STOL Performance Observations

[Battson]

I was looking for a thread summarizing STOL performance and slow flight, without success, so this could be such a thread.



We had a run of clear and calm mornings lately, so while out flying, I took the opportunity to do some slow landings to re-confirm the STOL performance of our Bearhawk 4-place. With the aircraft modestly loaded, around ¼ fuel and one person, the Bearhawk was running out of lift at ~37 kts GS (almost stalled) and touching the ground at ~33 kts GS allowing a half-second for measurement. Approaches can easily be flown below 40 kts indicated and GS. I'll explain the method below.

As a preface, this was an opportunistic performance check for interests sake, not a pre-planned exercise. So hold any questions on experiment design - this is only a practical check. My method was rounding out and holding off during the flare using a small amount of additional power, and using the GPS speed to confirm slow flight speed at steady-state pre-touchdown. I did landings on several different different vectors (the vectors are arranged in a triangle) to help identify the effect of any imperceptible breeze. All landings considered, it seems the practical results are +/-1 knot. Ambient conditions were about 200 ft AMSL, with temperature and pressures approximately at ISA for that altitude - which was convenient!

It’s hard to fly an aggressive STOL landing safely, with eyes inside the cabin monitoring speeds, while holding the aircraft at steady state pre-touchdown. Holding it a couple of feet off the ground, in other words. For that reason I didn't do an aggressive STOL landing, just an "average" STOL landing with a small margin for safety. So these results are not the best possible result, like you would expect in a STOL contest scenario. The plane can certainly land slower if pushed to the limit.

Note that our aircraft has extended hoerner wingtips fitted, not the common rounded wingtip.

So to summarise:
Approaches 38 kts
Rounding out / flaring at 37 kts
Touchdown somewhere just above 33 kts, without pushing it too hard.

Of course the safety caveat - each homebuilt aircraft is different, has different performance characteristics, and air conditions do vary. Please fly to the ambient conditions and your own aircraft's limits!

I think this is awesome performance for an honest 4-place aircraft.
Over the years, the 4-place has certainly allowed us to keep up with Super Cubs, in terms of landing on the shortest gravel bars etc. Given the high cruise speed and low-end STOL performance, it's hard to beat.

The best fun is had when very lightly loaded, with a cold high pressure morning, and a steady headwind. The plane can do some truly amazing things on those special days. For instance, a 30% climb angle with decent headwind is practically achievable from a standing start - measured over the ground, not through the air. Again, this is about practical or "useful" STOL performance, rather than a highly accurate measured result. Being able to land two full grown men and a decent load gear in a 60 metre (200 ft) space is a lot of fun, and challenging. Lately this has proven useful, on a couple of occasions.

[Utah-Jay]

Great stuff, I would be more than happy with an approach at 45mph and touching down at 38mph.

Can you give us some ballpark numbers on takeoffs?

[Mark Goldberg]

I would like to caution everyone that Jonathan is an exceptionally capable pilot. With 800+ hours in his plane. His BH also has VG's as well as the Hoerner style wingtips which both help lower stall speed some. My point - don't assume you can safely use the numbers Jonathan reports. Pilot skill is the most important factor in this STOL flying.

Another situation where pilot skill made a big difference: Bob Barrows & I were coming back from a trip to the kit factory in N303AP. When we got back to my home airport, the winds were 60-90 degrees crosswind from the right 25 kts gusting to 30 kts. Bob was flying fortunately. He planted that right wheel on the runway and made it look pretty easy. If I had been flying it would not have turned out as well. The plane is plenty capable. In a lot of cases, pilot skill is the limiting factor.

Just a cautionary note for you guys to practice before exploring the edges of what is possible. Mark

[Battson]

Can you give us some ballpark numbers on takeoffs?

Well - that's a tougher one to investigate for a casual observer like me Having full power applied is a large part of the take-off equation, so holding the aircraft at steady state to make observations isn't an option. It happens very fast in a IO-540 powered ship.

Horsepower is a huge factor here, but ultimately thrust (ergo horsepower plus prop selection) and weight have most influence for given set of ambient conditions. Unless there's long wet grass or similar reducing acceleration...

In the conditions mentioned in the original post, I would expect to see the aircraft breaking ground somewhere between 30 and 40 metres (100 to 130 ft) ground roll, and developing a strong positive rate of climb almost immediately. During takeoff, airspeed is changing so quickly that its hard to make any coherent observations about airspeed. We are using the Hartzell Trailblazer prop with two blades. According to the Lycoming performance charts, at our measured manifold pressure and RPM limit, we are seeing between 270 and 275 horsepower under standard atmospheric conditions. We do get noticeable ram air effect, which helps.

Clearly takeoffs become a lot of fun as conditions become more favourable. A headwind kinda breaks the conversation, as in theory you can take-off on the spot with a steady 35 kt headwind.
With a steady headwind in that comfortable range between 10 and 20 knots and favourable ambient conditions, takeoffs can be shorter than 25 metres (80 ft), or about three plane lengths - as lightly loaded as can be.

A three-blade carbon prop with wide paddles should further improve both take-off and landing distances, offering both more thrust and more drag.

[Utah-Jay]

Battson thanks for the response

[BradW1062]

Battson - is your Bearhawk an A model or a B?

[Nev]

Jono's is an A model.

[BradW1062]

OK, good to know, thanks.

[Battson]

Battson - is your Bearhawk an A model or a B?

It's an interesting question, for me. Does it actually matter?

Increasingly I am wondering which model (A or B) has superior STOL performance - or indeed, if there is any appreciable difference in the aerofoil alone. Other factors like weight, VGs, wingtips, build quality, and pilot inputs probably make a greater difference than the model does. I am increasingly convinced about this.

I would also note for completeness, I modified my aerofoil to increase camber, by lowering it at the trailing edge. My trailing edge sits about 1.5-inches lower than the template, from memory.
This isn't a big deal in practice, the ailerons are reflexed up slightly in flight due to wind loading, and the flaps are obviously deployed when landing. The wing roots and tips are the only parts not subject to changing effective camber.

[jaredyates]

I would think new airfoil differentiates more at the top end of the speed range than the bottom, and I do think the other factors Jonathan talks about, especially weight and pilot input, are easily overshadowing the airfoil difference at low speeds.

[BradW1062]

Battson, that is exactly why I asked the question.... I would like to know how they actually compare, but it's a difficult comparison given all the builder variables.

I would be interested also to know a little more about why you further modified your airfoil. And how, since just "lowering the trailing edge" isn't something everyone would do the same way. How far back into the wing did you take this mod? It would appear to me that your airfoil is really no longer a 4412. The camber is a good bit more with 1.5" of trailing edge droop - that probably translates to a couple of percent at least, making it more like a 6412. Now I will have to experiment with the computer some......thanks. This is actually good to know, because now I can't use your results in the original post as representative of the normal A model 4412 wing.

But I can say this with a fair amount of confidence - there's a actually pretty big difference in the airfoils at low speed, where the Riblett airfoil gives up a good bit of lift compared to the 4412, all for a small gain in "trim drag" at high speed - about a 35% lower pitching moment, maybe 400ft-lbs, or about 27# at the tail at 150mph roughly. I was really sort of shocked that Bob made this change given the already proven performance of the original Bearhawk, especially in slow flight, but I have never had the opportunity to talk to him about it.

I guess the reality is, as Jared says, that most guys probably won't notice this difference at low speed because all the other factors are masking the real differences at low speed. Further than that even, since they don't fly the two models side by side, and they compensate for the loss of lift by using the huge flaps to land anyway. I would be interested in knowing how the two models perform without flaps, and slipping to land instead. What is the difference in a power-off clean stall? I guess most probably never really practice this, since you are typically programmed to practice power-off stalls with flaps in landing configuration (Vs0), and power-on without, in clean climb condition (Vs1). But it really would be interesting to know.

Someone, I don't remember who or what thread now, posted that they had a concern with their loss of elevator effectiveness without power on slow approach. So they always used power on approach. That is pretty alarming to me. If you are having to make a dead-stick landing in a field somewhere, the last thing you want is to loose elevator authority as you slow as much as possible. My gut is that this could be an effect of the change to the lower moment Riblett airfoil with it's different incidence, and the reduction in the standard incidence of the horizontal stabilizer along with that. So the glide pitch stability is maybe not as good? At least it appears the flaps may be blanking the elevator some. But since I don't remember who it was, I don't know if theirs was an A or B or even a Patrol.

Anyhow, inquiring minds *always* want to know...

Brad

[rodsmith]

In your airfoil comparison, don't forget the A model uses a 4412 airfoil modified with leading edge droop. Bob changed to flaps that extended to the fuselage concurrently with airfoil shaped horizontal and vertical stabs to maintain elevator effectiveness.

[Battson]

Naturally elevator authority reduces with airspeed, but there's always control. Control is relative, in a seriously turbulent day - serious windshear can easily overpower any Bearhawk control surface (or other light single engine GA plane). But you need a LOT of weather.

I lowered the trailing edge using the flaps and ailerons, wing root piece, and my custom-made wingtips. The profile forward of those parts is unchanged.

[jaredyates]

Interesting thinking Brad. My short answer would be you're worrying about things that don't present as problems in the real world, but it's good to think things through.

Here are some additional thoughts. I have around 450 hours in one Bearhawk which had the original wings, and then around 100 hours in a model B. I routinely/usually practice power off stalls without flaps and without engine input in the entry and recovery, but the reality is that it can be really hard to know what is actually happening with airspeed in that regime. As you know, pitot errors really start to stack up at high AOA, because we don't optimize our pitot tubes for those lowest speeds. I would think any differences in Vs are going to be smaller than the pitot errors, even with the best CAS correction that we can do.

In both airplanes, elevator effectiveness is highly variable based on thrust/propwash/airflow over the tail. Elevator needs in the flare are also highly variable based on CG. This is something that is very easy to play with and experience at altitude. At a forward CG, if you establish a steady engine-idle glide, then maintain altitude until the point of a stall, you will probably not get a clean break and pitch down. Instead it will mush, which I propose is because the airflow at the tail has separated. The wing hasn't fully separated, but we don't have any more elevator effectiveness available to further increase wing AOA. However, if instead you run the engine up about where you would usually do a magneto check, and then slow to a stall (you'll probably have to climb) you will get a very pronounced break. The elevator has a totally different feel. I don't perform power on stalls with much more power than this, and certainly not full power. The deck angles get extreme.

We did some tuft testing on our original airplane just to see what the tail was looking like during power off stalls, with and without temporary gap seals. Here is the video:

https://youtu.be/NB0kh3Z4OVM

It's not all that scientifically rigorous but it supports my theory.

As the CG moves even a little aft, the pitch situation begins to change. At the aft CG limit, it's a totally different airplane. The elevator force and pitch stability are reduced, and the stall is easier to achieve. At forward CG you really have to work to make it happen. This is why it is crucial do to a thorough flight test program in Phase 1 testing or otherwise. We approach the limits incrementally.

Something else to note is that the Model B changes also included a new horizontal stabilizer airfoil shape, which Bob estimated to have around 20% more effectiveness. Some folks have made this mod even with the original airfoil and there doesn't seem to be any disadvantage.

Regarding the pitch authority in a dead stick landing, here are some thoughts. The short version is that there are many different forced landing scenarios and each will require its own decision making. Thankfully the airplane does provide us with several options, compared with some of the slick little composite airplanes where plan A is bailing out with a parachute. Off airport crashing capability is one of our key mission elements in owning a Bearhawk.

The Bearhawk has a very handy high-drag/high-sink mode that for me starts around 55-50 knots indicated, and the slower you get, the "more" it is. I usually demonstrate this in transition training. We remain intentionally very high on final, then slow way down to just above stall speed. We are way back behind optimum L/D so we have high induced drag. Also, because we are moving much more slowly across the ground, any headwind aloft has more time to apply its effect. The sink rate gets quite alarming. If you try to carry that all the way to the ground without any engine power, it's going to be a hard landing, maybe breaking something or maybe not. There isn't any flare energy left because you are so close to the stall, so you need to bring in some extra energy. This can be done by adding some engine power in the flare, or by increasing the speed on final before you get down low. If the intent is to steepen the glide angle, this is highly effective. You can be back at 50 knots and high on the final, and if you dive down to 60, you'll gain some energy that you can later devote to the flare, but you'll also lose more altitude in that process. If the engine was not available, the second option would be the only choice.

If we apply that to an engine-out situation, particularly off-airport, you have some choices to make. If the wheels aren't going to be able to roll, then reducing kinetic energy is the most important. Say you are setting down in a wide-open swamp, or even open water- you will want to keep best glide (65-ish) all the way into the ground effect, then hold it off until you run out of stick. You don't care how long you float in the flare, you just want to get rid of every knot you can before the wheels grab and dig in. Maybe this happens at something like 45 knots (totally hypothetical). When the tailwheel hits, the mains might be a foot or two over the surface. Contrast that to an intentional ocean ditching with engine power- if you enter the ground effect at 50 knots and 1500 RPM, then hold it off until you run out of stick, then maybe the tailwheel hits at 40 knots, with the mains three feet over the surface. The pitch angle is higher, the AOA is higher, and the speed is slower for the same amount of lift. Both of those are totally survivable scenarios, but it is true to say that because you didn't have any thrust/propwash available, you didn't fully minimize the speed as much as you could have if you did have power available.

Both of those scenarios are also highly improbable unless you are flying over the ocean. Where I fly, a far more common scenario is landing in a spacious agricultural field or a road. In those cases, I don't want to put the airplane into the high-drag 50 knot mode up high. I want to enter the ground effect at best glide so that I'm sure I have plenty of control getting there. I still want to minimize airspeed before touching down, but I want to use skills and luck to position myself in a place where I can still afford to float some.

What if the only option is a tiny field? In that case, maybe you do want to consider a high-drag high-angle approach, but I'm not so sure. I would be planning on an airframe-consuming collision. Maybe you drag it in really slow and plan to splat, probably breaking the gear and but also risking spinal injury due to the vertical vector. In that case I start to wonder what the alternatives are to that tiny field. If it is trees, maybe the trees are a better choice. They are also going to consume the airframe, but they are going to spread the deceleration over plenty of time.

The Bearhawk flaps are primarily drag creators, which makes them very useful for steepening an approach over an obstacle. But like all flaps, they do also increase lift and reduce stall speed a little. I can't imagine a forced landing scenario where I wouldn't endeavor to get the flaps extended before ground contact, but maintaining control of the flight path would be the primary task for sure. I wouldn't want to get distracted from that just to put in flaps.

Those are just a few of an infinite number of possibilities, but the point is that while yes, you do lose elevator effectiveness at low speed, I think there is a very limited need to be concerned about it being a measurable safety risk in a forced landing.

Also, editing to add, you are more than welcome to come fly with me sometime if you are in the area and we can explore more of these scenarios.

[BradW1062]

jaredyates - thanks for that reply - that's very good stuff to know. I don't presume to dispute any of your points, and since I have zero time in a Bearhawk (other than once as a backseat passenger in Mark's) I really have no idea how they fly other than what people say.

I'm not really "worrying" about any of it, I was just surprised that lack of elevator authority was reported, it is not something I would have expected. I would have expected strong elevator authority with the large area back there, and easy to get a full stall power-off. I guess I missed that point somewhere along the line. Maybe the ribbed horizontal stabilizer and elevator helps that some? I am well able to do that on my airplane if it's worth the effort.

I would like to come get some time with you someday, probably when I get closer to done - but who knows when that will really be? My most recent and current experience is in my Champ, which of course, I almost always slip to land unless I am low. And because I like to fly that way . Since it's such a light plane to begin with, and I usually fly by myself, elevator authority has not really ever been an issue for me.

I also wasn't aware of the leading edge droop in the airfoil that rodsmith mentions - that's the first I have heard of it. It's not obvious from the plans. I guess I need to call Bob to pick his brain a bit...

Brad

[Nev]

Brad, I have discussed the lack of elevator authority on occasion. I have a model 4B. To clarify, it becomes a factor when the CG is forward of 14", and the airspeed is below 55 KTAS, when power is at idle. If any of these conditions are not met then it is unlikely you would notice it. Having the airspeed above 55 KTAS or power on, gives sufficient airflow over the elevator to retain authority, and with CG further aft then less elevator required anyway.

It's noticeable when flying a slow approach at forward CG, particularly if high on profile, and the power is reduced to regain profile. The nose begins to drop in a rather docile manner, but it can certainly get your attention at low altitude, and it feels similar to a docile stall. Likewise, when performing power off stalls, it's very easy to confuse a loss of elevator authority with a very docile stall. Usually, with a forward CG, it is difficult to get the main wing to stall at all without adding power. Obviously at mid to aft CG it will stall perfectly well and a normal wing drop is experienced.

This phenomenon is not isolated to the Bearhawk. The early C182 was known to experience the same characteristic, and it is common among STOL aircraft due to the lower approach speeds, especially with wide CG envelopes. With an aft CG envelope it is unlikely to be observed.

I carry a 50lb kit in the aft baggage area when I'm operating light, and that solves the issue. I haven't flown a BH5, but I believe they have a ballast rack installed, probably for the same reason.

[BradW1062]

Nev - thanks for that reply, it was probably your post that I remember.

After thinking about this some more, it seems that maybe this is just where you are already sort of "in the stall"? Where the controls become mushy, or less affective in general? But there's not enough elevator left to actually get a break and full stall? This may actually be a very good effect in a way. I think I remember Budd Davisson's pireps where he described there being a mushy stall in lightly loaded conditions, but that you could continue to fly the airplane around with a little power. Maybe this is the same sort of thing he described back then. I need to go back and read those again.

If you have this condition on a slow approach, where the nose is dropping power-off and you can't raise it with the elevator, I understand, to clarify what you said, that you can just put the nose down a bit, or let it down a bit (relax the elevator some) and pick up some speed (you mention 55 KTAS) to regain authority? So, effectively, it would result in your approach and landing being at just a little bit higher speed? Or, as an alternate method, a bit of added power actually allows a slower approach because the improved elevator authority allows a higher AOA? Sorry if I'm not clear I'm just trying to understand.

Thanks
Brad

[Nev]

What engine will you install Brad ?

[BradW1062]

I have an O-470-M and CS prop off of a Cessna 310. It has the PS-5C throttle body injection - but I have to do something else for the Bearhawk, so I was thinking a carb or maybe aftermarket fuel injection. So yeah, I know, it will be heavy - maybe even a little more than a O-540, more like an angle-valve IO-540 probably.

[Nev]

There's no silly questions Brad. There's no harm is being conservative with the flying, especially early on. The BH is very capable, and will land plenty short enough without having to fly very low speeds.

[Nev]

Nev - thanks for that reply, it was probably your post that I remember.

If you have this condition on a slow approach, where the nose is dropping power-off and you can't raise it with the elevator, I understand, to clarify what you said, that you can just put the nose down a bit, or let it down a bit (relax the elevator some) and pick up some speed (you mention 55 KTAS) to regain authority? So, effectively, it would result in your approach and landing being at just a little bit higher speed? Or, as an alternate method, a bit of added power actually allows a slower approach because the improved elevator authority allows a higher AOA? Sorry if I'm not clear I'm just trying to understand.

Thanks
Brad

I should add that engine weight has a large effect on this. I have an IO540, and my empty CG is forward enough that elevator authority can become an issue. With a lighter engine it's likely that the empty CG will already be far enough aft that elevator authority is maintained throughout the flight regime. Most IO360 equipped Bearhawks won't see this issue.

If elevator authority is lost say on approach, the easiest way to regain it is to simply add power, and the nose will lift. If you attempt to regain elevator authority without power, I personally wouldn't lower the nose further deliberately because you'll already be pointing at the weeds. Unlike a stall, you don't have to "unstall" the wing, you just have to increase airflow. Because the nose has already dropped (like a pendulum) it becomes self correcting. The issue is having sufficient height to recover. It can take 2-300ft to safely increase airspeed to a point where authority is regained, and the nose can be lifted. As you mentioned above, flying an approach with CG forward of 14", at a very slow airspeed, is fraught with risk because if the engine fails you may be too low to safely recover. Because of this many of us fly approaches at a higher airspeed and only reduce airspeed when on short finals for minimum exposure. My own technique when practicing a power off forced landing is to fly 70-80kts throughout the approach, and never let the airspeed decrease below 60 kts until flaring.

There are 3 ways to mitigate against this.

1. Keep the CG aft enough. Typically installing a lighter engine will ensure this. Alternatively adding some sort of ballast well aft like a survival kit in the baggage area will do it as well.
2. Fly at a speed that ensures sufficient airflow is kept over the elevators. Typically an approach speed of 55 KTAS or higher works well. Edit to add that with a glide approach it would pay to keep a minimum of 60kts and preferably higher.
3. Keep some power on during the approach, which keeps prop wash over the elevators.