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lgold(at)quantum-associat
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 Posted: Sat Sep 27, 2008 1:06 pm Post subject: BRS balance issue |
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Bob,
I don't understand why our 701 balance is so different. My 701 with a Rotax 100-HP (old style mount) and a BRS behind the baggage compartment weights 654#. My empty CG is 373-mm behind the front of the slats. (500-mm is max.). My weight and balance sheet (an Excel spread sheet) is attached. I did the W&B twice to make certain it was OK. I have had up to 40# of baggage, 400# of people, and 20-gal of fuel in the plane (plus the 20# of tools, see below) with no problems. In fact, the plane flies really well with no tendency to go nose-up. The elevator trim usually stays mid position and I have not had to add VGs for slow flight.
FYI, my battery is on the fire wall and, just to play it safe, I keep 20# of spare parts and tools in a 6" aluminum tube under my feet mid-way between the peddles and the seat. However, based upon your problems, I plan get to a really accurate weighing at the Cafe' HQ. in Santa Rosa Ca.
After 110 hours flying, including trying to see if the nose would come down in extreme nose-up/ idle stalls, I see no balance problems with my plane. How much weight did you add to the plane with the structure you put in to trying to stop the oil-canning? (I just glued on some very light wood "L" moldings for this purpose).
Les
| Quote: | From: owner-zenith701801-list-server(at)matronics.com [mailto:owner-zenith701801-list-server(at)matronics.com] On Behalf Of BokKat
Sent: Saturday, September 27, 2008 6:41 AM
To: zenith701801-list(at)matronics.com
Subject: Re: Re: After 5 hours flight time, questions
Not a dumb question regarding CG issues at all! In inches (the DAR wanted it figured out in inches) the CG was about at the 19 inch mark with 20 being at the edge of the envelope. From 11 to 20 inches according to ZAC.
After the first almost disasterous flight, our EAA group including two A & P's concluded that though the plane was withing the CG limits, albeit at the edge, probably as one gets closer to the aft limit of the cg things get a lot more critical. In fct, we carefully went over the plane and ran plumb bobs where we all agreed the various weights might be, the fuel tanks, the seats, etc. and they didn't agree 100% with numbers from the factory or what another builder in northern Minn had used! An inch more or less makes a small difference in overall W & B which gets more critical when you get closer to the edge of the CG envelope! The moral of the story is to be awfully careful as you approach the edge of the envelope.
"All the mods" in my airplane are ONLY the placement of X braces and their rivets in the tail section, a mod that many people have done. This adds pretty well no significant weight to the tail (a couple of pounds, maybe) and no other mod was made to the entire airplace other than long range tanks (four 10 gallon wing tanks instead of two) Othersise NO mods at all! Standard kit!
My DAR was insistent that the CG be in the front half for the first flight! He said his boss in the Minneapolis office insists on that. After working the math for half the afternoon, we decided that the only way to get the CG in the front half of the envelope would be to remove the BRS and even add a few pounds of lead in the forward part of the cockpit, and only put 10 gallons of gas in the tanks. This would put the CG into the forward half.
I thought this was overkill as I had seen pictures of planes with a BRS against the baggage comprtment bulkhead, although in retrospect those were the earilier baggage compartments which were shorter than the extended one that is now used. I thought I knew better than the DAR as my plane was within the envelope so launched with the BRS on the baggage compartment bulkhead, 20 gallons of gas, no lead, etc. With the cg in this position the plane was almost uncontrollable at low speeds and almost ended up in disaster! ;I escaped with only a bent nose fork and bruised ego!
It looks to me like your BRS will end up even behind mine, and unless your engine mount is longer or a heavier engine you could be setting yourself up for problems. But every airplane is a little bit different even though they are standard kits and who knows? You might be OK!
I have a standard FWF package from Skyshops and a Rotax 912 S which obviously would not balance that 25 - 30 pound BRS way back there in my airplane. One can't beat the laws of physics, unfortunately. The laws of physics almost beat ME, instead! Never again.
Which is why I'm cautioning anyone who contemplates putting their BRS way back there. These airplanes are light and a few pounds way back there might make quite a difference. If the builder has a heavier engine or longer engine mount that I have, it might work out OK. Even with a 912 you MIGHT get away with it, depending on all the other little factors involved in building a plane, although being kits they are pretty well much the same. But even your strengthening the fuselage in that area will shift the CG a little bit back. Every little bit makes a difference on these light airplanes.
I built a Starduster 2 before the 701 and they tend to have an aft CG at best which had to be overcome by either a longer engine mount or a heavier engine. I love power and aerobatics, so put on a heavier engine to kill two birds with one rock! Hah!
After sending your pictures around to several other EAA'rs who have all built at least one airplane including 2 A &P's and who have all inspected my airplane before and after the first sort-of-flight and almost had the opportunity to analyze the wreckage, the most poignant one I recieved back is quoted:
This is BIG TROUBLE!
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b.carl@sympatico.ca
Joined: 20 May 2008
Posts: 77
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| Posted: Mon Sep 29, 2008 4:18 am Post subject: BRS balance issue |
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Enjoyed the discussion of CofG factors affecting the flying characteristics of the 701. I suggest that one important factor was not mentioned, the vertical position in the fuselage of the weight change.
A/C are usually weighed in the level flight position to measure the CofG. Tipping the a/c away from the level position will result in a different CofG position. In the 701 it will generally move forward (stabilizing) if the vertical position of the added weight is low on the fuselage and rearward (destabilizing) if high.
The position of a BRS chute if high in the fuselage would be destabilizing and therefore flight testing for CofG should be approached with due caution.
Another change that could be destabilizing is addition of larger fuel tanks in the wings. These tend to be extended chordwise moving the CofG rearward. Because of the high wing, when the angle of attack is increased, the weight of the fuel and tank shifts rearwards, in addition if the tanks are not full the fuel flows to the back of the tank giving a double whammy to the CofG. This factor is further accentuated in the 701 because it can achieve angles of attack well above 20' before stall.
I agree that the affect on the CofG of any changes should be carefully measured on the ground and deviations from the norm corrected and assed before start of flight testing.
One good rule is to fast taxi and try to rotate the nose to get a feel for elevator response before going airborne. During flight testing I always make sure I get a good positive increase in stick back pressure with decreasing speed right up to the wing stall. Any levelling of back pressure before the stall is a danger signal that the CofG is too far back.
30 hrs this summer, trying for 40 before freez-up.
[quote]Carl[b]
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