Flap Speeds - 4 place Bravo

[Nev]

I’m getting placards made for the flap speed limits.

The plans show 3 flaps settings (as below).
My flap mechanism has 4 detents as well as the fully down (retracted) position.

Can someone shed light on this for me ? I realise that the first position takes a lot of slack out of the cables, but it probably should also have a speed limit.

712A7488-90F2-4F21-90A8-D262A09055C6.jpeg

[Nev]

Update

I received an update from Bob this morning for the 4-place Bravo model.

15° - 100 mph
30° - 85 mph
40° - 75 mph
50° - 65 mph

For simplicity I’ll be calling them :

Flaps speeds in kts
1 - 85
2 - 75
3 - 65
4 - 55

I think in the big scheme of things this will help to prevent me from having a serious flap over speed.

[PaulSA]

I think I will steal that - I had worked out numbers based on interpolationg the figures on the plans but this is simple, and simple is always good!

[Battson]

These are the same speeds which I use. As noted, the first notch is useless and practically I always go straight from zero to notch 2, so I started my white arc on the ASI at 75 KIAS.

[jaredyates]

Bob has just put out an update confirming these speeds for the 4-Place Model B, Patrol, and Companion.

[PaulSA]

From what I have seen in the forums, the higher flap settings are pretty much self-limiting in terms of the muscle power required on the lever!!

[Nev]

These are the same speeds which I use. As noted, the first notch is useless and practically I always go straight from zero to notch 2, so I started my white arc on the ASI at 75 KIAS.

Thanks Jon, that’s a good idea too. Doesn’t seem much point having the first notch actually. Except maybe like a gym warmup before going for the heavy weights

[Battson]

In case anyone is interested, if (when) you overspeed the flaps, you will stretch out all the cable eyes in the system, which reduces the flap angle next time you use them. Worst case an eye might break, but I highly doubt that. You can take the slack out of the system with the turnbuckles.

I don't believe it's a safety concern - worst case the flaps would slam up, possibly asymmetrically but more likely symmetrically. This would most likely happening during high speed flight, or possibly during flap deployment. Noting that asymmetrical flight in the Bearhawk is easy with such large control surfaces, stick forces remain light. The whole situation should remain easily controllable.

[Nev]

Incidentally, my cable eyes all got stretched when they were tested for integrity.

[Pbruce]

Does anybody ever use the first notch of flap,for takeoff?

[zkelley2]

Never. From me if I'm at an airport/anything longer than 1000', it's no flaps. If I'm short/rough/soft, I go with the 3rd notch. Clean up the second I'm clear of obstacles and if none, accelerate and clean up in ground effect. It climbs better clean.
The bear hawk flaps don't add much lift, no plain flaps do. Just big drag devices for the most part. There's a tiny bit noticeable at the 2nd and 3rd notch, but nothing like a 172 or 180. And just like Battson said below, the actual ground roll is generally pretty similar. With the flaps, I'm truly just trying to coax it into ground effect a bit earlier to save the abuse to the gear.

[Battson]

Does anybody ever use the first notch of flap,for takeoff?

A vexed question, there are two key considerations:

1. How did you rig your flaps?
2. Why does the pilot use flaps for takeoff?

How to answer that...

1. Normally the first notch is between 0 and 15 degrees with air loads applied.
2. So far as I can tell, the flaps' main purpose is to improve takeoff performance - namely, to get the plane clear of the ground and accelerating, clear any small obstacle (50 ft tall you say... hmmm, that depends how far away it is), and finally allowing an earlier clean-up of flaps and transition to an efficient climb out. Bearhawk flaps don't improve climb performance, to my knowledge. There is a secondary purpose, reducing weight on the tailwheel and allowing the tail to lift sooner, which reduces damage when off airport.

This is where it all gets difficult. Here's an opinion for your digestion:

Setting 15 degrees or less would achieve next to nothing, except perhaps making the pilot feel better. The plane is so quick off the ground, the pilot needs a lot of timing and skill to improve it significantly. In our testing and competition flying with official measurements, a lightly loaded plane is off in 29m with "set and forget" 25 degree flaps, and off in 26m with flaps timed to perfection and a bit of aileron into the bargain to really trick the plane into flying before it's actually ready. Flapless takeoff in the three point attitude, well that's probably around 31m... Not much in it.
But with pilot 'intervention', that distance can easily be extended to a much, much longer take off run.

Pilot technique is more important than any given flap setting for maximum takeoff. But this takes a lot of practice to get it to work. It's not easy to tell when the plane is in the transitional phase when it can be 'coaxed' off the ground. That phase moves around a lot with weight, air conditions, and wind. Get it wrong and you'll extend the take-off roll considerably. So flaps are not helping reduce ground roll or improve obstacle clearance either in that case - it's just bad piloting.

Its easier to roll three point with 25 degrees flap and let the Bearhawk and horsepower do the magic. Don't even bother lifting the tail, the plane will start to fly in the 3-point attitude, and it'll do it very close to the earliest possible moment. If the pilot cannot recognize that moment, then going up onto the mains will only reduce lift and delay the take-off.

[Sir Newton]

Battson seems like a grey beard sharing his knowledge.
Thank-you Sir

[haribole]

Wow!! Thanks so much Battson!!

[Mark Goldberg]

Greetings from Oshkosh. I have always considered the 1st notch of flaps to only take the slack out of the cable system of the flaps. With airloads that first notch does not deploy the flaps at all.

I have heard Bob say that he lowers the flaps 2 notches for take off because it makes the ailerons more effective more more quickly.

And I would never argue with ace STOL champion Jonathan about flying the plane. But I think the ability to sense when the plane is ready to fly on take off is a skill that is not something everyone has or even can develop. I have flown with Bob a lot and think he also "feels" when the plane is ready to fly.

But for the rest of us mere mortals, trying to obtain the take off distances Jonathan does is perhaps a way to end up with a bent airplane. Takes a ton of practice and that ability to sense when the plane is ready. If you double or triple Jonathan's numbers you are still getting great performance compared to most other planes. Mark

[Gerhard Rieger]

The numbers regarding landing speeds even with the model A wing is amazing, one of the voice comments from a pilot/instructor from JAARS on the Bearhawk aircraft web page says, even if you do your approach at 55 mph you might see two airplane lengths longer role . To change the flap system is so complicated and to many structural parts that will have to be altered. Add vortex generators, more efficient wingtips and gap seals on the elevators, and the airfoil kit for the Model A Bearhawk, the wood kit that I got from you Mark looks awesome, might add a little extra weight, but I thinks the benefit will be much more. I have made a couple of changes on my plane like adding bolt on axles and had a mechanical engineer help me with all the alterations needed.

[Pbruce]

Really interesting comments. Thanks all. Especially that you can actually extend the takeoff roll by improper application of flap. I really enjoy exploring the min speed corners of light airplane flying, and am so looking forward to doing so on my Patrol. I’m not a very experienced tail wheel pilot though, so this will be done at altitude and the plane will be flown very conservatively near the ground. Notwithstanding the rough airstrip issue, seems like the takeoff performance exceeds landing performance on this plane anyway, so there should not be any imperative to get into that near the ground. Man I can’t wait to fly this plane!

[zkelley2]

Jonathan can correct me if I'm wrong, but I don't think he's talking about improper flap application. Just set it(or not) and forget it. With only a 2-3kt difference between full and no flaps(and the corresponding couple meters of roll difference), there's no point in putting flaps in during the roll. If you saved a whole foot when doing it perfect I'd be surprised.
I think what he means is an improper 2 point takeoff, something like putting the tail 4ft in the air on the roll and then hauling back on the stick. That's a lot of added drag, and then a lot of people bang the tailwheel back down when rotating. Also bad.
Or I've seen people just haul back on the stick for the whole roll from the 3pt attitude. That'll also add a lot to your ground roll, in my experience a whole 30% more or so.
Either trim it so it just flies off the second it can in the 3 point attitude(or thereabouts, if you're in the 3 point attitude it's unlikely you're somewhere you need absolute max performance) or if you're soft field, the tail should be 2-3" in the air. Only enough for the tailwheel to be clear of obstacles and not making drag via the boat anchor it is, while keeping as much AoA on the wing as possible.

Feeling it isn't that difficult. Most people get to that in an airplane in just a handful of hours. Literally any off airport flying requires you to be head outside and to feel when the aircraft wants to fly. Numbers are meaningless and you shouldn't be looking at them anyways for TO/Landing(wings don't stall at airspeeds, they stall at angles of attack). Basic attitude flying. Same goes for the approach and landing, you'll have wildly more stable approaches with a constant AoA rather than a constant airspeed. Get that grease pencil out like your CFI did if you need to until you figure out the sight picture.

[whee]

Great discussion!

Thoughts from a lackluster pilot. The first handful of hours I put on my plane I didn’t use the flaps at all. I didn’t want to introduce another system till I had verified other systems worked correctly. Flying the BH with no flaps is effortless. In my testing power off stall speed is not significantly different with any flap setting.

When landing I pull the first notch just before I turn base and the second notch almost immediately follows. This usually occurs somewhere around 80mph. Third notch comes on just before or after turning to final. 4th notch comes on during final so I can steepen my approach. If I lose the engine I can let out a notch of flaps, flatten the approach and make the runway for sure.

2 notches floor takeoff. Occasionally when heavy and at high DAs I’ll milk a little more/little less flap during initial climb out.

Important note: Something magical happens at around 220hp. Takeoff happens much faster and the plane seems to break some cruise speed barrier. Under that hp you have more time to use pilot tricks to make the play fly sooner. Above that hp most pilots are better off just going 2 notches and tailwheel skipping off the tops of the bumps during takeoff.

[svyolo]

Silly question but I haven't found the answer. The first flaps "notch" in the handle mechanism I am assuming is flaps "0". Just off the AL floorboards. Is that correct, or is zero below that notch.

[Nev]

Mine has the flaps retracted position, about an inch above the floor. Followed positions by 1, 2, 3, 4. So there is the retracted position, then F1 etc.

[zkelley2]

Silly question but I haven't found the answer. The first flaps "notch" in the handle mechanism I am assuming is flaps "0". Just off the AL floorboards. Is that correct, or is zero below that notch.

You are correct.

[nborer]

Since I've taken time off lurking to reply, I'll add what I've been able to discern. A while back I hand-measured the Mylar drawing and then generated numerically smoothed airfoil forms for analysis. I'm currently using XFOIL, which is a 2D vortex panel code with an integrated boundary layer solver - it has heritage back to the Eppler code that Riblett used, but can do fancier stuff. I don't really trust its flow separation model (so I have a hard time believing drag after the flow starts to separate, which it will behind more than about 10-15 deg on the flap), but it does a pretty decent job of estimating max lift, at least for modest flap angles (and 50 degrees is no way a modest flap angle). Still, I agree with the assertions that the flap is really adding drag rather than adding lift. I've attached an analysis of the section drag polars - section lift coefficient vs. drag coefficient - at a Reynolds number that is in the vicinity of where you'll be for approach/stall. If you believe this, maximum section lift coefficient increases by about 20% from clean to 50 degrees (and really not much at all above 35 deg), yet drag more than triples from the max lift point. Huge grains of salt here; this is 2d, so there are lots of 3d effects happening that aren't captured, but it goes to show that plain flaps do a little for lift but a lot for drag (and again, these drag estimates are wrong, but I suspect the relative magnitudes are decent).

TBH, I haven't even looked at my plans to see where the detents are, so if they're at 15, 30, 40, and 50, I'll have to rerun and see what 30 and 40 come out as. I don't suspect much lift improvement based on these results...
bearhawk_airfoil_polar_Re_4e6.png

[jaredyates]

Fantastic work, thank you. To make things more complicated, deflection changes with speed, as the system stretches under the load of the relative wind. One of my to-do projects that won't get done was to add a flap position sensor so that I could see how it changed in the recorded efis data.

[Battson]

Engineering 101 - model data must always calibrated with experimental data, before it can be used or published. i.e. We have no way of knowing if this is correct, or not. I suspect not.

[nborer]

Well, heck, Riblett wrote a whole book on airfoil performance without ever conducting an experiment, IIRC. He used the same type of vortex panel code that was used to generate these results, just a few generations earlier. Sure, folks have used his designs and they've performed well, but I haven't seen experimental validation of any of his results...

[nborer]

I got curious so I dusted off my scripts and ran the flaps 30 and 40 cases. I'm attaching a plot of lift vs. angle of attack. These are generated at a Reynolds number that are close to my estimate of the aircraft 1g stall speed at 2500 lb - if I'm generous, I estimate stall speed at 56 mph clean and about 53 mph with flaps 50, but it'll be higher than that because I'm not accounting for what are likely ample 3D effects that'll drop it further.

A few things jump out, and based on my reading of this thread and others, will come as no surprise to anyone here. First, the angle of attack for maximum lift is pretty high - right around 19-20 deg clean, and down around 13 deg with 30+deg of flaps (though you're basically getting close to the max lift benefit 5-10 degrees before, and at that point are really just adding drag = sink rate). I've read some of you talk about deck angle being high on low-speed approaches, and/or running out of elevator. This is one of the reasons why I'm playing with a simple slotted flap retrofit idea; it seems that many others have had that thought as well. That takes a different level of horsepower than the analysis codes I'm well-versed in, but I'm trying to learn a little from a few folks I work with that do flap design for a living. We'll see.

Next, the spread in stall speed from not flapped to fully flapped is only going to be a few knots. Stall speed will be proportional to the square root or the relative lift coefficients - we're talking 2.2 flapped vs. 1.8 unflapped, so a 22% change. That leads to about a sqrt(1.22) = 10% difference in stall speed for the flapped section, but only about 55% of the wing is flapped, so at most a 5.5% change (and probably not that). 5.5% of 56 mph about 3 mph. I bet a decent single-hinge-point "semi-slotted" flap would double that, but it's still not a real barn burner - how far should I go to get another 2-3 mph? It sounds like it's the deck angle and trim authority more than the stall speed...

Now, to get to the topic of this thread. I was looking at the flap speeds listed earlier, and I noted that the 50-degree setting was dropped from 75 mph (as per my plans) to 65 mph. Certified aircraft (at least for this type of airplane) are supposed to pick a reference approach speed that is at least 1.3x the stall speed in the landing config. If we have a 53 mph stall speed, 1.3x this is about 69 mph - above the stated 65 mph flap speed. I suppose no one really approaches with 50 degrees of flaps unless you're deep into bush territory (in which case an approach speed that is 1.3x higher than stall speed is silly), but as normal procedures go, 50 degrees of flaps shouldn't be used for any normal approach-to-landing. I don't know what speeds folks use, and I suppose lots of folks don't fly at gross weight or even close, but it's something I'll now add to my eventual flight manual - if I can ever stop playing with XFOIL and get my shop back (still waiting to clear it out after it became a "stuff" repository after our emergency house repair that turned into a giant remodel).

Is <65mph for 50 deg of flaps the new official guidance for the Model B? I may have missed it; I know some good stuff shows up in the engineering change notices. I'll need to be sure to log that one.

bearhawk_airfoil_lift_curve_Re_3e6.png

[Utah-Jay]

Am I missing something here? My plans/specs say a landing speed of 40mph for the Companion, and I have seem 35mph in print for the Patrol. Where is this 50-55mph stuff coming from? I have been flying a much more “slippery” Rans S-21 and I am seeing touchdown speeds in the 44-46mph range (43mph) was the lowest and I feel like there was always a bit more left.

I just can’t imagine flying 50+mph on final in a Bearhawk. but then again history shows that the vast majority of pilots fly final way too fast.

[Utah-Jay]

Thanks Jonathan, that is in line with what I expect. It seems too many fly too fast in my opinion.

[nborer]

I'll put it another way. The Bearhawk as the same wing area as a Cessna 172. The model A uses a very similar airfoil (the 172 has a NACA 2412, the Bearhawk model A a 4412, IIRC). Both unmodified airfoils have very similar maximum lift coefficients unflapped, though the 4412 should have more benign stall characteristics as the lift doesn't drop quite so much off a cliff post-stall. The Cessna 172 uses large, fairly sophisticated Fowler flap system - a slotted flap that also increases wing planform area - so I have to believe that it produces more lift than the Bearhawk's plain flap. My 172P POH shows that the stall speed, at rear CG and 2400 lbs and 30 degrees of flaps, is 46 KCAS - about 53 mph.

I have a C172N POH that shows for 2300 pounds and 40 degrees of flap deflection (they reduced the deflection by 10 degrees when the introduced the P model, more on that some other time), that airplane will stall at 44 KCAS - not quite 51 mph.

I believe Battson has a model A, but also has a bunch of VGs. That'll certainly delay the separation. I suspect significant separation exists on the plain flap after the 15-degree notch. If the VGs can energize the flow, I can see it hanging on for longer - out of curiosity, where do you locate them? Just ahead of the flap, or a little further up?

Otherwise, I just don't see how a "stock" Bearhawk wing, particularly a model A wing, can beat the stall speed of a Cessna 172 wing - a wing with a similar airfoil, the same wing area, and a flap system that should be much better at producing lift than a plain flap. (Fowler flaps are great, but they're a lot more complex.)

The Riblett airfoil is good, and has a better max lift coefficient clean than the 2412 or 4412 - not dramatically better, but 10%+ish. Nothing to sneeze at. Still, a plain flap is a plain flap. At more than about 15 degrees deflection, it stops doing anything more for lift for you, and basically becomes a drag device. If it did produce more lift, you'd see people with ailerons that deflect 50 degrees for better roll control.

[Battson]

I missed the above comment earlier - but there are so many variables not being considered here... The C172 and Bearhawk wings are so different, and cannot be compared so easily without huge errors being introduced.