Making your Bearhawk too light - the best way to ruin a good airplane!

[svyolo]

I agree on a technical level. It is called "polar moment of inertia" in US lingo. Not sure the rest of the world uses for it. Putting 1/4 of the weight as the baggage compartment all the way in the tail should have little effect on spin recovery. All that being said, I would only configure an airplane like that for a STOL contest, which I won't be doing. I would increase my survival and tool kit in the cargo area.

I have a very light cargo tube for long light stuff. 8" round CF that only weighs 22 oz. I also have a second one mounted on the floor behind the cargo bulkhead. I don't know if I will use it because of my first statement. I would rather have more camping/survival/tools.

[jaredyates]

I'm having difficulty with the physics of this one. I would expect this to be a moment issue. To move the CG aft the same amount, exactly the same moment must be added. How does it matter where the mass is if the moment is the same? ...except there there might be an advantage to the full system being lighter?

Imagine an extreme case. If you had a broom stick with a balance point in the middle, with 5 pounds on each end, it would be balanced and weigh a little over 10 pounds. Move the weights as close to the middle as they will go, and the total mass and balance point are the same, but twirling the two sticks would be quite different.

[kestrel]

...adding weight to the tailwheel area was undesirable because he feared extra weight at the tail could inhibit the plane's ability to recover from a spin. So he added weight at the back of the baggage area.

I'm having difficulty with the physics of this one. I would expect this to be a moment issue. To move the CG aft the same amount, exactly the same moment must be added. How does it matter where the mass is if the moment is the same? ...except there there might be an advantage to the full system being lighter?

[Mark Goldberg]

Nev, when contemplating the forward CG of the prototype Model 5, Bob examined a few options. He decided that adding weight to the tailwheel area was undesirable because he feared extra weight at the tail could inhibit the plane's ability to recover from a spin. So he added weight at the back of the baggage area. Something to consider. Mark

[zkelley2]

I also prefer the approach with drag and power on. This is how every aircraft that can carry much of any load flies. Be it a C206 or a B747.
It's not like a 172 or a Pa18, but neither of those can haul much.

[Nev]

All good points there Jon. I really like the idea of a baggage tube and if I figure out a way to install one it may well be in my future (thanks !).

A couple of interesting points worth noting :

My normal empty CG of 8.3" is a full 1.5" forward of where yours was (before it moved aft (as is the discussion point of this thread).

Currently with the additional aft ballast, mine is now at the same point that yours used to be (9.8"). I definitely agree with keeping it slightly forward as it does offer protection against a stall when on a STOL approach. Losing elevator authority is much preferable to stalling, and usually a mere inconvenience.

So I think I'm now at the point where yours used to be, and I like how it flies - and noting that yours is now further aft.

If I can use a ballast tube to achieve the same thing then it'll be a win-win But first I had to find where that point was that it flew well on approach, and I believe I've now found it.

Regarding the engine failure scenario, if below 500ft, 50 KTAS, CG forward of 14" and the engine fails, the only option is to lower the nose. That's because if you keep the stick back where it is, the elevator loses authority anyway and the nose lowers itself. Or, you can chose to ease the stick forward and thereby lower the nose. Below 500ft both will likely result in ground impact in a nose low attitude as the aircraft accelerates downward. The only way I've found to protect against this is by using power. With enough altitude it will gain speed and elevator authority, and can be flown normally again into a glide approach.

With the CG at 14" or more, there is enough elevator authority to maintain the pitch attitude. However the descent rate does still increase. It doesn't completely mitigate the risk, but should give more control.

[Battson]

Building with the CG too far forward

Why not install a baggage tube? If you put the 50lbs of gear inside that - then your CG will be even further aft than you currently have it.

The beauty of a baggage tube for CG management, is you can still enjoy all the benefits of having a forward CG (where you want it) if you have to carry a heavy load. You can always balance the aircraft to whatever setting is important to you. Plus now you have a baggage tube! Which is good to have! Also, a baggage tube certainly should weigh less than the 12lbs of useless ballast on the tailwheel So you are lighter overall too, which helps STOL - which appears to be the ultimate goal here...?

I think you've achieved the perfect result with your CG location, as you have maximum choice and flexibility. The grass is always greener (noting you can never bring the CG forward by adding baggage / people / fuel)


As you know, I don't agree with the underlying hypothesis that a "well balanced" CG is desirable for STOL operations. I like the inherently safe design of the Bearhawk elevator. It's hard to stall during a STOL approach - you really have to force the plane into a stall using a lot of power, it's hard to make a big mistake. This design feature has surely saved many pilots a headache or worse. I have never found the lack of control authority with the throttle closed to be an issue

I think the best way to get a steeper, slower, "more STOL" approach and landing is adding drag (and lift!) by milking every last drop out of the flaps and using more power - allowing one to get further behind the drag curve while retaining a blown-tail for control, blown-flap for lift, and all without making it too easy to stall. I dread aft-CG landings, because I know if I jerk the stick backward for whatever reason, it could stall - and the stall recovery would not be as fast or automatic as with a forward CG. Those are just my personal views.

As an aside, I avoid closing the throttle completely during a low-power approach whenever I can. The engine temperatures crash suddenly to the coldest you will see with the engine running steady state. It sets off all kinds of alarm bells in my head, and on my instrument panel This is another reason why I like higher drag and more power.

Interesting point about the engine failure. I agree that a steep approach is not the approach to fly if the engine fails... If you are flying a steep STOL approach without backup landing choices available - and the engine fails - a classic steep STOL approach gives you options. Personally I would add airspeed and reduce drag flap, extending the landing zone options by improving the glide performance. The very worst option here is the low, threshold-stalking approach which one sees from time to time - that gives no time and no choice - you are going where fortunes dictate.

[Nev]

Building with the CG too far forward

I want to add another data point to this thread. During the build process I followed the normal advice to keep the aircraft light, and keep the weight forward. I've got an IO540 a Hartzell composite 3-bladed prop and I put the EarthX battery on the engine side of the firewall.

The day we weighed it I knew I'd achieved the goal of keeping the weight forward - my empty CG was 8.3" aft of the wing leading edge. By adding a pilot and 30 liters (7 gallons) I was right on the published forward limit of 10.5". It also mean't I had a huge CG envelope.

The background

The problem is, when flying light (which for me is a lot of the time) there is a down side to having the CG this far forward, and (it turns out) no real upside. Obviously when flying a STOL approach any reduction in airspeed gives an exponential reduction in energy and hence landing roll, so it's desirable to fly relatively slowly (keeping safe margins of course). My normal approach speed is around 50 KTAS.

However when on approach at 50 KTAS or less and with a CG forward of 14", if I reduce the power to idle, the elevator loses authority and the nose slowly drops. After losing 200-300ft (if you do nothing) the airspeed increases and elevator authority is regained - and the pattern repeats. This issue can be observed on a number of other STOL aircraft when at very low speeds. The obvious problem here is that if I had an engine failure below say 500' AGL while flying a STOL approach, I'd hit the ground in a nose down attitude while accelerating. Not something I'm keen on . With normal elevator authority I'd probably still make ground contact, but it would give a chance at flaring and control.

This means that it is difficult to fly a steep approach at a low speed, because it involves reducing the power to a point where elevator authority is lost. If I keep the power on enough to retain elevator authority, the aircraft doesn't descent steeply. If flying a normal approach profile at a speed of around 55 KTAS as you would at an airport, there's no real problem (but with an increase in energy and ground roll). It probably only effects those 4-place aircraft with a heavier engine/prop combination up front. I believe that an O360 powered 4-place would have the CG aft enough not to have an issue.

Comparisons

I've done many landings using various amounts of ballast to try different CG locations up tp 19" aft, and also finding the forward minimum CG conducive to safe operation all round. As a result I decided to keep my CG aft of 14" where possible (and preferring further aft) anytime I'm flying an approach at a low airspeed (or simply fly a little faster on approach). For the past month I've achieved this by carrying 12 liters of water in addition to my 50lb kit of tools, survival gear, and tie downs.

However I've now added just 12 lbs at the aft most point of the aircraft (tailwheel), and removed the water bottles (cargo compartment). This moves the CG 1.5" aft at light weights and allows me to be at 14" CG solo, and around 16" with 2 POB and gear. I'll fly this configuration for a while to see if there are any other effects, but early observations look promising. The added weight is directly over the tailwheel axle, so should have a minimal effect of the aft fuselage and stinger. Precession doesn't appear to be an issue.

Results

Empty CG has moved from 8.3" to 9.8.

Now, if I'm solo and half fuel, my CG is around 14.2".

At the other end of the envelope, 4 x 200lbs people, 60 lbs gear and full fuel gives a CG of 20.8", which I'm comfortable with.

[Low Friction]

My mag experience sample size is a couple sets on puddle jumpers. I'd really need to talk to one of the big accessory overhaul shops to figure out the truth. Wonder how that 200F over temp tape on Pmags works in hot climates? Seems like +200F post shutdown temps would be common.

[Battson]

For traditional mags the blast tubes help protect the copper windings/insulation and plastic parts. Bearing lube as well probably. It extends their life. For PMags it does the same but also keeps the micro electronics from going into over temp. Mil spec components are rated to 125C -> 257F. Other parts of the system could very well go out of spec before that temp. These are just assumptions. You'd need to consult the manufacturer.

Personally, I am not convinced that regular mags benefit significantly from blast air. The most common response from experienced people is "Sure, it's not going to hurt..."

The published manufacturer temp limits should never be exceeded in normal flight and ground ops. I am not aware of any heat-accelerated failure mode which limits the life of a traditional mag.
What's more, the mags are exposed to the internal temperatures of the engine through conduction and direct airflow. Convective heat loss on the outside of the case from a 1" tube isn't going to make a massive difference. The highest temps occur after shutdown, when the blast tubes are doing nothing.

[Low Friction]

For traditional mags the blast tubes help protect the copper windings/insulation and plastic parts. Bearing lube as well probably. It extends their life. For PMags it does the same but also keeps the micro electronics from going into over temp. Mil spec components are rated to 125C -> 257F. Other parts of the system could very well go out of spec before that temp. These are just assumptions. You'd need to consult the manufacturer.

I did see that PMags have thermal tattle-tail tape that changes color at 200F. Definitely monitor the tape to ensure thermal head space is maintained.

[Battson]

A lot of mags run fine without blast tubes, including the Surefly SIM electronic mag. I have limited experience but I assume less than half of all mags have a blast tube installed. I note P-mags specify blast tubes.

[500AGL]

We have had good results with the surefly as well. Any experisnces to the contrary?

[Low Friction]

No flame wars please. My comment is based on my experience working in the trenches designing and testing big jet flight critical avionics most of my career. The certification standards are extreme. I easily become nervous with uncertified flight critical embedded systems such as the PMag. That said I'm not disagreeing with your comment.

[PaulSA]

Pmags are now some 10+ years old - they are a mature technology. There were a couple of issues early on but they have been long since sorted. Yes, you need blast tubes but you do with mags as well. I think they are by far the simplest and most robust electronic ignition option - without wishing to start an "ignition war" .....