Flap Speeds - 4 place Bravo

Nev

Nev

Quick-build 4-place B #25
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.

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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.
 
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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!
 
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.
 
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.
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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 :rolleyes:
 
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.
 
Pbruce said:
Does anybody ever use the first notch of flap,for takeoff?
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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.
 
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.
 
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Incidentally, my cable eyes all got stretched when they were tested for integrity.
 
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
 
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!
 
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.
 
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.
 
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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.
 
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...
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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.
 
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.

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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.
 
I think there are two major factors at work.

One culprit is IAS vs. CAS/EAS vs. TAS. At low speeds, pitot-static systems notoriously under-indicate airspeed. I have a Cessna 172P POH here that shows that, at gross weight with flaps up and aft CG, stall is at 44 KIAS is 51 KCAS. With flaps at 30 degrees, stall is at 33 KIAS is 46 KCAS - that's a 13-knot spread (30% higher) from indicated to calibrated airspeed! The airspeed calibration depends heavily on the location of your static port and your pitot probe. Landing and stall tend to be the worst, since the pitot probe is at angle of attack (and not capturing full impact pressure), and the static port may have local flow that is not perpendicular to the face of the port. Indicated airspeed is based on the difference between the pitot pressure and the static pressure - the bigger the difference, the higher the indication. If you are getting less impact pressure (because of the angle of the pitot probe), and more static pressure (because you have some air "coming in" to the static port rather than passing perpendicular to it), you'll indicate a lower airspeed.

Another sizable culprit is the definition of "Stall Speed" vs. the speed you stall at. Stall Speed, which may be the bottom of the white or green arc, will be defined at max gross weight and forward CG, which will lead to the highest values. The speed you actually stall at will depend significantly on weight and CG. If you tend to fly mid-to-aft CG, the speed you stall at will go down a little. If you tend to fly lighter than gross weight, you'll stall at a quite a bit lower speed. (Yes, I'm omitting load factor for this discussion.) Like lift coefficient, the difference in stall speed will be related to the square root of the relative weight. For a Bearhawk Model B with a 2500 lb gross weight, if you build light and fly with one person and minimal fuel at, say, 1700 pounds, your stall speed will be (1 - sqrt(1700/2500))*100% = 17.5% lower than full gross weight. If my estimate of 53 mph (CAS) above is accurate, that is a 9 mph reduction - 44 mph, which will be even lower in terms of IAS. The Patrol, using the same wing area and flap ratio, is limited to a 2000 pound gross weight, so its stall speed (given the same wing and same airfoil as the Model B) should be about 9% lower at max gross weight - about 5-6 mph lower.
 
The Garmin Pitot in the plane I have been flying has an AOA speed sensor too, so I think it was very accurate. The ground speeds my GoPro was reading at the same time as the speeds listed above coincide with my field elevation as well relative to IAS. In my mind, 40mph landing speed is in fact 40mph. This 50+ Mph is student pilot stuff way over the needed Vspeed. I am a Vso 1.1 guy, not Vso 1.3. 1.3 and the plane is not ready to stop flying, 1.3 and you are not gonna hit your spot consistently. I may be wrong, but I won’t believe I am wrong until the plane I am building proves me wrong.
 
nborer said:
50 degrees of flaps shouldn't be used for any normal approach-to-landing.
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Who taught you to fly.... :rolleyes: Jokes aside, I am always concerned when I see people landing with less than full flap. It greatly increases difficulty, risk factors, and heightens the risk of a ground loop. Aside from special situations like crosswind landings / emergencies / training.

For instance, when was the last time you saw a commercial plane landing with half flap?
 
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My Bearhawk flies at 28 to 32 KIAS, real touchdown speeds are at 37kts GS. This with a moderate load aboard. Obviously the speeds are lower with a light load.
 
Is <65mph for 50 deg of flaps the new official guidance for the Model B?
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I wrote to Bob and he replied the speeds that I've copied early in this thread. Not certain if that's "official", so to clarify, it's second hand, via me.

nborer you make some very interesting observations via your charts. The one that grabs my attention is that with full flap the effect on lift is minimal, but the effect on drag is large. This makes sense on a Bush/STOL aircraft, for the ability to make steep approaches etc and I think it's probably a large factor in the short landing ability.

My understanding is that drag also obeys the dynamic formula in relation to being proportional to V² and the Bravo model has huge flaps. Hence the inflight loads would rapidly increase proportional to the square of the airspeed. In the previous aircraft I was current on we frequently flew approaches at a reduced flap setting as it lowered the flight loads and maintenance on the flaps (and reduced fuel consumption).
 
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.
 
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.
 
Lol. I'll agree with you in general, so I apologize in advance if this sounds argumentative. You have a lot more trophies than I do when it comes to landing where you want to. ;)

In a response on the previous post, I noted that the C172N has 10 degrees more of flap than the C172P. That's because they upped the gross weight of the 172P by 100 lbs. Our flying club recently adopted an STC for our N model that increased the gross weight - it basically involves a lip around the oil cooler (we already had it as a late-model N) and a reduction in max flap travel from 40 to 30 degrees. I was curious about this, and what it really came down to was the balked landing climb gradient. To meet cert requirements, Part 23 aircraft are supposed to have a balked landing climb gradient of at least 3 percent. That's with the aircraft in the landing configuration, which includes all approved flap settings for landing. Both the N and P model 172s have a 160hp engine, and I suspect that in some conditions they couldn't meet that balked landing climb requirement at the increased weight with 40 degrees of flaps, so away it went.

In the experimental world, we have the luxury of being able to pick our engines. I have yet to create a performance model for the Bearhawk, but it's on the list. (Has anyone come up with a drag polar, both flaps up and down? I can WAG one, but it won't be very good at low speed.) That said, the excess thrust (T-D) from a prop turned by a 180hp engine vs. a 230hp engine is going to be pretty dramatic - T is going to go up roughly proportional to the power, and D isn't changing much... and climb gradient is related to (T-D)/W. Given the very high drag of the Bearhawk flap at higher flap settings, I suspect that some engine choices could result in an anemic climb gradient in a go-around, particularly at max gross weight and a high/hot day.

Commercial planes (particularly big ones) have very complex, very effective flap systems - leading edge slots and multi-segment Fowler flaps that are very effective at increasing the maximum lift coefficient. They are designed to significantly lower the stall speed of the airplane in the landing configuration to get the approach speed (and therefore landing distance) down. It irresponsible for them to not use all that lift - brakes (and overruns) are expensive. That said, the speedbrakes and spoilers don't (generally) come out until the wheels are on the ground.

In the case of the Bearhawk plain flap, it doesn't seem to add much lift beyond 15 degrees, and beyond 30 it's a speedbrake - that is to say that all it does at the higher settings is increase your sink rate, not your lift capability (that experience may change with VGs). That's great for obstructed areas, but I fly out of a Class D airport with a 7000 ft runway. They want me on a 3-deg glideslope in a bomber pattern. I still approach high - I see white on white quite a bit - because if that spinny thing in front of me stops when I'm a mile out, I want to at least make the clearway.

All that said, I land with full flaps in our club planes unless winds dictate otherwise or I'm training. "Full" for the Bearhawk is just a bit more of an extreme than most planes. I've never flown one, and my technique will depend on my experiences if/when I finally get it in the air. I also have initially though I'd put in a 180hp engine, in which case my go-around climb gradient could be a bit anemic, particularly at gross weight. Lots can happen between now and then to change my mind.
 
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Yes, parasite drag is also proportional to V^2. Paradoxically, for level flight, induced drag is proportional to 1/V^2, hence why we have a "back side of the power curve." The loads we're talking about on the flaps will go up quite a bit with V^2 due to the parasitic effects. My thought/concern was that my plans list 75mph as the limitation for flaps at 50 degrees, which implies to me that he's cleared the structure for those loads at those speeds. If he's now saying that we should only fly 65mph with 50 degrees of flaps, I am curious if that is due to a revision in the structures calculations or if it's more related to experience/flight technique. It doesn't effect me now, but it'll make me think about what I placard/etc. when I get to your stage of the build... many years from now...
 
In the case of the Bearhawk plain flap, it doesn't seem to add much lift beyond 15 degrees, and beyond 30 it's a speedbrake - that is to say that all it does at the higher settings is increase your sink rate, not your lift capability (that experience may change with VGs).
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Well, I must say I think oversimplifying (overthinking?) it - but you clearly know your stuff, so good for you :)

You can feel it when you land. Drag is required to land an aircraft with maximum safety.

Landing with 25 degrees flap or below, the Bearhawk is not easy or fun to land. You have a higher deck angle meaning visibility is reduced unnecessarily, the plane is slippery, it tends to float and bounce. Control is more difficult. The tail is remarkably heavy and steerage in the three-point attitude is more difficult.

Lift is not the whole story. We are trying to descend, without building airspeed, we need lots of drag. When we reach the ground, we want to stop flying ASAP, floating down the runway in a semi-three point attitude is an accident waiting to happen. Especially if wind is present.

If your engine fails you need to react to the situation before you, wherever you are. I don't buy the argument that landing with half flap somehow improves your odds. Most engine failures happen on take-off anyway. This is for turbines but you get the idea:

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I have to say that my experience doesn't agree with your theoretical data, about 15 degrees creating maximum lift. Experientially, 15 degrees does NOTHING to improve lift. In fact, it does next to nothing at all.
Most of the lift happens around 30 to 40 degrees. The last notch to 52 degrees is pure drag.
 
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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.
 
I'll take real-world experience over theory any day. If I don't have an experience basis, I'll start with theory. It often misses some nuances. You clearly have the experience here, and it trumps any theory I may spout. I've never flown a Bearhawk, but I'm itching to find out!

Interesting that in your experience the first bit of flap deflection doesn't improve lift. I haven't tried modeling the A wing, but that should be straightforward given that it's a NACA section. One thing I don't have is the location of the hinge point. I can guess the chordwise location of the hinge point, but don't have the vertical position. I could probably WAG something, but if someone has a drawing handy and can let me know how far down vertically the hinge point is from the chord line, it wouldn't take long to model. Otherwise I'll guess something.

There will be some 3D effects - vortex lift from the flap edge - but I can't see this as why higher flap deflections would be so much better. For some crazy low-aspect ratio swept wing, absolutely. One of the local military contractors here flies F-21 Kfirs to "attack" the Navy and Air Force from the airport where we're based. I love watching them on approach to land - that plane ain't going where the nose is pointing at low speed!

As I recall, you have VGs. Do you mind sharing where they are located, approximately? I can trip the flow at a specified location. Did you notice that the first 15 degrees didn't increase lift prior to putting on the VGs?

Regarding an engine failure, the most important thing to do in a single-engine airplane is push to keep (or get to) the best glide and/or min sink airspeed (depending on your situation - usually best glide), and then configure as necessary. The more flap you have, the more you need to push when the spinny thing quits. The planes I currently fly all have electric, slow-acting flaps. I generally don't apply full flaps until I'm sure I can make the clearway if the engine quits. I suspect it's a bit different when you have a Johnson bar - I've only flown two airplanes that have had one (both Pipers). I actually kind of like them, and I see it as a selling point of the Bearhawk. But, I agree, approach to landing is not usually the spot where you need to worry about the engine quitting. Two exceptions - if you're bad a fuel management, or if you are susceptible to carb icing (which won't show up in the jet data). It's pretty humid here, and we've known a few folks that got a wake-up call when they got complacent and forgot that there's a carb heat knob they need out when landing.
 
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...
 
If you don't have a set of Bearhawk plans yet, you could buy some?

That will answer your questions about the real wing profile (which are not exactly the same as the NACA profiles), the hinge point, and set you on your way to perhaps flying a Bearhawk of your own.

VGs - use a search engine (Google?) over this forum, you'll find what you're after.

Lift is a practical thing in flight, best measured by checking stall speeds at different flap settings. I've done all the CFD stuff, it's fun but hard to get realistic results.
 
Thank you. I have a set of plans for a Model B, and was even doing well at making parts until my shop/garage became a storage area for stuff following a massive kitchen flood. Hence, when I'm not working at my job or working on the house, I look over my plans, come up with designs for a 9' brake, conduct analyses, or whatever else. I guess I'm really not all the interested in buying a set of model A plans just to satisfy a curiosity related to a hinge point. I can probably make an educated enough guess for the error bars I'm working with already.

CFD is great but has to be interpreted with a grain of salt, particularly in cases for high-lift. It's not an excuse to eliminate experimentation, but done right, really helps to inform it.
 
My thought/concern was that my plans list 75mph as the limitation for flaps at 50 degrees, which implies to me that he's cleared the structure for those loads at those speeds. If he's now saying that we should only fly 65mph with 50 degrees of flaps, I am curious if that is due to a revision in the structures calculations or if it's more related to experience/flight technique.
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I'm currently working on one of the flaps and I'm also very curious about the revision. The B model flaps are truly huge. Yesterday, my mentor and engineer took a look and was very interested in the attachment hardware, particularly the hinges. He expressed a concern of making certain not to overspeed the flaps in case damage occurs.

During flight testing I'll pay attention to the landing characteristics to see if perhaps it makes sense for me to use a lesser flap setting when runway length is not a limiting factor (most of the time).

Another consideration is the go-around scenario. Having 260hp will perhaps make the climb gradient a non issue, but it might be very easy to overspeed the full flap setting, especially with the full prop wash being directed at them.
 
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There are lots of reasons why airplanes land at less than full flaps, including large commercial A/C. I agree with Nev that 260 hp in a BH covers a lot of those up. Some of those reasons are certified requirements, to meet some performance minimum requirement, some are really idiotic and the local government requires it to reduce noise. Some are driven by bean counters to save .01 percent of fuel burn. Some, in my opinion, actually make aviating less safe.

For me, I always found it best to land one way as often as possible. You can't help but get better at it as time goes by.
 
Nev said:
He expressed a concern of making certain not to overspeed the flaps in case damage occurs..
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:rolleyes:

Charming, but not very useful. It's amazing how little metal it takes to hold something together. If lifted slowly, one control cable should lift a family car or a Bearhawk. Now compare the hinges to a cable.

I have accidentally flown with 25 degrees flap at cruise speed for fractions of an hour, on a couple of occasions. It isn't a problem. I have, of necessity in emergencies, considerably oversped with full flaps. Again, no issue.

I think it's a lot about usability rather than structural load. I believe Bob sets the flap speeds based on the handle and the quadrant, and how hard the pilot has to pull. I understand that's the first thing that is expected to break is the welds in the bottom of the handle, or break your arm more's the point...

Nev said:
Another consideration is the go-around scenario. Having 260hp will perhaps make the climb gradient a non issue, but it might be very easy to overspeed the full flap setting, especially with the full prop wash being directed at them.
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I don't tend to apply full power with full flap like you would in a cub or 172, unless I have a real emergency - in which case you're so slow and needing to climb so overspeed is not a concern - almost always the opposite, stall speed. If I am going around in a controlled way with no rush, I go up to about 65% and then bleed off some flap and re-trim, then increase power again and climb.
 
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I have to agree. Unless there are obstacles, on my go arounds I apply enough power to stop the descent, maintain altitude and controlled flight. I clean up the airplane then gradually add more power to begin the climb. The black knob on my 540 doesn't move forward very fast.
 
The limiting factor for flap speeds in certified aircraft is usually related to load at that speed at maximum load factor. If I recall, the limit load factor for most aircraft with full flaps is 2g, meaning they're designed for 3g assuming a factor of safety of 1.5. So, if it's not turbulent or you're not exceeding 60 degrees of bank in a turn, you won't start cutting into the structural margin.

The C172s in our club have exhibited some wear on the flap tracks, and one of them had cracking that required track replacement (not fun and expensive). That's usually the long-term effect of continual flap overspeed events - fatigue and wear, rather than sudden failure.

Point being, there should be some decent margin there. That said, (I think I read it on this forum, and I love this quote): "margin belongs to the designer." Meaning that it's there for the little oopsies that sometimes happen, not something anyone should normally count on in operations.
 
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For what its worth - we land 737's with "half flap" fairly often. And our max flap setting of 40 is only common when needed to land on shorter strips. Flaps 30 is by far the most common setting with flaps 15 preferred for high DA situations with long runways like Denver, CO. The primary rational for the lesser flap settings is improved performance in a go-around/climb-out, reduced flap wear, and lower airplane noise levels (you might be surprised how important that last point has become). Some pilots who are used to large airports find flaps 40 simply uncomfortable after using flaps 30 99% of the time and so reinforce the flaps 30 standard. I don't know how much of that pertains to a BH but since you mentioned commercial planes... :)
 
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nborer said:
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.
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Just reading some of your posts here - very interesting, and most of your assumptions are very close to my experience to date. I gather you probably have a lot of experience and knowledge in these areas.

On my Bearhawk, Flaps 1 & 2 appear to increase lift a small amount but not drag so much. Flaps 3 is a good all-rounder and Flaps 4 increases drag a lot - excellent for short landings at lighter weights. When combined with power on approach, flaps 3&4 reduce the stall speed significantly and at very low airspeeds result in a higher body angle. At altitude, with 20" MAP I've seen a pitch attitude of up to 28°.

Because the F4 speed restriction is only 55 KIAS, it becomes unusable at heavier landing weights. I frequently land using F3, particularly if carrying 3 POB or more. It allows me to carry a few extra knots on approach and gives better all round landing results - for a slightly longer landing distance. As others have mentioned already, it's very common for the air transport aircraft to use a reduced flap setting for fuel economy, lower flap wear, and where Landing Climb performance is an issue.

I placed my VG's at 5% MAC. (This was what the manufacturer recommended). My F4 power on stall speed at 2200 lbs is 38 KIAS (37 CAS) vs 40 KIAS power off - my IAS indicates higher at these high AoA's. Realistically I'm finding that when landing on the shorter rougher airstrips, I'm limiting my landing weight to 2200 lbs. This allows use of F4 and generally gives a very good landing performance. I use an approach speed typically of 50-55 KIAS depending on weight. This equates roughly to VS1.3. On short approach when at light weights the speed will commonly reduce to 42-45 KIAS.
 
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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.
 
Battson said:
How do you figure?

I have used full flap on almost every landing for a decade, so it cannot be "unusable"! ;)
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Well several reasons :

My B model plans state 50° - 75 mph. I clarified with Bob that this was an error, and should be 65 mph (56 kts, although I use 55 for simplicity).

Although the plans don't make mention whether the flap limit speeds are IAS or CAS, I assume them to be CAS to account for position error, otherwise it doesn't really mean much. nborer gives an excellent explanation in 15.1 of this thread where he states : "One culprit is IAS vs. CAS/EAS vs. TAS. At low speeds, pitot-static systems notoriously under-indicate airspeed." He uses the example from a C172 POH of a typical 13 kt spread at the stall between IAS and CAS at F30.

So in order to compare speeds we need to quantify the position error and use CAS.

On my own aircraft, when I'm flying 55 KIAS the CAS is (unusually) 1 knot lower (so yep, I could pull F4:cool:). However on many aircraft when flying at low speed, the CAS is often higher by 5-10 kts. So it would be easy to be flying an approach IAS of say 48 KIAS, but have a CAS that exceeds the flap limit speed.

I typically use an approach speed of 50-55 KIAS depending on weight. So at heavier weights I find myself right on the F4 limit speed and therefore use F3.
 
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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.
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I'm not sure about this. I just noticed that my plans had 3 flap speeds whereas the aircraft had 4 flap settings, so I called Bob to discuss. His reply was that the F4 limit speed should be 65 mph.
 
Nev said:
Well several reasons :

I typically use an approach speed of 50-55 KIAS depending on weight. So at heavier weights I find myself right on the F4 limit speed and therefore use F3.
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The crux of the discussion is around what speed the Bearhawk 4-place will fly safely, vs. pilots preference. Flown with care in the STOL mindset, approach at 1.1 Vso is certainly realistic, and for most 1.2 Vso should be very safe. Speed control should be within 1 or 2 knots on a stabilised approach profile on a fair weather day. I suppose 1.3 Vso is probably appropriate for a fast twin...? Not my area of expertise!

For a Bearhawk 4-place, say the stall speed is 44 kts @ 2500lbs (conservatively, it is probably less):
At 1.1 Vso the approach speed is 48 kts, full flap is usable.
At 1.2 Vso the approach speed is 53 kts, full flap is usable.
At 1.3 Vso the approach speed is 58 kts - you should have bought a C182... :p

I don't think CAS vs IAS is important at these airspeeds, unless an aircraft has huge instrument + position errors. At these speeds, it should be a non-issue. Having watching the GS for years, my CAS / IAS must be within a knot until I get below 42 KIAS - below that the errors ramp up quickly. It seems like we're the same in that regard, albeit yours reads over and mine reads under.

Before we get too scientific, consider the margin for error around the 55 kts full flap speed....
Also consider, are you actually achieving full flap...?
 
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Not meaning to derail this thread, wanting to open up the texture a bit. A purpose directed stabilized approach
is needed particularly for back country operations. It is derived from aircraft specific performance and applied based on runway environment, weather conditions and pilot skill and confidence.
I cannot apply one condition from my experience where I plan or reflect based on what Vs 1.x was used.
It is always a performance attribute that gives me feedback on my aircraft control and environmental variables. Often my go, no go decision is from fly through, turbulence, obstacles, respiration, stick grip,
raised neck hair, or the observed glide path.
A high priority is how does the movie out the windscreen look? Airspeed is a historical look at the past when seconds matter. If my glide slope, descent rate and aiming point converge in an unsatisfactory conclusion not sure what the Vs 1.x matters if the mission failed. Or asked another way, how happy would I be walking away from a pile of rubble knowing I maintained Vs x 1.2?

Kevin D # 272
KCHD
Marking calendar days till COVID goes away!
 
Completely agree - 1.x Vs is used for discussion purposes only.
Practically, a backcountry pilot seldom knows what airspeed the approach was flown at, too busy flying the approach.
I would hasten to add, that a slight overspeed on the Bearhawk flaps has no practical consequences for the airframe, so monitoring speed to ensure Vf4 isn't exceeded, isn't a flight safety consideration when compared to flying the aircraft.
 
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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.
 
Bob has just put out an update confirming these speeds for the 4-Place Model B, Patrol, and Companion.
 
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