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

jaredyates

jaredyates

4-Place Kit Builder
Staff member
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 update was originally issued in December 2022, but it has been revised/clarified as of March 2023, with the most recent language here:

Engineering Change Notice
December 2022 - Clarified March 2023

These are the maximum airspeeds for having the flaps extended, based on deflection in flight:
10 DEGREES DOWN ---100 MPH
20 DEGREES DOWN ---85 MPH
30 DEGREES DOWN ---75 MPH
40 DEGREES DOWN ---65 MPH
Deflections are in flight, with air loads. On the ground, deflections will be greater. Systems built as drawn on the plans, with flaps fully retracted on the ground, will deliver very close to these deflections. These speeds apply to the Bearhawk 4-Place Model B, Bearhawk Patrol, and Bearhawk Companion. The original Bearhawk 4-Place uses a different flap design which allows for higher speeds.​
 
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If you have something different on the first page of the plans, make it these instead. Some sets of plans for the Companion may have shown speeds for the original Bearhawk 4-Place (when they should have matched the Patrol), and some of the Model B plans only showed flap speeds for three different deflections, though most builders use a quadrant with four notches.

Note that these speeds are the same as those passed along by Nev back at the beginning of this thread:
https://bearhawkforums.com/forum/bearhawk-general-discussion-news/66471-flap-speeds-4-place-bravo
 
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I assume these apply to the original Bearhawk as well as the Model B. In knots they are what I have printed on my panel. My plans updated 8/09 list 100mph-15deg, 85-35deg and 75-50deg. I thought there was an engineering change earlier lowering the 50deg speed, but all I could find was one dated July 2001 lowering the 35deg speed to 80mph.
 
rodsmith said:
I assume these apply to the original Bearhawk as well as the Model B. In knots they are what I have printed on my panel. My plans updated 8/09 list 100mph-15deg, 85-35deg and 75-50deg. I thought there was an engineering change earlier lowering the 50deg speed, but all I could find was one dated July 2001 lowering the 35deg speed to 80mph.
Click to expand...

I'm not positive about the answer there, but I know that Bob was concerned about the Companion in particular because he had copied the 4-Place speeds when he first made the Companion plans, but the Companion has the larger flaps from the Patrol. That was what instigated the update. I don't know of any other updates related to the flap speed, which isn't to say there aren't any. The speeds in this most recent update are slightly more restrictive than those on the first page of the plans for the original Bearhawk, so there would certainly be no harm in observing the new speeds. Sometimes in flying very slippery airplanes there are times when the flaps need to go out right below the limit speed to be able to make a restriction or get slowed for pattern entry, but the Bearhawk isn't one of those kind of planes. My opinion is that the whole system appreciates deploying flaps at much lower speeds than the limits.
 
Jared,
Thank you for posting this Engineering Notice.

I am a bit confused and concerned with the implications of this engineering notice. Our Model B 4 Place has flaps set up and placarded as per the plans.

15degrees - max 100mph - (87kts)
25degrees - max 85mph - (74kts)
40degrees - max 75mph - (65kts)
50degrees - max 65mph - (56kts)

The engineering notice requirements apply to the Model B.

10 DEGREES DOWN -100 MPH
20 DEGREES DOWN - 85 MPH
30 DEGREES DOWN - 75 MPH
40 DEGREES DOWN - 65 MPH​​

Is this just new Model B's built after Dec 2022 or all Model B's including ours?​

If it includes ours, then it is surely not just a matter of changing the placard but also adjusting the whole flap setup, notch locations and reducing the maximum flap to only 40degrees. Following on there would have to be test flights to revise stall speeds with amendments to the approved flight manual and so on.

This seems to be be quite a major change with no reasoning or explanation. I was expecting to see more comment in the forums, as it presumably affects a number of aircraft. Hopefully I am missing something?

I am no engineer and am looking for some guidance as to what I have to do. Can you or anybody else help?
 
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We would need to ask Bob, as I'm just the messenger.
Let me know if you'd like for me to call and ask or if you'd like to.
Part of the problem is that notches only loosely correspond to actual deflection degrees, with deflection at a particular notch being variable with airspeed. I don't get any impression that he is making an effort to say that the first notch must correspond to 10 degrees, and I don't see how yours could be rigged to deflect them to 15 degrees at all speeds or even the limit speed for that setting. If you are getting 15 degrees at 87 knots in the first notch, you must have something like 10-15 degrees of deflection at zero airspeed and the zero notch. This is how Russ Erb has is set up, more or less.
 
Why does the airspeed change the flaps so much? The cable stretching that much? Or does the return spring not pull them tight against the cable? This sounds really odd.
 
schu said:
Why does the airspeed change the flaps so much? The cable stretching that much? Or does the return spring not pull them tight against the cable? This sounds really odd.
Click to expand...

There has been prior discussions where people have done mathematical proofs that it is cable stretch. My Bearhawk has zero cables in its flap system. I don't have any return springs either. It has an electric actuator in the ceiling where most would have the manual trim wheel. It has the same compliance as everyone else's. I believe that ~80% of the compliance is the torque tubes in the wings. I lose 10-15 degrees, even when near stall.
 
kestrel said:
There has been prior discussions where people have done mathematical proofs that it is cable stretch. My Bearhawk has zero cables in its flap system. I don't have any return springs either. It has an electric actuator in the ceiling where most would have the manual trim wheel. It has the same compliance as everyone else's. I believe that ~80% of the compliance is the torque tubes in the wings. I lose 10-15 degrees, even when near stall.
Click to expand...

Whoa, so if the flap system is deflecting that much (torque tube or cable giving) and Bob is lowering the flap deployment speed, does that mean someone had a failure? What failed?

What about the A model? I wish the text was a little more definitive in specifically saying the A model is not affected.
 
Mine is an A model. They have said that the A model isn't effected due to the smaller flaps that don't reach to the fuselage. I've not heard of any failures.
 
It may be relatively easy to adjust the maximum flap using the cable adjustments behind the bulkhead. But I think it's important that we get confirmation from Bob that it is his intention that the maximum deployment is now 40°, and importantly, the reason for the change. I wonder if it's possibly related to the elongated flap handle bolt holes found in one of the higher hour A models recently.
 
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
 
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.
 
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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.
 
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.
 
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.
 
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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'.
 
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'.
Click to expand...

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.
 
Nev said:
He did clarify that 40° was his intended maximum deflection with airloads.
Click to expand...

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.
 
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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.
 
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kestrel said:
Why do you think it is cable stretch?
Click to expand...

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


Nev said:
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.
Click to expand...

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.
 
Bcone1381 said:
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.
​
Click to expand...

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

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.
 
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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.
 
That's good information Nev. With my original Bearhawk I will be setting up F4 for 50 deg on the ground.
 
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.
 
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Nev said:
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.
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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.
 
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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.
 
jaredyates said:
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​
Click to expand...

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 :p (Haha)
 
Nev said:
Flap force and deployment angles

In the event of a flap overspeed the air loads would be exponentially greater.
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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.
 
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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.
 
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.
 
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.
 
Mark Goldberg said:
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
Click to expand...

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 ?
 
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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.
 
I am building a 4-Place with Model A wings. I really enjoyed the recent paper by Russ Erb that discusses his testing of inflight flap deflections vs. on the ground. He did a great job of explaining how cable stretch is a major contributor to the inflight deflection being less than half of the deflection on the ground. He states at the end of the article that Bob Barrows suggested that flap torque tube twist should also be examined. Since the formulas for calculating twist are outside of my current knowledge, I decided to simply measure it. My measurements show with 40 ft-lbs of torque on the flap torque tube results in 10 degrees of twist. I used 40 ft-lbs based on my estimate of 80 lbs of pull at the flap lever ratioed up by 3 to find lbs pull, divided by 2 to get the per flap pull, and then dividing by 3 to get the torque acting on a 4" flap arm. This reinforces what Kestral has observed on his cable-less flap system. Kestral's system also has a cross plane torque tube which add an additional 4 degrees of twist totaling 14 degrees. Very significant!

Does this make sense or am I way off?
 
Given that I have 45 degrees on the ground and 30 degrees at stall (less at any higher speed), your numbers look reasonable.
 
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