Bearhawk STOL mods

[Battson]

Pardon my ignorance but can you please explain what you are saying about the first notch not being “blown” due to the pocket being imperfect? Do you mean that the slot won’t get extra airflow at that setting as the lower skin blocks it or something to that effect?

Yes you have it Ken, I assume that both sections of the flap needed attached airflow right around their surface to generate maximum lift. However the truth may be different, it all depends on how the design has been done though.

I have always thought they worked like this, so that the airflow was blowing smoothly over both segments of flap:

image_121.png
However, in researching this response, I found this literature which suggests that the double-slotted flap still works without a streamlined flap pocket. So-call Douglas double slotted flap.

I love this stuff

A26_Av_4505_flaps_dwg_p160_W.png

[AKKen07]

Awesome! That is a great illustration. Thanks for the information. I’m in the edge of my seat waiting to see how these things work!

[Battson]

Considering we have the "plain flap" shown above, I am expecting good things. Cubs have a slotted flap and it makes a big enough difference for them.

[rodsmith]

With the Bearhawks plain but huge flaps, I have always wondered if VGs placed near the front of the flaps would give you partial flow attachment and improve performance. Might be something I try once flying and comfortable with the plane.

[Battson]

With the Bearhawks plain but huge flaps, I have always wondered if VGs placed near the front of the flaps would give you partial flow attachment and improve performance. Might be something I try once flying and comfortable with the plane.

This has been tried and the results are known.
It makes a large difference, but there were some unintended consequences which were more annoying than the benefits were worth. I believe it made the plane really floaty in the flare.

[zkelley2]

Any other side effects than floaty in the flare?

[Battson]

I am excited to confirm the new Airframes Alaska double-slotted fowler flaps have flown on the Bearhawk 4-place.

Positives:

• There was a significant improvement in approach speed. Yet to be confirmed, but we are talking about a reduction of 6 knots or more and a 'rock-solid' approach.
• Reduced stall speed, improvement yet to be confirmed.
• There was an even more significant improvement in deck angle and forward visibility when landing. Angle is yet to be confirmed, but more importantly the point of touchdown was clearly visible even at the stall speed, no neck-craning required.

Negatives:

• There was a small reduction in elevator effectiveness at extremely low airspeed. This only occurred at speeds previously unreachable by a 4-place on final approach.
• When the plane stops flying, it really stops. The transition was sharp.

I am looking for permission to post the first-hand account, possibly video which I also have.

[AKKen07]

Fantastic! Looking forward to the rest!

[Shopperly]

Happy New Year to all! I’m hoping for an update on the testing and would really like to see some video. Is there anything more that you can share?

[schu]

Thanks for the update. Would love to know more.

[Shopperly]

This is excellent and exciting news, and is truly a great example of the experimental aviation world. Kudos to those involved in this project!! I’m very much looking forward to hearing more. Thank you!

[svyolo]

Excellent. I can't wait to hear the whole story.

[Isilverone]

Great news, are you waiting on permission from the builder?

[Cguy]

This has been tried and the results are known.
It makes a large difference, but there were some unintended consequences which were more annoying than the benefits were worth. I believe it made the plane really floaty in the flare.

I talked with MicroAeroDynamics about this (more vg's in front of the flaps, a month ago, as my Rans S-7S also has "plain" flaps, like the BH designs. I have been using their vg's for 20 years on my wings, and had my credit card out and was hot to trot. They talked me out of it, I'd be messing with the ALREADY vg-ized (energized maybe was the term) airflow caused by the existing vg's on the LE, no need to put more on plus there was something said about "control reversal possibilities", and that's when I tagged out. The idea comes up from time to to time, but near as I can tell there is a good reason why we don't see vg's in front of the flaps.

Way to go Battson on the Keller flaps! I too look forward to your numbers on how they work.....

[Battson]

For avoidance of doubt, this is not my plane we are talking about. This is a STOL modified machine in the midwest USA.

[Battson]

I have gone through the details more carefully and revised the approach speed reduction from 4 to 6 knots less.

The stall attitude appears to be about the 3 point attitude, or slightly tail low - same as a Maule with slotted flaps.

The plane is question is about 1,630 lbs empty weight. Initial flight tests were done with 20 US gal of fuel.

The results I have shared are quite literally from the first three approaches and landings. They are subject to change as test flying continues.

The pilot remarks that while it was possible to approach in the mid thirties [knots], it was a sporty regime and was accompanied by a loss of elevator control and reduced visibility. High thirties was rock solid with good visibility.

[zkelley2]

Is this the angle valve 300hp bearhawk in utah?
That's a porker for sure at 1630.

[whee]

Great results. The deck angle decrease is a welcome improvment.

Question: I often see these flaps refered to as Fowler flaps but I don’t see any aft movemt of the flap which is what defines a fowler. Are these not just double slotted flaps?

[Battson]

Great results. The deck angle decrease is a welcome improvment.

Question: I often see these flaps refered to as Fowler flaps but I don’t see any aft movemt of the flap which is what defines a fowler. Are these not just double slotted flaps?

There is some aft movement, chord length increases by several inches, so technically they are semi-fowler, like a Cessna flap. It's an advertising thing I guess.
I have laid this out before over at BCP, and it was argued out by the panel. Same conclusion was reached.

[whee]

Thanks Battson. I couldn’t tell that they move aft in the videos I’ve seen so I always wondered.

[Battson]

I tend to agree with your original conclusion though. I don't think it's a true fowler action. I have always thought it more of a sales pitch than a technically correct title.

Whatever you call them, it seems that it works.

[svyolo]

I don't think there is a carefully worded definition on Fowler flaps. "True" ones I guess have sliding tracks, and translate aft as well as rotate down. Very complicated and heavy, and usually have external tracks, but not always.

Jon;
Your description makes it sound like it stalls at a much lower AOA. Does that seem to be the case?

[zkelley2]

That's what flaps do. Lower the critical AoA.

[Battson]

One of the primary advantages of fowler or slotted flaps compared to other varieties of flap, they generate extra lift behind the CoP and lower the deck angle, creating increased visibility over the nose.

[svyolo]

Hopefully it turns out good. Do you happened to know if he has VG's, profiled tail, or gap seals on the elevator already?