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

[gregc]

Well we are the black sheep of the family......

[jaredyates]

The language for this update has been revised slightly to clarify that the deflections are based on air loads. I have modified the original post with the updated language and also updated the entries at http://bearhawksafety.com.

[jaredyates]

Richard, I think the best course of action may be to contact Bob directly so that you can tell him what you need and have him provide it. If the international logistics are a challenge, I'm happy to help, just let me know. In the USA we don't have the regulatory concerns that you do there.

[Richard E]

My understanding is that this clarification of the flap speeds came about because of a Companion builder seeing only three flap speeds written on his plans while the actual aluminum flap guide had four positions. Bob had copied the speeds from another model on the plans and it did not correspond to the actual four positions of the flaps. This was a plans correction for that reason. Mark

The same anomaly appears to be on Bearhawk 4 Place model A plans as a well, but for some unknown reason the Notice does not apply to this model ?

[Richard E]

There has been a lot of talk since I raised my original queries on the Engineering Change Notice, but they have not been resolved.

Bob issued a Notice which NZCAA consider to be an Air Directive. The FAA are probably the same. It contains a significant change in the flap arrangement which is a 'structural change to the design'. NZCAA require a reasoning for the change and then want to see any affected aircraft satisfy the requirement.

It is great that Nev has spoken with Bob and has a better understanding. However, if:

• the max flap angle is now 40degrees,
• the max speeds are reduced by 10mph and
• the terminology of 'air-loads' and 'non air-loads' is introduced,

this needs to be clearly in writing from Bob Barrows as the designer. Not just a bit of hearsay from a phone call...sorry Nev. The Notice is an important legal document which affects many owners.

Mark Goldberg indicates that the Notice was prepared to rectify a fault on the first page of plans for a Companion in the building stage. The Notice affects all Model B, Patrol and Companions and is non-optional. This gives any completed build a problem.

As Bob's representative I hope that Mark Goldberg can communicate this uncertainty to Bob so that an amended, updated or additional Notice can be issued to enable us to comply.

[Battson]

Thanks - that is interesting - it sounds like up to 10 degrees of the strain in the Bearhawk flap system could be coming from the torque tubes alone, but of course it's hard to comment definitively without knowing your system.

[kestrel]

To better nail down my own data that I've presented, I went out to the plane and rechecked my flap deflection marks. First to check their accuracy because I have a better tool than the last time i did it, and to remind myself what the numbers were. I have makes on the root of the flap that indicate on the bottom surface of the wing. This works on the original 4 place because the flap doesn't extend all the way to the fuselage.

My electric flap install with no cables or return springs has a full flap deployment of 45 degrees on the ground. My next mark on the flap is 35 degrees and it is visible just before stall. So, I am losing 10 degrees at stall speed. Somewhat more than that at any speed higher than stall.

This is how it was setup when I bought it almost 10 years ago. I haven't made any changes other than the gauge marks I put on the flap.

[Battson]

Flap force and deployment angles

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

In case anyone does overspeed their flaps, e.g. reach cruise speed at 2 notches deployed for instance, there is a follow-up inspection which I would recommend.

Let's be honest - this kind of thing happens to lots of people (forgetting to retract flaps after takeoff).

Overspeeding the flaps will almost certainly result in extremely high stresses in the main drive cable from the flap lever connection to the cable splitter tang. This can cause the cable to break strands where the cable bends, such as the pulleys with a 90 degree corner, and the cable eyes at each end where the cable bends around the eye.

The best way to check for cable defects, in my experience, is rubbing the cable down with a lightly coloured cotton rag. The cotton will snag on any kind of strand damage and highlight the area, it's also very easy to feel without hurting your skin. Visual inspection alone is certainly not effective.

If I had to guess, achieving cruise speed at full flap could potentially break that main drive cable entirely, probably at a cable eye.

[Battson]

Bob has issued an Engineering Change Notice (this is the non-optional type of update) about the maximum flap speeds for the Bearhawk 4-Place Model B, Bearhawk Patrol, and Bearhawk Companion. The speeds are as follows:
10 DEGREES DOWN -100 MPH
20 DEGREES DOWN - 85 MPH
30 DEGREES DOWN - 75 MPH
40 DEGREES DOWN - 65 MPH

To be fair, and to Richard E's point, unless Bob actually publishes a document - this notice will probably not be considered official by the local aviation authorities. Certainly in NZ.

For all they know, Jared has invented this (Haha)

[Battson]

It's unlikely to be a linear increase in load / force for two reasons. 1, air is notoriously non-linear, and 2, additional deflection of the flaps does not increase the flap area exposed to the airflow in a linear way.

[Battson]

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.

Just a clarification here, in case anyone was interested.

There are two kinds of "stretch" in play here. The pre-stretch in the cable is to prevent the "stretch" (creep) as the cables wear in. The other "stretch" (more correctly known as strain) is what happens to all metal parts when it experiences a force. Metal parts will always stretch (strain) and spring back to their original length when they are loaded up with a force, unless the force exceeds their elastic limit. This is what causes the flap system issue we are discussing, when we see 50 degrees on the ground and 40 degrees in the air.

So all metallic materials "stretch" (strain) when force is applied, and they do so directly proportionally to the force they experience. Double the force, double the stretch (strain). Thus all parts of the flap system will "stretch" a bit when a force is applied, and then spring back when the force is removed, unless the force is too large and you bend something. In fact, the wings flex a little more when you lower flaps, because they are all metal too.

Longer metal parts will experience more strain (stretching) than small ones, because their length is also directly proportional to how much they strain. Double the length, double the strain.

So in summary, we might assume the flaps air load situation is caused by the tolerances in the system, slop in the connections in other words, as well as primarily cable from the flap handle to the tang which experiences the most force, as well as the torque tubes for the flap drive in each wing. All other parts of the aircraft will move a little bit too, but I assume it will be less than the aforementioned components.

[Nev]

Rod, on the B model and possibly on the A model, to do this requires having the cables pre-tensioned enough that the flaps are already deployed a few degrees in the up position. It's the only way to get the full 40° inflight. So on the ground the flaps will "droop", and when airborne the air loads are enough to retract them fully. The force required on the flap lever is then significantly higher than the 85 lbs that I measured. Just saying. People will be able to identify a BH pilot by the size of their right bicep.

[rodsmith]

That's good information Nev. With my original Bearhawk I will be setting up F4 for 50 deg on the ground.

[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.