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by0ung(at)brigham.net
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 Posted: Tue Nov 06, 2007 7:03 am Post subject: W&B |
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I guess I had thought things through a bit more and was not going to send
the last email until I had changed things a bit...... SORRY
Unless things move inside an aircraft,, or fuel is burnt.. the cg does not
change by changing the pitch or attitude of the aircraft..... what we
need to do is make sure our planes fit into the same model that the factory
has set... examples.
If you were to raise the tail till the bottom of the wing was level,,,,
then drop a plum bob from the leading edge to the seat. Measure the cg of
the pilot using the belly button method. The results would be greatly
different than,,, with the tail wheel on the ground, drop a plum bob from
the leading edge and using the belly button method of obtaining the pilot
cg....
The aircraft kit manufacture, KOLB, had determined a method of obtaining
the cg in a manner that ALL that follow their instructions will have a
plane with predictable results. If you do it some other way,,, your results
may vary.
I think this is what I was going to change things to before I hit the send
receive button.
Boyd
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mdnanwelch7(at)hotmail.co
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| Posted: Tue Nov 06, 2007 9:16 am Post subject: W&B |
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Kolber Guys,
Hey!! I'm back (from my visit to Alabama) What a beautiful state! And the highways. I've never seen such great highways. I'd like to take this opportunity to thank all of you that took the time to respond to my "where to retire" question. Thank you, guys.
If I'm not too late to join this W & B party, I'd like to clear things up a bit. This is an apples and oranges thing.
Here is the apples:
First of all, Boyd's response is the most accurate (along with a couple of others), so far. It is a fact. The center of gravity (or mass) does NOT change, if nothing else has moved. By taking various weight measurements, and picking an arbitrary datum, a person can calculate the "center of gravity". And you can twist and turn the object all you want, it doesn't alter its center of gravity".
This is where Ray is correct, but that's all we know at this point. But we need more information. We also want to "balance" our object. And it has been decided by others (Homer?) a long time ago that IF the wing is held at 9 degrees , AND we could suspend the airplane by a string (attached to our specific c.o.g.), then we will come up with the specific balance point. And they say this balance point should be located at a particular percentage of the main wings chord. Let's say....28-32% from the leading edge of the main wings.
So here is the oranges:
In physics, this imaginary string is known as the "NORMAL" force. It is a line at which 100% of the weight of an object is pulling down, with respect to gravity. It is "normal" to the earth's gravity. Now, if you can picture our imaginary string supporting our airplane, with all loads correctly in place.....and this string goes up through the main wings at a point between 28% and 32% (I don't know actual percentages at this moment), then we can say we are "balanced". ...."you're good to go!!"
Now, if we drop the tail down to rest on the ground (but not touch, because remember, our imaginary string is supporting the entire weight of the plane). And we still want 28-32% to be our "normal" force, we would see the iimaginary string roll forward toward the leading edge. Obviously we are rotating the plane backwards, there fore the supporting string is lining up with a "more forward" point.
So, with our tail end low, we see our new string support position is way ahead of the 28-32%. But we want 28-32%. We have to have 28-32%! That's not going to change. And you have the tail-end almost touching the ground, right? The only resolution now is to add lots of weight to the back end of the airplane. This will shift our imaginary line more rearward (travelling rearward parallel from where it was) And now, we finally have finally our center of gravity that lines up with 28-32% of chord, WITH THE TAIL-END LOW!!! Now, even though we have achieved the 28-32% balance point, the plane is concidered incredibly TAIL HEAVY!!
This revised scenario of weight and balance may take off and land only one time!!! The reasons are obvious. When calculating weight & balance for you aircraft, you are furnished certain "GIVENS". One of these givens is that the plane be put in a certain configuration (main wing 9 degrees up). This is not a variable. Another "given" is the final balance point must be within 28-32% of chord. This also not a variable. The next factor is the "NORMAL" force (our balance point).
If you go and lower the tail, then you will need a NEW point to call your final balance point. You can NO longer use 28-32% of chord. That is someone else's figures, not your's. You need to find out what the new "GIVEN" is with tail low. It may be the leading edge. Who knows? You'd have to experiment, if you live to tell about it. In other words, you are coming up with your own criteria to do a weight and balance. But, then don't get to go back and grab someone else's "GIVENS". It doesn't work that way!!!
As Boyd said. "Why reinvent the wheel??" For a KOLB AIRCRAFT, there is only one prescribed correct process to calculate weight and balance and have it be reliable, and that is with the main wing held to 9 degrees flight attitude.
Best to all, Mike Welch
BTW. Did you know, the center of gravity in a banana is not even in the banana? It is out somewhere in the middle, in mid-air, inside the curve. (Does this make this an apples, oranges, & bananas comparison? Hmmm?)
| Quote: | From: by0ung(at)brigham.net
To: kolb-list(at)matronics.com
Subject: W&B
Date: Tue, 6 Nov 2007 08:02:33 -0700
I guess I had thought things through a bit more and was not going to send
the last email until I had changed things a bit...... SORRY
Unless things move inside an aircraft,, or fuel is burnt.. the cg does not
change by changing the pitch or attitude of the aircraft..... what we
need to do is make sure our planes fit into the same model that the factory
has set... examples.
If you were to raise the tail till the bottom of the wing was level,,,,
then drop a plum bob from the leading edge to the seat. Measure the cg of
the pilot using the belly button method. The results would be greatly
different than,,, with the tail wheel on the ground, drop a plum bob from
the leading edge and using the belly button method of obtaining the pilot
cg....
The aircraft kit manufacture, KOLB, had determined a method of obtaining
the cg in a manner that ALL that follow their instructions will have a
plane with predictable results. If you do it some other way,,, your results
may vary.
I think this is what I was going to change things to before I hit the send
receive button.
Boyd
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