment angle will give an increase in load through the system. I assume this would be linear but someone else might clarify this.

In the event of a flap overspeed the air loads would be exponentially greater.

Selecting flap just a few knots below the limit speed makes a large reduction (also exponential) in the force required to pull the flap lever.

[Bcone1381]

His advice was very clear : limit the speed to 65mph at 40° flap (with airloads).

An editorial, as opposed to a fact follows.

If built according to plans with no airloads, the first notch of flaps in flgiht does about nothing but remove system slack and elaticity.

Post #6 shows a data point....Inventive builders have been tightening up the flap cables so that cable slop and system elasticity is removed in flight...or maybe better said the flaps are up but loaded ready to deploy with no slack in the system. This gets ten degrees more flap travel at notch 4 and the loaded deflection matched the plans....lets say its 50. This safety notice reigns that practice in.

[Nev]

Brooks, on my plans it actually states 50° 75mph (65 kts) - which was very obviously a mistake and hence why I phoned Bob back then to get clarification on ALL the flap speeds. The stresses in the system would be very high if someone operated to 65 kts and had set the flaps to 50° with airloads. But I did wonder if this was why some builders had extended the flap handles.

[Nev]

So notch 4 gives you about 50 degrees on the ramp, and 40 with air loads? The downside to the notice comes to he who interprets it wrong.

Yep that's right. But because the original 50° (without airloads) was being misinterpreted occasionally, Bob has now made it 40° (with airloads).

If someone interprets it wrong, the overall loads on the flap system will now be lower rather than higher.

[Bcone1381]

Why do you think it is cable stretch?

I called it cable stretch, system elasticity is a better description. You already said its torque tube elasticity. Lets go with that.

My experience is that my cables were all pre-stretched during the cable testing. This also elongated the cable eyes - in other words if you're experiencing cable stretch it might be worth confirming that the cables were in fact tested prior to installation. I'd be very surprised if they stretched further after installation, and if they do I'll simply check and adjust as required periodically. But as far as Bobs engineering notice goes, he didn't mention cable stretch.

His advice was very clear : limit the speed to 65mph at 40° flap (with airloads). On my aircraft that corresponds very closely to the 4th notch on the B model when built as per the plans, and I'll fine tune it from there. I can't see any downside to conforming with his notice, only upside to reducing stress on the entire flap system.

His advice was very clear! Great! So notch 4 gives you about 50 degrees on the ramp, and 40 with air loads? The downside to the notice comes to he who interprets it wrong. Post 5 shows a reasonable interpretation prior to clarification that changes flap setting 10 degrees.

The notice needs clarification for future builders and those who don't visit this thread.

[Nev]

My experience is that my cables were all pre-stretched during the cable testing. This also elongated the cable eyes - in other words if you're experiencing cable stretch it might be worth confirming that the cables were in fact tested prior to installation. I'd be very surprised if they stretched further after installation, and if they do I'll simply check and adjust as required periodically. But as far as Bobs engineering notice goes, he didn't mention cable stretch.

His advice was very clear : limit the speed to 65mph at 40° flap (with airloads). On my aircraft that corresponds very closely to the 4th notch on the B model when built as per the plans, and I'll fine tune it from there. I can't see any downside to conforming with his notice, only upside to reducing stress on the entire flap system.

[kestrel]

"Notch four about 40 to 50 degrees airload dependent due to cable stretch."

Why do you think it is cable stretch?

[Bcone1381]

He did clarify that 40° was his intended maximum deflection with airloads.

Bottom Line:

1) I like to pull notches of flaps. I don't set degrees of flaps in a Bearhawk.
2) Why attach Max Flap speed limitations to an angle that is fluid, and unknown?
3) Why not attach Max Flap Speeds to correspond with Notch One thru Four?
4) I build according to plans and just let the flap angle find its home when deployed.

___________________
5) Non-Optional Engineering Change means mandatory to some builders and future owners.
6) It is reasonable for it to be interpreted as a design change but its not a design change.
7) Bob's clarified data point is authoritative and visible here in this thread but next month it will lack authority and visiblity if it stays here. I'm not aware of where else its documented.
8) So, Leaving out Bob's clarification from the authoritative notice may cause confusion to future builders and operators.

Conclusion;
I recommend Bob document the Plans to reflect static flap angles, flap speeds according to notches, and change the notice to say

1) "Build IAW the plans"
2) Define Max Flap Speeds using "Notches".
3) Say "Degrees of flap deployment reflect a range of angels that are possible under varying conditions." For example "Notch four about 40 to 50 degrees airload dependent due to cable stretch."

Then Plans have good data for promotion and good data in the safety notices for operations.

[Nev]

Do you know if Bob is intending to add to or amend the Engineering Change Notice? As it stands, our log entry will have to refer to your verbal clarification that you have posted above, which does seem a bit 'back door'.

He didn't mention that so I don't think so. But essentially the engineering notice states the flap deflection angles, and the corresponding limit speeds - so I think it's contained in the notice already. He did clarify that 40° was his intended maximum deflection with airloads.

[Richard E]

Nev,
I was just emailing Bob/Mark but you have beaten me to it. Do you know if Bob is intending to add to or amend the Engineering Change Notice? As it stands, our log entry will have to refer to your verbal clarification that you have posted above, which does seem a bit 'back door'.

[Bissetg]

Perhaps this could explain why some builders have experienced high load on the flap handle when deploying Flap stage 4?

[Nev]

Clarification from Bob

I spoke with Bob today and got clarification on his recent engineering notice.

Bob said that originally the maximum full flap deflection was set at 50° assuming no air loads.

His intention with the engineering change is to promulgate full flap as being 40° with air loads, at the prescribed maximum speeds.

I'll check mine inflight with a marker pen and tape on the rear window, then measure the deflection on the ground.

[Battson]

It doesn't really vary that much with airspeed within the Vf to Vs speed range, the main point is - what you see in your hangar and what you see in flight are rather different, for the reasons explained below. It seems to be widely understood.

[Battson]

Bob's notice describes the maximum flap speeds (Vf) and does not mention anything about maximum flap angle.

If you measure the angles on your flaps in flight, you will probably find they closely match the new angles Bob has described, assuming your system conforms to his design and hasn't been adjusted to achieve maximum flap deployment.

[Nev]

I think what Richard is pointing out is that the flap angles have now changed too. Instead of 50° at full flap it is now only 40°. So originally it was clarification on the flap limit speed that affected some of us, but according to the original post above it is now a reduction of flap deployment angles which would affect all of us.