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  #1  
Old 12-23-2018, 05:15 PM
fbrewer fbrewer is offline
 
Join Date: Sep 2017
Location: Leander
Posts: 99
Default G loading on RV6

Reading the discussion about g limits on the RV (let's use an RV6 as the baseline).

Condition 1

Let's say the plane is loaded to it's aerobatic weight of 1390.

The airplane has a 180 lbs pilot and 16 gallons of fuel (all in the left tank)

Condition 2

Now the plane is loaded again to it's aerobatic weight of 1390.

The airplane has a 100 lbs pilot and 29.3 gallons of fuel (all in the left tank).

Even though the airplane is loaded to the same weight, does condition 2 (100 lbs pilot) put less bending moment on the wing spar than condition 1 (180 lbs pilot)?

Last edited by scrollF4 : 12-25-2018 at 08:14 AM. Reason: Last paragraph had an 1800 lb pilot in it.
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  #2  
Old 12-23-2018, 06:36 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 5,070
Default

Sure, but it's a small difference. Best to ignore it.

In fact, on the RV-3B, Van's Support told me that fuel weight counts as part of the gross weight for aerobatics. That is, the low aerobatic gross weight limit is regardless of the fuel load.

Dave
RV-3B building the fuselage
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  #3  
Old 12-23-2018, 09:25 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,666
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Quote:
Originally Posted by fbrewer View Post
Reading the discussion about g limits on the RV (let's use an RV6 as the baseline).

Condition 1

Let's say the plane is loaded to it's aerobatic weight of 1390.

The airplane has a 180 lbs pilot and 16 gallons of fuel (all in the left tank)

Condition 2

Now the plane is loaded again to it's aerobatic weight of 1390.

The airplane has a 100 lbs pilot and 29.3 gallons of fuel (all in the left tank).

Even though the airplane is loaded to the same weight, does condition 2 (100 lbs pilot) put less bending moment on the wing spar than condition 1 (1800 lbs pilot)?
If the fuel was equally distributed throughout the full span of the wing the answer would be yes.

Since the tank is only on the inboard portion of the span, the portion of the wing outboard of the tank will not experience a reduction in bending moment.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

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Van's Aircraft Engineering Prototype Shop Manager
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RV-6A (aka "Junkyard Special ")
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  #4  
Old 12-23-2018, 09:30 PM
MConner MConner is offline
 
Join Date: Mar 2018
Location: Snead Island, Florida
Posts: 205
Default Wing bending weight

The question here is alluding to the concept of wing bending weight. All jet joc?s know this term as it states that alll weight in the fuselage acts to bend the main spar based on G loading. Weight in the wings (fuel) does not add loads on the spar to bend the wings. The problem with trying to use this logic is that the stress analysis on the airframe must be done to factor the wing fuel versus fuselage weight. Although it may have been calculated at some point, I do not know of it having ever been published. That makes sense if you consider the variables of how weight is accumulated in an amateur build process. I feel Van?s has always tried to keep us safe by publishing worst case numbers and I appreciate that perspective. I flash back to the Partenavia that shed both wings in an air show and hope never to see another sad ending like that. If you want to test your airframe load limits, that is your call.
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  #5  
Old 12-23-2018, 09:53 PM
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AlexPeterson AlexPeterson is offline
 
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Location: Ottertail, MN
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Quote:
Originally Posted by MConner View Post
The question here is alluding to the concept of wing bending weight. All jet joc?s know this term as it states that alll weight in the fuselage acts to bend the main spar based on G loading. Weight in the wings (fuel) does not add loads on the spar to bend the wings. SNIP
Weight in the wings which is not evenly distributed span-wise does indeed add to the spar bending loading. The fuel tanks on an RV are an example. They add less bending loads than the same amount of weight put into the fuselage would. However, one would have to calculate the moment diagram for different scenarios to get a sense for the magnitudes involved.
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  #6  
Old 12-24-2018, 04:21 AM
Capt Capt is offline
 
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Who weighs a 45kg's? I'm 183cm tall 90kg's, pretty standard for a lot of RV drivers I never fly around wth such uneven tanks!
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  #7  
Old 12-24-2018, 12:31 PM
sblack sblack is offline
 
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Location: Montreal
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Who in their right mind would fly with all their fuel in one tank and the other empty?
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  #8  
Old 12-24-2018, 12:52 PM
Kyle Boatright Kyle Boatright is online now
 
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Location: Atlanta, GA
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Quote:
Originally Posted by sblack View Post
Who in their right mind would fly with all their fuel in one tank and the other empty?
Where do you find an RV-6 with 30 gallon wing tanks? ;-)
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  #9  
Old 12-24-2018, 01:02 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by sblack View Post
Who in their right mind would fly with all their fuel in one tank and the other empty?
?????????

Fuel not evenly distributed among the two fuel tanks has nothing to do with the evenly distributed comments.....

Fuel weight during G's acceleration loads the wing downward, countering the upward bending moment being induced in the wing. The problem with assuming that this reduces the bending moment of the wing during G's (aerobatics) is that the entire wing is producing lift to counter the load of airplane, but the fuel in the tanks is only countering the bending load on the inboard portion of the wing where the fuel tanks are. There will still be an abrupt increase in bending load in the wing at the point just outboard of the fuel tank.

So in the context of evenly distributed.... if the fuel was stored in the entire span of the wing (and not all at the inboard end because of dihedral angle), then fuel would have a valuable counter effect on the bending load of the wing during high G's.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Hubbard, Oregon
Van's Aircraft Engineering Prototype Shop Manager
FAA/DAR
RV-6A (aka "Junkyard Special ")
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  #10  
Old 12-24-2018, 02:32 PM
Christopher Murphy Christopher Murphy is offline
 
Join Date: Feb 2007
Location: colorado
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Default More fuel in one tank

Quote:
Originally Posted by sblack View Post
Who in their right mind would fly with all their fuel in one tank and the other empty?

When I fly an airshow routine at high density altitudes I dont want full tanks but I want the tank Im feeding from to be nearly full. I might have 16 gal in one and 5 gal in the other.

Cm
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