REFLECTOR:CG Test
Scott
reflector@tvbf.org
Sun, 25 Jan 2004 07:51:11 -0700
When I re-weighed my airplane to establish the CG Box I weight it empty,
then weighed it with me sitting in the front seat(establish front seat
arm), then sitting in the back seat(establish back set arm), then weighed
it with 10 gallons of fuel, then 20, then 30, (establish fuel arm).
This was all per the POH instructions.
My front seat arm as more than factory specs but I knew it would be as the
builder had moved the front seat up 1.5 inches. Everything else was nominal.
Scott
At 09:27 PM 1/24/2004, you wrote:
>Keith,
>I completely agree with you and plan on using my new data to modify my W&B
>calculations. Jim S. points out that the factory published pilot arm may
>be off a bit. (A 2" change in the pilot seat location changes my cg by
>0.2") Perhaps the calculation can be simplified for the standard Velocity
>(I'm not familiar with the XL) because both the rear seat and the fuel are
>close to aft cg. Therefore changes in the reat seat weight and fuel
>weight will not change the cg much when you are near aft cg and that is
>the only time to be concerned about balance (with the exception of too
>much weight in the front seat which is a forward cg issue). Only changes
>in the front seat weight make a big difference. So what it boils down to
>is to determine how much front seat weight is needed to avoid being aft
>cg. From my test it appears that my weight is sufficient for this purpose
>and that makes it a no-brainer as long as the plane doesn't leave without
>me or I don't become anorexic and lose 50 lbs. My only remaining concern
>is how much the fuel can change cg by shifting back during a climb with
>the tanks 1/2 full. Moving the fuel arm back 2" moves the cg back
>0.2". However if I move myself forward 2" I can cancel out the fuel
>shift, so I think I'm covered. I know there are ballfes, but over several
>minutes the fuel will get past them. Again I appreciate all comments lest
>I have overlooked something.
>
>John
>
>KeithHallsten wrote:
>>John,
>>
>>I think the point of doing the weight and balance measurements and
>>calculations is to have documentation of the acceptable C.G. range. Then
>>some proposed loading can be checked before flight to assure that is
>>within the acceptable range. This is both good practice and required by
>>regulation: it's the "W" in the "AROW" required documents list.
>>
>>Maybe because of my engineering background, calculating the center of
>>gravity location seems like a very logical thing to do. I plan to
>>calculate W&B for all of my proposed loading configurations, then compare
>>those numbers to the flight characteristics experienced. That way I will
>>know what flight characteristics to expect when I'm proposing to fly at
>>some different loading configuration for which I calculate the W&B numbers.
>>
>>Keith Hallsten
>>
>>>----- Original Message -----
>>>From: <mailto:aminetech@bluefrog.com>John Dibble
>>>To: <mailto:reflector@tvbf.org>reflector@tvbf.org
>>>Sent: Saturday, January 24, 2004 10:50 AM
>>>Subject: Re: REFLECTOR:CG Test
>>>
>>>Jim,
>>>I appreciate your comments. I suspect a weight/balance would confirm an
>>>aft cg loading according to the numbers that Velocity provides. What I
>>>am trying to do here is establish a limit specific to my plane which may
>>>be different from the standard assuming there are small differences in
>>>how each plane is built. Scott S told me that, for a standard Velocity,
>>>typically 170 lbs in the front seat is enough to put you within the aft
>>>cg limit. So perhaps my plane is different, or maybe the original cg
>>>measurements are off. In any case I think an actual flying stall test
>>>is more meaningful than measurements on the ground. Of course with
>>>flying there is risk involved and the weight/balance measurement is
>>>essential before first flight when you know nothing about how it will
>>>fly. My plane has 280 hours (75 of which are mine), so I know it flies
>>>ok and I'm looking to fine tune (determine with more certainty) my aft
>>>cg limit and hopefully avoid adding weight to the nose. From recent
>>>reflector discussions I felt I understood the causes and onset
>>>indicators of a deep stall as well as how to recover from it (altitude,
>>>prop pitched for power, moveable weights) and felt sufficiently
>>>comfortable to stall the canard near the aft cg limit. I think doing a
>>>weight and balance measurement now is like measuring the length of the
>>>bungee cord after having already jumped off the bridge. If anyone
>>>disagrees, please tell me.
>>>
>>>John
>>>
>>>Jim Sower wrote:
>>>>
>>>>Having done what you have done, I would be inclined to muster up some
>>>>friends and do a
>>>>weight/balance with the fuel load you tested with, you *personally*
>>>>sitting in the
>>>>pilot's seat, and the weights you used just as close as you can place
>>>>them to where
>>>>they were in the back seat.
>>>>Just to be sure .... Jim S.
>>>>
>>>>John Dibble wrote:
>>>>
>>>>
>>>>>
>>>>>I decided to do a test to determine my aft cg limit as suggested by
>>>>>Scott B. My balance calculations for my SRG indicate I will be aft cg
>>>>>with less than 250 lbs in the front seats. I weigh 170 so this needs to
>>>>>be resolved and nose weight added if needed. I put 8 x 10lb weights in
>>>>>the copilot seat. At 7500' I trimmed for 75-80kn and pulled back until
>>>>>the canard stalled at 60kn. It took all the aft stick and I had to trim
>>>>>to about 70kn in order to stall the canard. The canard motion was
>>>>>gentile and the plane was a bit wobbly, but stable. One-by-one I moved
>>>>>the weights to the rear seat and repeated the stall. Each time the
>>>>>canard motion was less pronounced. With all the weights in the back
>>>>>seat the canard just kind of mushed down at 58kn. In all cases I had to
>>>>>maintain aft stick pressure to get slow enough to stall and never had to
>>>>>push the stick forward. I should add that I have 3 leading edge
>>>>>vortilons and one trailing edge fence inboard of the aileron on each
>>>>>wing. So it appears that I do not need to add any nose weight. Anyone
>>>>>agree/disagree?
>>>>>
>>>>>John
>>>>>
>>>>
>>>>
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