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Such an interesting read, thank you to all for your input. This is exactly why I added 10 brackets/attach point for a cargo pod that will be able to carry 250 lbs. it will mainly be under the pilot seat , and lighter stuff under the rear seat. This way you can load smarter, nor more. I added the aux tanks and with them full I would like to be able to load 2700 lbs, and keep it away from the rear cg limit. The cargo pod will come with a penalty of 25 lbs ( carbon fibre ) but the benefits will be more stability at max gross weight. With more of the weight on the mains and less on the tail wheel.
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I’m putting a pod on my airplane too but mostly because bodies fill the cabin so I need a place for cargo. Because of the land gear configuration the pod has to be push back quite a bit so the majority of the cargo will be under the middle row passengers. This is still better than in the cargo area but I’ll still be limited by CG. What I need is a nose locker or a heavier engine. -
You need a IO540, I think its the perfect combination for a balanced aircraft
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Expanding some on Zach's comments above and revisiting the classic ground school topic, stability has at least two flavors. The first is the static stability. Considering pitch, if you trim the airplane for a certain steady airspeed, then make a pitch change and release the stick, what happens? A plane with positive static stability will pitch opposite of the disruptive input. A plane with neutral static stability (or an Airbus) will stay at the same pitch, and a plane with negative static stability will pitch further in the same direction as the input. If the static sability is anything other than positive, then the next discussion about dynamic stability is moot. Dynamic stability relates to how the corrective pitches oscillate over time. If the plane corrects from the initial disruption, but oscilates to smaller and smaller pich changes, then it is also dynamically stable. If the oscilations remain constant, then it has neutral dynamic stability. If the oscillations get bigger, then it has negative dynamic stability. For a plane to have any type of dynamic stability it must have positive static stability, otherwise how could we talk about the nature of its self-corrections if it dodn't self correct in the first place.Originally posted by Utah-Jay View Post
The same concepts apply in all three axes. For example, the Bearhawk, like most planes its shape and including the Cessna trainers, tend to have variable roll stability depending on bank angle. At low bank angles they are stable, but at high bank angles (60 degrees maybe?) they are unstable and will tuck over into a tighter turn.
The Bearhawk still has positive yaw stability in the air, but much less than a trainer. And on the ground, well, you know how that goes for any taildragger.
As CG moves aft from the forward limit, pitch and yaw stability decrease. Continuing aft, eventually pitch stability will reach zero and then go negative. Depending on how bad this gets, if you are experienced and have the bandwidth to process it, you can probably still keep it in the air, but imagine having a workload in pitch sort of like your rudder workload on the ground at 40 knots. In IMC, you have an emergency. This is why we test incrementally. Move the cg back a little and fly some, then repeat the process.
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Negative stability is when the aircraft is upset from a steady state, and it will just keeps going with that upset, getting worse.Originally posted by Utah-Jay View Post
You're trimmed, level, and at a constant airspeed. You push the nose forward without touching trim until you get +10kts. The airplane should oscillate some and come back to the attitude it was at, probably close to the speed it was at as well
Negative means that the speed just runs away, and gets worse, in a dive(or decays in a climb) and it never gets back to anything, and the pitch gets worse.
What creates pitch stability is the C.G. in front of the center of pressure. As we load aft, you're moving the C.G. towards the center of pressure, eventually they're at the same place, and you get neutral stability, and then with the C.G. aft of the center of pressure you get negative stability. Flaps, slats, AoA and a few other things do effect the center of pressure in flight.
I've got a book from the college days that has the formulas to calculate this, but it's math I'm no longer capable of doing, or rather can't be bothered to re-learn. Even then, you still flight test, as most everyone knows when engineering meets the real world, it's not always exactly the same.
This applies in all three axis, though the most common instability is in pitch, but you'll see varying degrees of roll and yaw stability in designs as well. For example, most all aerobatic aircraft have neutral and sometimes negative roll stability, while most small civil aircraft have positive stability, and in the Bearhawk as well as Cessnas and Piper's the primary way that is done is with the wing dihedral. Lots of fighters are negative in all axis.
This is explained in the EAA flight test manual and they have you do a series of tests to determine where your airplane is stable and not. You're not supposed to just declare a cg envelope on paper, you're supposed to actually figure out where it is. They have you do this for all three axis.
What it comes down to is positively stable airplanes(in all axis) are easier to fly. They will more or less fly themselves in cruise. The further you get from that, the more difficult it becomes to keep the airplane going where you intend it to, the more you(or some computer) has to actively manage it at all times. Eventually you get to a point where a human more or less can't control it.
In the bearhawk, like any airplane, when you get to the neutral or negative pitch stability, you have to really be on it and manage it every second to not be all over the sky. Most people can manage this on a VFR day. Most people would struggle in MVFR or IMC in that condition, add upsetting gusts and we're really in trouble. This is why part 23 airplanes mostly require positive stability, and I'd imagine LSA as well.
First google result - https://www.youtube.com/watch?v=Q2DOus05Qso&t=60sLast edited by zkelley2; 01-25-2022, 09:20 PM.Comment
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Today I got the CAA sign off and this topic was raised as part of that. With an IO 360 Lycoming it’s not possible to load to 2700 lbs and remain within the Cof G limits in my aircraft. I’ve limited it to 2500lbs MAUW for Take off and landing for that reason. Our CAA queried me on that and when we discussed the flight test data and the maths involved we agreed that 2500lbs was a sensible and safe limitation for my configuration. Thing is that gives me a stupidly good payload (1169lb) at a controllable envelope so I’m very happy. If I put floats on I might reconsider the MAUW for TO. I have to say that in a regulatory environment that can be pedantic and confused our CAA guy is a standout clear thinking practical chap.
At this stage I don’t envisage the need for a cargo pod but I do see that as a sensible option if you need the capacity.
As part of my 40 hrs of flight testing I did fly at the aft C of G limit, my opinion is that it’s doable but not fun. The Bearhawk 4B is such a delightful thing to fly but like all aircraft will protest when you push it outside the envelope. The above explanations by Zach and Jared are spot on in my view.Comment
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I was recounting to Bob Barrows the comments of you guys concerning aft CG flying. He 100% agrees that at the aft CG limit of the 4 place it is not fun at all to fly. He told me he would not fly at the aft CG limit unless it was really important. He says it is controllable, but not fun at all. He agrees that we should test our own planes sneaking up to aft CG limits to familiarize ourselves with how our own plane flies. And see where your comfort level is. He said you should NEVER exceed the aft CG limit on a four place BH. MarkComment
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Empty Forward CG
This might be considered a thread drift, but since it's still CG/weight I'm posting here.
I've spent the last two days doing circuits while lightly loaded and near the forward CG limit. To recap, my empty CG is 8.3" and the FWD limit is 10.5", so I need to have myself and around 30L of fuel to be aft of the forward limit. All fine so far because I'm not going flying without either.
Initially I had thought I had been able to trim the elevator back to cover this scenario, but I've rechecked my CG envelope and found my earlier efforts had still been around 12.0". As the fuel burnt off and the CG moved further forward I reached a position where it was no longer trimmable.
On the forward limit it now requires quite a lot of back pressure to hold. Some of this is probably attributable to my approach speeds being lower now as I gain comfort with the aircraft, and a commensurate reduction in airflow over the empennage.
If I reduce power to idle, the elevator will lose authority and the nose drops. If I fly with power on, I can hold the nose up for a stable approach, but it requires a flatter approach profile (and quite alot of back pressure) to do this.
Next, I landed and put 50kgs (110 lbs) in the cargo area and went flying again. Such a different experience on approach. I was able to easily trim the forces out, speed control was improved, and the video showed some approach speeds in the mid to high forties. I wasn't aiming for those, and actually I was largely using the AOA, but overall the aircraft seemed to be in a much happier place. Same for the Pilot too in fact.
My understanding is that most A model 4 place Bearhawks have an empty CG 2-3 inches aft of mine - I'm estimating around 13 inches making the forward limit somewhat academic. However the last couple of B models have a similar empty CG to mine, possibly due to the new wing and its position. It's probably worth bearing in mind if you're building a new 4-place Bravo model, that while building with the CG that far forward does in theory give a large payload envelope, in practice there are some definite trade offs.Nev Bailey
Christchurch, NZ
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