Tweet
Perhaps this could explain why some builders have experienced high load on the flap handle when deploying Flap stage 4?
-
-
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.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'.Nev Bailey
Christchurch, NZ
BearhawkBlog.com - Safety & Maintenance Notes
YouTube - Build and flying channel
Builders Log - We build planesComment
-
Bottom Line:Originally posted by Nev View Post
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.Last edited by Bcone1381; 01-12-2023, 01:14 PM.Brooks Cone
Southeast Michigan
Patrol #303, Kit buildComment
-
-
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.Last edited by Nev; 01-12-2023, 02:26 PM.Nev Bailey
Christchurch, NZ
BearhawkBlog.com - Safety & Maintenance Notes
YouTube - Build and flying channel
Builders Log - We build planesComment
-
I called it cable stretch, system elasticity is a better description. You already said its torque tube elasticity. Lets go with that.Originally posted by kestrel View Post
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.Originally posted by Nev View Post
The notice needs clarification for future builders and those who don't visit this thread.
Brooks Cone
Southeast Michigan
Patrol #303, Kit buildComment
-
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.
-
-
Yep that's right. But because the original 50° (without airloads) was being misinterpreted occasionally, Bob has now made it 40° (with airloads).Originally posted by Bcone1381 View Post
If someone interprets it wrong, the overall loads on the flap system will now be lower rather than higher.Nev Bailey
Christchurch, NZ
BearhawkBlog.com - Safety & Maintenance Notes
YouTube - Build and flying channel
Builders Log - We build planesComment
-
An editorial, as opposed to a fact follows.Originally posted by Nev View Post
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.
Brooks Cone
Southeast Michigan
Patrol #303, Kit buildComment
-
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.Nev Bailey
Christchurch, NZ
BearhawkBlog.com - Safety & Maintenance Notes
YouTube - Build and flying channel
Builders Log - We build planesComment
-
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.Last edited by Nev; 01-14-2023, 12:55 PM.
-
Just a clarification here, in case anyone was interested.Originally posted by Nev View Post
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.Last edited by Battson; 01-15-2023, 05:41 PM.Comment
-
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.Originally posted by Nev View Post
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.Last edited by Battson; 01-15-2023, 06:11 PM.Comment
-
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.
Comment
-
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.Comment
-
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 ?Originally posted by Mark Goldberg View PostComment
(Haha)
Comment