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G loading on RV6

fbrewer

Well Known Member
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)?
 
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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
 
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.
 
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.
 
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.
 
Who weighs a 45kg's? I'm 183cm tall 90kg's, pretty standard for a lot of RV drivers:D I never fly around wth such uneven tanks!
 
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.
 
More fuel in one tank

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
 
The airplane was certified to a gross weight of 1800 lbs.

I've been told, but I'm still in the process of verifying the g limits of this RV6

Aerobatic category +6 / -3 when the aircraft weight is less than 1390 lbs

Between 1390 and 1600 lbs, the g limits are utility category: +4.4 / -1.75

Between 1600 and 1800, the g limits are standard category: +3.8 / -1.5

If the g limits are verified, would the following weight / g limit chart be approximately accurate? Note, I only plotted positive g limits.

G%20Limit%20Box%20for%20N664SB.png


Does anyone have an RV6 VN giagram?
 
The airplane was certified to a gross weight of 1800 lbs.

I've been told, but I'm still in the process of verifying the g limits of this RV6

Aerobatic category +6 / -3 when the aircraft weight is less than 1390 lbs

Between 1390 and 1600 lbs, the g limits are utility category: +4.4 / -1.75

Between 1600 and 1800, the g limits are standard category: +3.8 / -1.5

If the g limits are verified, would the following weight / g limit chart be approximately accurate? Note, I only plotted positive g limits.

G%20Limit%20Box%20for%20N664SB.png


Does anyone have an RV6 VN giagram?

Once you're above Van's supplied weights, you don't know what the G limits should be because you don't have access to the engineering data Van's used to set the original limits. It may be that (for instance) the gear is the limiting factor and not the aircraft structure.
 
I've been told, but I'm still in the process of verifying the g limits of this RV6
Told by the owner, some random Internet person, or the factory?

Between 1390 and 1600 lbs, the g limits are utility category: +4.4 / -1.75
Your graph shows (eyeballing here) a 5g limit at about 1470lb. Which is in excess of the numbers you state here.

If the g limits are verified, would the following weight / g limit chart be approximately accurate?
Only if the factory verifies them, or if you hire an aeronautical engineer to fully analyze the structure and confirm the design. Without one of those things happening, all you have is wags telling you their *opinion*, which won't keep the wings on when you exceed the limitations of *physics*.
 
Only if the factory verifies them, or if you hire an aeronautical engineer to fully analyze the structure and confirm the design. Without one of those things happening, all you have is wags telling you their *opinion*, which won't keep the wings on when you exceed the limitations of *physics*.

Technically, that wouldn't be enough....
Calculations and analysis is important but standard practice is to then (at a minimum) do a static load test to prove that the results of the analysis are valid. Engineering standards for metal aircraft structures is well defined but the real world result still doesn't always match even the most detailed analysis (just ask Boeing:rolleyes:).
 
The explanations of why fuel load must be included in the aerobatic weight calculation have finally made it clear to me why it is necessary to include fuel weight. Would the experts comment on some other aerobatic weight limit questions?

I've got an old Citabria manual from 1979. It shows load factors of +5/-2 at the 1650 lbs "normal and acrobatic" category and appears to have been approved for loops, snap rolls, spins and other maneuvers.

So, did the load factors for aerobatics change after 1979? Does the tendency of the 6A to build up speed when you point it down cause the RV to need higher load factors? Is it safe to perform aerobatics in a Citabria with the +5/-2 load factors compared to planes with +6/-3 load factors?

Also, I've read and heard that the 6/6A has the strongest wing of any of the RV's and that it withstood up to 12g in testing. What part of the 6/6A is most likely to fail first as a result of high G maneuvers?.

John
 
Also, I've read and heard that the 6/6A has the strongest wing of any of the RV's and that it withstood up to 12g in testing. What part of the 6/6A is most likely to fail first as a result of high G maneuvers?.

John

You're not going to get engineering answers to that question here. If anyone has done those calculation and tests, it is Van's, and if they have, their most definitive statement is going to be "It is safe at 6 G's and 1375 lbs. No further comment."
 
I see you're point Kyle. Maybe a better question would be "have been any in-flight structural failures in 6's and what failed." I think I remember a horizontal stab. failure with one of the 8's and I guess that would be a possible issue with the 6's given service bulletins regarding that structure. I've never gone higher than 4.5 g in my 6A. It's the tail that gives me the most concern when looping. My engine/prop combination are pretty light. John
 
The explanations of why fuel load must be included in the aerobatic weight calculation have finally made it clear to me why it is necessary to include fuel weight. Would the experts comment on some other aerobatic weight limit questions?

I've got an old Citabria manual from 1979. It shows load factors of +5/-2 at the 1650 lbs "normal and acrobatic" category and appears to have been approved for loops, snap rolls, spins and other maneuvers.

So, did the load factors for aerobatics change after 1979? Does the tendency of the 6A to build up speed when you point it down cause the RV to need higher load factors? Is it safe to perform aerobatics in a Citabria with the +5/-2 load factors compared to planes with +6/-3 load factors?

Also, I've read and heard that the 6/6A has the strongest wing of any of the RV's and that it withstood up to 12g in testing. What part of the 6/6A is most likely to fail first as a result of high G maneuvers?.

John


The Laminated plate main spar of the 4, and the 6 are quite robust in terms of absolute strength and fatique resistance /load cycle life.

However, simple G load and exceeding G limits are only a tiny part of what can bring any aircraft to structural grief. One could be doing a gentle 3G pull up , but encounter a vertical shear wind gust (crossing a mountain ridge, or even in certain turbulent clear air conditions) that were well beyond the limits.
The faster your forward speed, the less the time the shear effect is distributed over and a higher the peak load results from encountering a sharp vertical gust.

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25_341-1.pdf

It is simple and easy to fail any GA aircraft in 1 G flight by simply speeding past the Vne and exceeding the flutter speed limit. You don't need to be pulling any G at all.
 
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