Gross Weight Aft Loading Limit

[Baloo]

I am just updating my W&B program. The Cof G envelope for my 4 place is 10.5"-22.5" MTOW 2700 MLW 2500. When you calculate your CofG do you put any restrictions on the rear most CofG when operating above a certain weight or close to Gross. Id love to hear your personal loading limitations and techniques.

[Blackrock]

I just load the plane with heavier items as far forward as possible and the lighter stuff aft. Sleeping bags, tents, clothing and dry goods go aft. People, coolers, etc go forward. I do load and fly at 2700 lbs MTOW, but when I do, that normally includes 70 gallons of fuel on board. Also, due to the servo nature of the flap tabs, aft loading tends to increase pitch sensitivity so that's another good reason to keep weight as far forward as possible. I do not place any other restrictions on loading other than staying within the CG range.

Everything that goes into the plane gets weighed if I'm getting anywhere near gross weight or if I'm unsure. A hand held fish scale works well for gear. Making sure the weight cant shift is of course hugely important, too. the rear seat anchor points can be used to secure gear as well as the rear bulkhead anchor point on the floor.

[Bcone1381]

Many folks like to keep the CG away from the rearward limit. I suggest this idea. Try different locations, and explore the entire envelope. You won't scare yourself if you honor the envelope. Once upon a time I flew an aircraft with one of its five fuel tanks in the tail. In cruise flight the fuel system would automatically manage the fuel in the tail tank to give the aircraft a cg that was at 98% of the aft limit. This design reduced fuel consumption.

I've flown an aircraft with the CG outside of the aft limit. It was unstable, but very controllable. I recall thinking "So this is what its like to have an aft CG." (The situation was a legal flight at a commuter airline that used standard weights for suitcases of 23.5 pounds each. After landing we learned that we had a Tour on board and everyone had those huge bags that weighed nearly 70 pounds each.)

If your airplane is loaded within the CG limits, it will fly just fine. Discover the benefit to flying with higher cruise speed for a given power setting or lower fuel consumption. I'd like to see Bearhawk cruise performance numbers with it loaded with the Fat Guys in the front seat compared with the fat guys in the back seat. I think you'll be surprised.

On last thing. Back in the day, we did a weight and Balance on every flight by hand using a form the company had made up. We are talking 6 to 8 a day, 5 days a week. Everyone could complete the W/B, and plot the CG in about 30 seconds. I tell you this, because they can be made very easy, and its worth doing to fully understand your aircraft's capabilities.

[jaredyates]

Personally, I don't consider Bob's published limit to be a conservative value with extra margin. I used ballast to approach the aft limit a little bit at a time, and 1" forward of his limit was as far as I cared to go. The airplane was controllable, but to give a specific example, I trimmed for level flight, took my hands off of the stick, and gave it a little "pop" with my hand. That led to a 1.6g pitch up that I intervened to stop at 20 degrees nose up pitch. I could still control the plane, but it was a constant hassle, and while I'm no Chuck Yeager, I'm also not a low-time pilot. The TC aircraft manufacturers may have a lawyer-inspired fudge factor in their published envelopes, but don't expect that in Bob's numbers. The best way to have confidence in your own plane is to load it up a little at a time and work your way towards the limits incrementally. This can be a safe way to explore stall characteristics and indications also. Be sure to secure the ballast well.

[SC Patrol 253]

I appreciate you sharing this experience. I have wondered about the aft limits and if the airplane would diverge or not with sharp inputs.

[whee]

Personally, I don't consider Bob's published limit to be a conservative value with extra margin. I used ballast to approach the aft limit a little bit at a time, and 1" forward of his limit was as far as I cared to go. The airplane was controllable, but to give a specific example, I trimmed for level flight, took my hands off of the stick, and gave it a little "pop" with my hand. That led to a 1.6g pitch up that I intervened to stop at 20 degrees nose up pitch. I could still control the plane, but it was a constant hassle, and while I'm no Chuck Yeager, I'm also not a low-time pilot. The TC aircraft manufacturers may have a lawyer-inspired fudge factor in their published envelopes, but don't expect that in Bob's numbers. The best way to have confidence in your own plane is to load it up a little at a time and work your way towards the limits incrementally. This can be a safe way to explore stall characteristics and indications also. Be sure to secure the ballast well.

I'm pretty much in agreement with Jared. I loaded a BH up to gross (2500) and right at aft CG and I don't care to ever do it again. I was scared for my life. 21.5" is as far aft as I ever want to load the plane; that last inch made a big difference. When I flight test my plane I will go through the same process and if it feels good at 22.5" then I'll be willing to fly it there.

[Bcone1381]

I was wrong! Don't do what I said. Thanks correcting me and for giving the group real life experiences, Jared and Whee. My experience is limited to conservative numbers from certificated airframes.

Brooks

[Baloo]

Thanks for the info guys!

[Battson]

Personally, I don't consider Bob's published limit to be a conservative value with extra margin.

I'm pretty much in agreement with Jared. I loaded a BH up to gross (2500) and right at aft CG and I don't care to ever do it again. I was scared for my life. 21.5" is as far aft as I ever want to load the plane; that last inch made a big difference. When I flight test my plane I will go through the same process and if it feels good at 22.5" then I'll be willing to fly it there.

Strongly agree - I would not assume you can flaunt this limit.

The handling of the is very different when you are at the published limit, I would not want to go past it. There are very real physical reasons why that is a bad idea.... :P

I have been flying at the aft limit at lot, and Jon, it sounds like that plane was past the limit.
Perhaps the W&B was a little off, or perhaps the homebuild was a little different somehow? Am I right in recalling one BH you've had experience with was 2" longer in the fuse that the plans call for?

[whee]

That plane was built according to plans; the one I am building now is 2" longer. It is possible that the W&B was slightly off.

[Baloo]

Twice now in the past few months I have gone flying with a moderate load 2300-2500 lbs and run out of nose down trim in cruise. My calculated cofg for both flights were near 21.5". Have any of you run out of nose down trim and at what arm? I just want to cross check my observations and calculations with yours. (I did not weigh everything and used approx weights)

[Battson]

I don't think that's normal (!). Even when my plane is at the hairy edge of the published limits, I still have a LOT of trim to spare.

I would cease flying in that configuration until you find out why that's happening. Per the previous posts, I also suggest resisting the temptation to use approximate weights, particularly if you might be near the limit. Human senses are not good at accurately judging weights, as has been proven time and time again.

A few questions to help us provide some more info to you here:
- How far do your trim tabs physically move in that direction?
- What design of trim tab do you have?
- Is your trim electric or manual, and where is the stop in the system?
- What method are you using to determine the CG location?
- When you were flying in that configuration, was the aircraft unstable in the pitch axis (i.e. highly sensitive to control inputs, moreso than usual)?

You might also like to think about when your last real weighing was done. If not recently, then perhaps a re-weigh is in order?

[Baloo]

Battson, As usual thanks for taking the time.

I will have to do some further investigation when I get to the airport next in order to answer your questions. But to answer your last question yes it was very sensitive in the pitch axis. A small bump of the stick went a LOOOONG way especially at ``higher`` speeds. (100kts+). The trim was not sufficient and I had a bit of constant FWD pressure. The 2 instances where this has happened recently my load was as follows:

SCENARIO 1
Empty: 1562 Empty
Front Seats: 307lbs
Back Seat: 320lbs
Baggage: 35lbs
Fuel: 180lbs
Total: 2404lbs

SCENARIO 2
Empty: 1562 Empty
Front Seats: 230lbs (Pilot + 25L of Water in Copilot Seat)
Floor Between Front & Back Seats: 80lbs (Propane Tanks)
Back Seat: 90lbs (Fuel & Supplies)
Baggage: 50lbs
Fuel: 240lbs
Total: 2252lbs

I`m almost thinking that something went out of whack and it is not getting its full travel for trim. I have very little travel left on my trim wheel in the forward range using takeoff neutral as a reference.

[Mark Goldberg]

Just as elevator authority/response increases with aft CG - so should trim. I do not understand why you would run out of trim at aft CG. If you have plenty trim authority at normal or forward CG loading, you should have more at aft CG. Mark

[whee]

Large or small trim tabs? The plane I flew had large tabs with one disabled. With an aft loading it would almost be out of forward trim; not to the stop but almost.

[Baloo]

Well I think I may have solved the problem. Troubleshooting is always a bit more of an adventure in a plane that you didn't build becasue you don't have the intimate knowledge of how it all went together. I wont know if it did the trick until I can test fly it.

It seems that the Trim tabs were out of rig. In the before state the desired takeoff setting was very close to the dose down travel limit of the trim wheel thus leaving me not much room to trim nose down in cruise.

Long story short I removed and lengthened the rod ends where they attach to the trim tabs on each elevator and calibrated them to give "neutral / takeoff" trim further from the forward travel limit. Hopefully this will give me some more trim authority. Here are some numbers if you care:

Trim Wheel Position / BEFORE / AFTER

Full Nose Down.............................................. .../ 7.0 Up / 14.8 Up
Neutral Takeoff (Old Temp Marker).............../ 3.9 Up / 13.4 Up
Neutral Takeoff (Perm Placarded Marker).../ 0.7 Dn / 4.0 Up
Full Nose Up................................................ ...../ 27.8 Dn / 18.0 Dn

Reference point is the Elevator when it is secured to the Horizontal stab using control locks.

I'll post an update once I have a chance to fly it in this config.

To answer Whee's question there is a trim tab on each side measuring 13" Wide x 4 5/8" Deep

[Battson]

But to answer your last question yes it was very sensitive in the pitch axis. A small bump of the stick went a LOOOONG way especially at ``higher`` speeds. (100kts+).

You are welcome!

^ This means you were *potentially* outside of the CG range. Of course we couldn't say anything with certainty, but that is the symptom to look out for.

The revised measurements after the change look a lot better to me. The nose up might be too little at forward CG / empty plane. You do want more trim nose up than nose down, but the 7 degrees you started with was not much.