Gross weight between patrol and bearhawk

[zkelley2]

The four place is the same in some ways. I cannot exactly measure the aft CG, but I suspect I have been more than close to the limit. Weight is harder in the 4 place, I put mine at 2,700 T/O max weight and I haven't been able to get that heavy. I would need to start over-eating.

But in other ways, its different to the Patrol as reported. When flying near the weight limits, the takeoff performance degrades remarkably at 2,600lbs compared to 2,100lbs. Every horsepower counts.

CG limitations aside, I've yet to read why the bearhawk was designed in the utility category. I haven't seen anyone pulling G's or doing anything acrobatic in them yet. Certainly not outside the normal category 60 degrees of bank and +/- 30 degrees of pitch. 3.8 is standard category, 4.4 is utility, and the bearhawk is 4.5 and I assume with a 150% ultimate factor. Ergo, at least while in flight the bearhawk should have the strength, in standard category, to fly at a weight of 2960lbs (2500*4.5/3.8).
There's usually a smaller C.G. Envelope for utility category as well.

[Battson]

CG limitations aside, I've yet to read why the bearhawk was designed in the utility category. I haven't seen anyone pulling G's or doing anything acrobatic in them yet. Certainly not outside the normal category 60 degrees of bank and +/- 30 degrees of pitch. 3.8 is standard category, 4.4 is utility, and the bearhawk is 4.5 and I assume with a 150% ultimate factor. Ergo, at least while in flight the bearhawk should have the strength, in standard category, to fly at a weight of 2960lbs (2500*4.5/3.8).
There's usually a smaller C.G. Envelope for utility category as well.

The weight is limited to 2,500 by the undercarriage travel. The springs will bottom out above 2,500lbs.

I bet you could take off at 2,960 if you put enough weight and kept it forward, but you need to lose it before you land.

Normal climb out on a light-weight flight is about 45 degrees pitch. Advanced stalls start at about 55 degrees nose up. Turning stalls we practice at some bank angles and nose attitudes which have to be seen to be believed. So I guess that is technically utility category!

[Battson]

The other comment I would make, is the fuselage obviously has to bend as you add weight. All metals / composites need to bend because of the stress to strain relationship. Above 2,500 lbs, the doors might not close as smoothly as they normally would....

[zkelley2]

The weight is limited to 2,500 by the undercarriage travel. The springs will bottom out above 2,500lbs.

I bet you could take off at 2,960 if you put enough weight and kept it forward, but you need to lose it before you land.

Normal climb out on a light-weight flight is about 45 degrees pitch. Advanced stalls start at about 55 degrees nose up. Turning stalls we practice at some bank angles and nose attitudes which have to be seen to be believed. So I guess that is technically utility category!

Ya, that's nuts. And at least in the US, you'd need to be wearing a parachute to legally execute that maneuver. (Anything outside 60* bank and +/-30 pitch). Some jets I've flown empty will do that to and it's not very comfortable from a - hey this doesn't seem normal perspective.

And on that note, how does it spin? Full power on stalls at 55 degrees, I'd think you'd get a torque roll at the stall break, with not enough rudder to counteract once stalled. Like the PC-12 does if you've seen the videos on that.

As far as the frame bending with more than 2500lbs in it, wouldn't that preclude it from the G-loading required for normal flight? 2500lbs at 4.5G's means the whole aircraft is experiencing 11,250lbs. It should not permanently deform or break at all at that weight. Not just the wings either. The frame certainly shouldn't be bending and putting stress on doors.

[Battson]

And on that note, how does it spin? Full power on stalls at 55 degrees, I'd think you'd get a torque roll at the stall break, with not enough rudder to counteract once stalled. Like the PC-12 does if you've seen the videos on that.

In short it doesn't, provided you build it straight.
We can stall at 65kts in a 80 degree bank turn with the nose 35 or 40 degrees up, and it just shudders and keeps turning. It really is a shockingly safe design. You can do incredible things and it lets you off the hook every time.

You can find videos of Eric spin testing his one on YouTube, but I think you'd have to force it in. You can stall with maximum unbalance and hold it there for three or four seconds and nothing happens, just a lazy drift toward the down-aileron side.

[PaulSA]

I know each country works under different rules, but I must say that I always find it difficult to accept the way in the US that people (as the "manufacturer") can declare the aircraft at whatever weight they wish. For example, I currently fly an RV-10. As 4-seaters go, it's pretty good in terms of weight. Nothing is going to let you take full fuel, baggage and 200lb people. The -10 comes close. I built with a reasonable compromise between lightness and comfort and ended up with a useful load of 1065lb. So I can take full fuel, 3 decent size people and 100lb baggage. Or cut the fuel load to 2/3rd (still enough easily for 400nm) and I can take 4 and baggage. Yet people build the Cadillac version, turning into a 2-seater, and then just up the weight to compensate. I've heard of one declared at 400lb more than recommended - 3100lb against 2700........

There is rarely, if ever, any proper work or testing done to justify this. People simply eat into the 150% safety factor and find some excuse as to why it is OK ("I'll limit the G to 2.5 until below 2700lb and use that as MLW" or "I've put a 300hp IO540 up front so I have plenty of performance"). In general, we are not aircraft engineers. We are reasonably competent with our hands and can follow some plans and/or instructions. We have no real idea about how the testing for our aircraft was done or exactly why it has a particular limit or what it is that fails first to limit that or what the critical component is. The 150% safety factor is there for the designer - not for the builder or pilot. It allows for less than perfect building and/or pilot technique. The Bearhawk family, in particular, does very well in terms of payload so my advice (as a 20,000hr pilot and Mech Eng graduate on his second build) would be build light and stick to the numbers. Or build the Cadillac if you must, but accept the payload limitations. But just arbitrarily adding 1 or 200lb to the Gross Weight? No ..........

Personally, I am going for a fairly "utilitarian" interior using fabric and a 200hp IO375. I hope for an empty weight of 1250-1300lb. This will easily give me full main tanks, 4 large people and 100lb baggage (W&B allowing, of course). If I can get towards the bottom end of the estimate and have a couple of skinnier pax, then I can fill the aux tanks as well. That is one REALLY useful aircraft - within the designer's weight limit.

OK - flame suit on .......

[zkelley2]

You have to remember, there's more than just plans and kit built aircraft being built. You can dream up any design you want, build and fly it. You can modify any plans or kit however you want. As far as the FAA is concerned, a 51% kit built aircraft and a 100% from scratch, dreamed up one night and written on a napkin are the exact same thing. Experimental - Amateur Built. They both come with a special airworthiness certificate and mostly the same operating limitations.
As the manufacturer you can do far more than just increase the gross weight on paper, you are the manufacturer, you can do literally anything you want provided a DAR will sign it.
If I wanted to buy a bearhawk fuselage and cut into it and make it have low wings, I can.(A Pawnee Bearhawk!) If I wanted to mount cub style fabric wings I could. Very quickly it's not a bearhawk anymore and the original design and margins are well out the window. People actually do this with wrecked cub fuselages a lot. Make something crazy out of them.
But when you go to get an airworthiness certificate for your contraption, you're the manufacturer, so you should know best. If you're not comfortable with that much leeway, that's why there's placards all over the thing warning passengers.

Increasing the weight is just the tip of the iceberg when it comes to modifications to experimental aircraft. And that's what's great about them.

[zkelley2]

The very different thing about the bearhawk though, is there's already a 18% "safety" factor(and then 150% on top) by it being slightly stronger than utility category, at the normal MTOW. There's the weight penalty for building it that strong, why not take it back? I can't come up with any reason to need 4.5Gs in this plane, it's not aerobatic. The limiting factor of course being the landing gear.

If it's just the shock travel at the G loading of a crap landing required by testing, that's really easy, land better(haha) or go with something like an AOSS or TK1 setup and call it a day. But holy crap are those things draggy. What needs to be figured out is the strength of the two points the shock strut connects to and make sure they're up for it as well.

[Thyfish]

" I can't come up with any reason to need 4.5Gs in this plane, it's not aerobatic. The limiting factor of course being the landing gear."

You can always use more Gs positive or negative regardless of whether you're doing aerobatics in my opinion. It gives you a better safety margin and rather than many people that operate at the weight limits and put themselves in real danger in an unforseen upset. No matter how good a pilot is, they can't fly an aircraft when the wings fold up or they lose flight controls. Read this article on a bearhawk 4 recording +4.8 and - 3.5. Sure in hindsight they could have slowed down but there were no indications of turbulence. I'd rather the structural integrity for something like this.

https://www.kitplanes.com/mr-bearhawks-wild-ride/

[zkelley2]

I have thousands of hours in aircraft with a +2.5G limit. It's more than fine. Almost every other GA airplane out there is +3.8G and the wings aren't folding up on those. You get a 150% of that before anything happens anyways, so even with the +4.8 event, should it have been a Cessna or any other GA airplane, the plane should have still held together.

[PaulSA]

The very different thing about the bearhawk though, is there's already a 18% "safety" factor(and then 150% on top) by it being slightly stronger than utility category, at the normal MTOW. There's the weight penalty for building it that strong, why not take it back? I can't come up with any reason to need 4.5Gs in this plane, it's not aerobatic. The limiting factor of course being the landing gear

That may well be the case. In which case, it would be up to Bob as the designer to say that it is acceptable to increase the gross in the utility category to (say) 2700lb. It's not (IMHO) something that an amateur builder is qualified to do. Are you SURE it's the gear which is the limiting factor? - in which case it would be very reasonable to increase the gross with a LW of 2500lb. Or is it a fatigue consideration on a wing strut? Or a whole number of other things? You see my point? The problem is that people increase the gross on the basis of hearsay or pseudo-science without PROPER investigation or testing.

Now, I don't intend to do aeros so I would love it if Bob would publish a document that raised the gross to 2700lb (2500 LW) with a 3.8g limit. Bob? Mark?

[Battson]

Note that Bob has APPROVED a 2,700 lbs take off weight for the 4-place Alpha model.
It's worth knowing all the facts before going too far down the rabbit hole...

[Mark Goldberg]

Paul, I do not disagree with your thinking or comments. As a kit manufacturer we have to tell builders to build it like Bob designed it and stick to the plans. Not talking about doors/windows, or non structural items. Just structure which an allowed gross weight is in there somewhere.. But it is also true that the design criteria of utility category strength at full gross does give a little more strength margin than almost everyone else who has gross weight calculated to standard category. But within reason.

Rollie's 100 lb gross weight increase over Bob's spec'd 2000 lbs is not too far off of what Bob recommends.Rollie is a professional pilot whose Patrol has 31" Bushwheels which cushion landing much more than standard aircraft tires. My point is that Rollie's decision was unique to him and his airplane's specific situation.

But builders who just up their gross weight with no regard to the designer's criteria and have lesser pilot skills are taking risks that they probably shouldn't. Even if they have the right to do so under the regs. Mark

[PaulSA]

With respect, Mark, you have just proved my point. “31” tyres cushion the landing”, “Rollie is a professional pilot”, “2100lb is not far off what Bob recommends”. None of this has any engineering basis.

I understand, of course, that this is all allowed under US regs ....... I’m not trying to criticize any individuals - I’m just saying that the Bearhawk aircraft already have a great payload capability and you don’t need to screw with it. There are no light aircraft that can carry full fuel, pax and baggage but for some reason home builders feel they should be able to.

My RV-10 is not a Cadillac - maybe a BMW. If you want comfort, IFR, autopilot etc then this is what you build. To my mind, the Bearhawk is a STOL bush aircraft - build it light with just enough comfort so it keeps you happy and comfortable. Anywhere between a classic Land Rover and a Toyota Land Cruiser.

[Mark Goldberg]

Your are correct in your response except for one thing. The big tires for sure do make a difference in the load the plane feels when landed. They cushion impact quite a bit. We are talking loads and stresses on the landing gear and fittings etc. So while I am not quantifying how much load the Bushwheels absorb, it for sure does decrease the chance of damage on landing. But of course it is still possible to damage a plane even with Bushwheels. Mark

[zkelley2]

With respect, Mark, you have just proved my point. “31” tyres cushion the landing”, “Rollie is a professional pilot”, “2100lb is not far off what Bob recommends”. None of this has any engineering basis.

I understand, of course, that this is all allowed under US regs ....... I’m not trying to criticize any individuals - I’m just saying that the Bearhawk aircraft already have a great payload capability and you don’t need to screw with it. There are no light aircraft that can carry full fuel, pax and baggage but for some reason home builders feel they should be able to.

My RV-10 is not a Cadillac - maybe a BMW. If you want comfort, IFR, autopilot etc then this is what you build. To my mind, the Bearhawk is a STOL bush aircraft - build it light with just enough comfort so it keeps you happy and comfortable. Anywhere between a classic Land Rover and a Toyota Land Cruiser.

Paul, I agree that you need math to support any increase, not feel good stuff, that probably adds margin, but who knows how much. Certainly not pilot skill.

And that's where I'm coming from. If the airplane is built to 4.5G's (.1 over the utility category), that means, by definition, that in flight it can withstand a weight of 11,250lbs and not permanently deform or break.(no rivets pop, nothing.) There's a 150% margin on that, called the ultimate load, where between 11,250 and 16,875 things can deform past where they go back to their original shape. Elastic to 11,250, and then plastic deformation is allowed from there on, but nothing should break until 16,875 or more.

Those weights are the weights/force the aircraft experiences. 4.5*2500=11,250*1.5=16875. The aircraft, every single one of it's components in flight must be strong enough to withstand that force to call it 2500lbs@4.5G.

If we increase the weight to 2960@3.8G, here's the math again. 2960*3.8=11248*1.5=16,872. The same force.

Then if we want to keep things to a certified standard, you'd need to look at climb rate at those weights. Part 23 requires 8.3% climb gradient out of ground effect. At 60kts 8.3% is 888ft/min, from most accounts that would still be possible at that weight. (Note this is usually the limiting factor on a lot of small planes, not the structure, which is why bigger engines so often come with gross weight increases.)

Testing for landing gear is different however and involves specifically vectored forces acting upon the gear and frame. I'd have to look up the testing requirements and load factors on this. Van's actually has a good video of them doing this testing on youtube.

https://www.youtube.com/watch?v=KbFMogBNUa0

Now that all said, the bearhawk is not a part 23 airplane, and I don't know for certain that it was engineered entirely within the scope of part 23. For example, if the wings can withstand the full 11,250(these are the most stout built wing's I've ever seen on a airplane this size, I have no doubt they can) but the attach points cannot, well then, you'd have wings coming off or attach points deforming if anyone actually managed 4.5Gs in the thing.

[whee]

Personally I don’t see that there is anything to talk about in regard to the 4-place gross weight. Bob designed it for 2500lbs and has approved a 2700lbs takeoff weight. There is no SWAG going on, no builder doing something that the plane wasn’t designed for. If Bob was ok with a gross weight higher than 2700lbs he would have said so. We know the reason for the max landing weight; if you want to know the reason for the max takeoff weight (2700lbs) then call Bob. I personally don’t care what the reason is because it’s going to be hard enough to burn off 200lbs of fuel as it is.

[Zook]

Sorry. I didn’t mean to start an argument. Thanks for all the input. I’m sure from what I am hearing here that the patrol will do what I need. Most of my flying will be on floats so I can store some things there. Happy building

[zkelley2]

I hope this isn't an argument. That's certainly not the way I intend to come off. Just a discussion on the merits for/against an increase in gross weight. That's what forums are all about. Experimental is way more fun because of all the cool things people come up with.

[zkelley2]

Personally I don’t see that there is anything to talk about in regard to the 4-place gross weight. Bob designed it for 2500lbs and has approved a 2700lbs takeoff weight. There is no SWAG going on, no builder doing something that the plane wasn’t designed for. If Bob was ok with a gross weight higher than 2700lbs he would have said so. We know the reason for the max landing weight; if you want to know the reason for the max takeoff weight (2700lbs) then call Bob. I personally don’t care what the reason is because it’s going to be hard enough to burn off 200lbs of fuel as it is.

Experimental is for more than just building. If there's 2960lbs worth of strength in the fuselage and wings, I'm not going to pay the weight penalty of carrying around that strength without using it's benefits. It seems like the landing gear, and specifically the shock struts are the weak link. That's very easily fixed.

I know I saw a beartracks article on load testing, I'll need to go back and re-read it.

But that said, my floats are 2700s.

[zkelley2]

Found the article in 2002 Q4 beartracks.

They tested the wing and found no permanent deformation at 5.7G, failure at 6.4G.
6.4G/1.5= 4.2G
4.2*2500/3.8=2807lbs for standard category and a 150% ultimate load factor while not having any problems at that 3.8G.

Seams reasonable if everyone is already comfortable with that 2700lbs, which gives you 3.95G limits.

[jaredyates]

There was an interesting article by the designer of the Vans airplanes about builders specifying max weights above his limits. I think it ran in Sport Aviation but here is a thread about it over at Doug's place:

http://www.vansairforce.com/community/showthread.php?t=73492

The meaty bits:
Who Owns the Margin?

It seems common practice among homebuilders to second-guess the factory engineers, particularly regarding gross weight increases.* Because of all of the “I gotta have” added features, empty weight creep erodes the aircraft’s useful load.** The simple solution for the homebuilder is to “pencil in” a new gross weight limit.* It’s only 100 lbs. (3.7%) more; how much effect can that possibly have?”* Imagine this example: you are on a mid-size airliner with a gross weight of 270,000 lbs.* Just before leaving the gate, the captain comes on the PA system and says: “we’ve overbooked more than usual today, so we’re going to assume that the factory engineers over-designed this airplane and allowed an abundant safety margin. We’re going to take off at 280,000 lbs. instead.** So move over, there are 50 more passengers coming on board.”* Run the numbers; it’s the same over-weight ratio as simply pencilling in an additional 100 lbs to the gross weight of an RV-10.

Along with gross weight increases, some builders take the same liberties with horsepower increases and speed increases, betting their lives on the assumption that the airplane is designed with a huge margin of safety---it is really far stronger than in needs to be.* This is not really true.* Certificated aircraft, and well-designed kit aircraft, are designed to withstand limit loads at specified maximum weights.*** During testing, they are subjected to ultimate loads, which are higher than design limit loads by a specified margin.* Yes, there is a margin between the design and ultimate strengths.** But that margin belongs to the engineer.* He owns the margin.* It is his insurance against the things he doesn’t know or can’t plan for, and the pilot’s insurance against human error, material variations, and the ravages of time.* Wise pilots respect this design safety philosophy and leave this insurance policy in effect by operating strictly within established limits.* They don’t try to steal the margin from the designers.

[svyolo]

Couldn't agree more. The margins also represent a statistical scatter that if you build to a certain safety factor, the chance of a failure is slightly above nil if you stay within the limits. As you exceed them, that chance of failure goes up dramatically.