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G forces and all up weight

dave_091

Active Member
Hi guys,

A question about max G’s with max gross aerobatic weight in aircraft.

Aircraft are stressed and rated for a certain weight and a certain amount of G.

If an aircraft is rated for 6 G at 2,000 lbs. that would equal 12,000 lbs. max while experiencing a 6 G load.

Would that same aircraft be safe to fly at 2,250 lbs at 5.3 G ?
12,000 / 2,250 = 5.3
 
Hi guys,

A question about max G’s with max gross aerobatic weight in aircraft.

Aircraft are stressed and rated for a certain weight and a certain amount of G.

If an aircraft is rated for 6 G at 2,000 lbs. that would equal 12,000 lbs. max while experiencing a 6 G load.

Would that same aircraft be safe to fly at 2,250 lbs at 5.3 G ?
12,000 / 2,250 = 5.3

The math isn't that simple. Bending loads depend on how the weight is distributed on the airplane, and to grossly overload an RV, the excess weight has to go in the fuselage, which disproportionately increases the bending loads at the wing root.

So, no. ;-)
 
The math isn't that simple. Bending loads depend on how the weight is distributed on the airplane, and to grossly overload an RV, the excess weight has to go in the fuselage, which disproportionately increases the bending loads at the wing root.

So, no. ;-)

Kyle, can you provide an example of that, with calculations? My understanding is that the OP is correct, that the loads imposed are calculated accurately.

Where the loads are placed within the fuse would influence handling, but not g force. In aeronautics, the load factor is the ratio of the lift of an aircraft to its weight  and represents a global measure of the stress ("load") to which the structure of the aircraft is subjected:

n = L W
where

n is the load factor,
L is the lift
W is the weight.

Since the load factor is the ratio of two forces, it is dimensionless. However, its units are traditionally referred to as g, because of the relation between load factor and apparent acceleration of gravity felt on board the aircraft. A load factor of one, or 1 g, represents conditions in straight and level flight, where the lift is equal to the weight. Load factors greater or less than one (or even negative) are the result of maneuvers or wind gusts.
 
NzG (weight times g) is how most modern military fighters determine max g over their weight range. 9g aircraft up to the equivalent of "max aerobatic gross weight" , then a linear decrease in max g when the weight is above that limit. There is a lot of experience to support this simple type of g reduction, but the final call should come from Vans.
 
One also has to consider landing loads at the increased weight, particularly, reserve landing loads addressed by drop tests that represent hard landings.
 
IMHO, the comments about bending moments and landing gear loads are valid.

There was even one RVer who extended his wingtips, not realizing that the extension was generating excessive bending moments on the outer wing panels. (!) I sure learned something from that!
 
Kyle, can you provide an example of that, with calculations? My understanding is that the OP is correct,

Here's an example:

24' span airplane with a 4' wide fuselage (and 10' wings).

Lightweight example (minimum fuel):

Fuselage weight (including pilot, luggage, etc): 1000 LBS.

Wing weight (including fuel): 100 lbs each. Assume weight is equally distributed along the wing and lift distribution is even as well (makes the math easier).

Total aircraft weight is 1200 lbs. Bending moment is driven by the fuselage weight because the wing structure has an even weight distribution.

So the bending moment at the wing root and 1 G is 1/2 the fuselage weight x 1/2 a wing's span, or 500 lbs x 5' = 2500 foot-lbs.

What if we add a 200 lb passenger?

The bending moment becomes 600 lbs x 5 ft = 3,000 foot pounds. That's a 20% increase in bending moment with only a 16.7% increase in aircraft total weight. 20/16.7 = 1.2, which is the ratio of how quickly the bending moment went up relative to the aircraft's total weight.

Point is, if the excess weight is in the fuselage, the wing root bending moment goes up at a disproportionate rate vs total weight.

And the only place to "overload" a stock RV is by putting fat people, sandbags, or something else of mass in the fuselage, because the fuel tanks have a fixed capacity.
 
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Sounds like people are looking for a "maximum fuselage weight", or more commonly known as a Zero Fuel Weight. So what's a reasonable ZFW to assume for an RV? I'm going to use the 7/8 weights because I'm building an RV-8.

Take off with 45 minutes fuel for a quick circuit with a big pilot/pax and left some baggage in there just because with an O-320 powered plane, that's 7gal fuel, or 42lbs. Lets call our ZFW 1760lbs or round down to 1750 to err on the safe side and for easy numbers. That's the sort of load we can expect.

Can also use the same logic for aerobatic loads, but higher fuel load for time to complete some aerobatics.

Landing loads can be dealt with by imposing a maximum landing weight. It's reasonable to assume a 1800lb RV-7/8 can take off at 1800lbs, do a circuit and land safely at 1790lbs, having burnt 10lbs of fuel. Then with a maximum landing weight, we're talking about extending the time required to return if an issue is encountered shortly after take-off. While not applicable, there are regulations that can be used to guide this.

There's a post earlier about fighter jets. I'd take this with a grain of salt as fighters store a lot of their weight suspended from the wing, and don't impose additional wing root bending in the same way fuselage weight would.

After all that rambling, I suppose the point is that if we keep weights and loadings reasonable, I don't see why there shouldn't be a direct correlation between G loading and all-up weight.

On another note, Van's published aerobatic weight limits for all applicable models. These being 1600lbs for the RV-7/8. What category of operations are expected between this and the max gross weight (1800lbs)? Are we now in normal category (typically 3.8G limit) or utility (typically 4.4G limit)? Extrapolating linearly, we're at 5.3g at 1800lbs (1600 x 6 = 9600 / 1800 = 5.3).
 
... I'm building an RV-8 ...

On another note, Van's published aerobatic weight limits for all applicable models. These being 1600lbs for the RV-7/8. What category of operations are expected between this and the max gross weight (1800lbs)? Are we now in normal category (typically 3.8G limit) or utility (typically 4.4G limit)? Extrapolating linearly, we're at 5.3g at 1800lbs (1600 x 6 = 9600 / 1800 = 5.3).

For the RV-8, between 1600 pounds (or 1550 pounds for those with the non-Dash-One wing) and 1800 pounds, Utility Category limits apply.

Ref: "Flying an RV" - https://www.vansaircraft.com/flying-an-rv/

"The RV-3B, RV-4, RV-7/7A, RV-8/8A and RV-14/14A have been designed for the operational stress limits of the aerobatic category (+6.0/-3.0 G) at and below their aerobatic gross weights. The operational stress limits for these aircraft between their aerobatic gross weights and their maximum design gross weights are utility category (+4.4/-1.75 G). The RV-9/9A, RV-10 and RV-12 are not designed for aerobatic flight.

The design operational stress limit for the RV-9/9A is utility category (+4.4/-1.75 G) at less than 1600 pound gross weight and is standard category (+3.8/-1.5 G) between 1600 pounds and the aircraft’s design gross weight. The design operational stress limit for the RV-10 is standard category (+3.8/-1.5 G).

No RV should ever be operated above its design gross weight limit."​

Stay with the limits the designer (Van's Aircraft) specifies, or do all the engineering calculations and testing necessary to validate your numbers, taking into account (for all parts of the airplane) flight and ground loads, gust loads, stress and fatigue, and flutter. Depending on what you're doing, all of those disciplines may or may not be affected.
 
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Here's an example:

The bending moment becomes 600 lbs x 5 ft = 3,000 foot pounds. That's a 20% increase in bending moment with only a 16.7% increase in aircraft total weight. 20/16.7 = 1.2, which is the ratio of how quickly the bending moment went up relative to the aircraft's total weight.

Point is, if the excess weight is in the fuselage, the wing root bending moment goes up at a disproportionate rate vs total weight.

And the only place to "overload" a stock RV is by putting fat people, sandbags, or something else of mass in the fuselage, because the fuel tanks have a fixed capacity.

Well, yes and no. 1) one of the questions the OP asked is whether the g's imposed are as he postulated, and they are.

You are arguing that depending on the placement of the weight, those g's might not be safe at weights above what Van's cites as its aerobatic weight limits, which is some of what the OP asked, so I agree in theory with that part of your comment, which leads us to...

2) Unless you're flying an -8/8A or 10A, I have serious doubts about the possibility of significantly altering the CG as much as indicated by your calculations given the seating and storage sizes, though given a will, there will be a way. Maybe if you used 3' instead of 5', but then the weights may have to be significantly more than the max weight of the aircraft?
 
There's a post earlier about fighter jets. I'd take this with a grain of salt as fighters store a lot of their weight suspended from the wing, and don't impose additional wing root bending in the same way fuselage weight would.

Just because I'm not a fan of being dismissed with a hand waive, the F15 can exceed basic fighter design gross weight (the equivalent of max aerobatic weight) with centerline tanks and fuselage weapon loadouts with nothing on the wing. The F16 can exceed it with just a centerline tank and very little on the wing. The 22 and 35 can exceed it with full internal bays based on fuel loading. While fuel in the wing does help decrease root bending moments for symmetric loading, it's also normally the first tanks to be emptied because it increases rolling moment of inertia.

The original question was "is this a good theory on how to limit g" and the answer is yes, it's a good theory because it's in use by a lot of planes today. Only Vans has all the data for exactly where you start hitting 100% load limits, which is why I ended my first post saying it was up to them. If the article linked says use a step down limit from 6 to 4.4 above 1600 lbs, then you should abide by that. But that doesn't mean the theory is bad.

Vans also doesn't publish a roll rate limit for 6g operation. I would be more worried about assymetric loading then symmetric loading slightly overweight. They couldn't have done the calculations at zero roll rate. Even though you're not supposed to roll and pull, the roll rate is never exactly zero. So does the 6g limit apply up to 10 deg/sec, 20, 50? That also affects root bending.
 
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Just because I'm not a fan of being dismissed with a hand waive, the F15 can exceed basic fighter design gross weight (the equivalent of max aerobatic weight) with centerline tanks and fuselage weapon loadouts with nothing on the wing. The F16 can exceed it with just a centerline tank and very little on the wing. The 22 and 35 can exceed it with full internal bays based on fuel loading. While fuel in the wing does help decrease root bending moments for symmetric loading, it's also normally the first tanks to be emptied because it increases rolling moment of inertia.

Didn't mean to be insulting, I apologize. Just that a small single engine piston aircraft is a bit different than a fly-by-wire jet fighter. Maybe not quite apples to oranges, but maybe Golden Delicious to Granny Smith.

Other aerobatic planes that I've flown that are certified in multiple categories also have baggage compartment limits for aerobatics. For example, put 100lbs in the compartment for 3.8g (normal) or 4.4g (utility), but it needs to be empty for 6g aerobatics. I haven't seen restrictions like this in the Van's aircraft family, but am happy to be proven wrong. Not that I'm intending to go against Van's recommendations. Curious why a fighter jet has the linear relationship discussed by agent4573 but Van's has a sharp change in G limit between 1600lbs (aerobatic) and 1601lbs (utility). Maybe they just say it that way for simplicity?
 
Thanks everyone

After reading your replies, and searching other posts, I’ve found the answer I’ve been looking for.

Speaking in layman’s terms, yes the original post about taking 2,000 lbs at 6 G’s would be the same as 2,250 lbs at 5.3 G’s. The math works.
However, that would only work in a perfect world where that extra 250 lbs is spread evenly throughout every inch of the entire aircraft, from wingtip to wingtip and nose to tail.
We don’t live in that perfect world. The extra 250 lbs would actually be put in a passenger seat which is most likely going to bring the C of G further aft. When pulling G with the extra 250 lbs in the rearward location extra twisting forces would be exerted on the wings which could cause the wings to fail, among other not so great things.

So, therefore fly within the envelope the aircraft was designed to fly within. Simple.
 
Speaking in layman’s terms, yes the original post about taking 2,000 lbs at 6 G’s would be the same as 2,250 lbs at 5.3 G’s. The math works.
However, that would only work in a perfect world where that extra 250 lbs is spread evenly throughout every inch of the entire aircraft, from wingtip to wingtip and nose to tail.
We don’t live in that perfect world.

This is exactly the issue with operating over the prescribed gross.

A simple example: You are allowed 6G aerobatic, let's say that's with a 200 lb pilot (just for example, probably not the actual limit). That means the seat structure is designed for a 1200 lb load on it before it bends (6G) and 1800lb load before it fails (9G). If you happen to weigh 300lb, you can carry less baggage and fuel to get your gross weight within the overall airframe limit, and the wings aren't going to come off. But you're still putting 300lb on a seat structure designed for 200lb. At 6G, that's 1800lb, the failure load at 9G.

The limit isn't all about the wings.
 
This is exactly the issue with operating over the prescribed gross.

A simple example: You are allowed 6G aerobatic, let's say that's with a 200 lb pilot (just for example, probably not the actual limit). That means the seat structure is designed for a 1200 lb load on it before it bends (6G) and 1800lb load before it fails (9G). If you happen to weigh 300lb, you can carry less baggage and fuel to get your gross weight within the overall airframe limit, and the wings aren't going to come off. But you're still putting 300lb on a seat structure designed for 200lb. At 6G, that's 1800lb, the failure load at 9G.

The limit isn't all about the wings.

It is even more complicated than that.....

The effective result is different depending on whether you are carrying less baggage, or less fuel.

Example - If the additional weight is actually more fuel in the tanks, this will reduce the bending moment at the wing attach point because the additional weight in the wing will add additional downward bending moment which will counter / oppose the upward bending moment on the wing that results from the wings lift supporting the load of the fuselage under positive G's.
But this influence is only present where the additional weight is located, so on the portion of the wing that is just outboard of the fuel tank, the bending moment will have increased because of the additional fuel weight.
As already mentioned, if all of the additional weight is within the fuselage, then the bending moment on the wing attach point at any given G value will be increased.

But I agree that localized loads on structure other than just wings have to be consider.
 
Just because I'm not a fan of being dismissed with a hand waive, the F15 can exceed basic fighter design gross weight (the equivalent of max aerobatic weight) with centerline tanks and fuselage weapon loadouts with nothing on the wing. The F16 can exceed it with just a centerline tank and very little on the wing. The 22 and 35 can exceed it with full internal bays based on fuel loading. While fuel in the wing does help decrease root bending moments for symmetric loading, it's also normally the first tanks to be emptied because it increases rolling moment of inertia.

The original question was "is this a good theory on how to limit g" and the answer is yes, it's a good theory because it's in use by a lot of planes today. Only Vans has all the data for exactly where you start hitting 100% load limits, which is why I ended my first post saying it was up to them. If the article linked says use a step down limit from 6 to 4.4 above 1600 lbs, then you should abide by that. But that doesn't mean the theory is bad.

Vans also doesn't publish a roll rate limit for 6g operation. I would be more worried about assymetric loading then symmetric loading slightly overweight. They couldn't have done the calculations at zero roll rate. Even though you're not supposed to roll and pull, the roll rate is never exactly zero. So does the 6g limit apply up to 10 deg/sec, 20, 50? That also affects root bending.


Fighters tend to be a different animal in that they carry far more fuel in the fuselage than the wings and when load is added a significant amount is often hung on the wings.
Most non fighter jet aircraft burn the fuselage tanks first and then the wings to reduce long term fatigue on the wing structure.
 
It is even more complicated than that.
Definitely! ;)

Example - If the additional weight is actually more fuel in the tanks, this will reduce the bending moment at the wing attach point because the additional weight in the wing will add additional downward bending moment which will counter / oppose the upward bending moment on the wing that results from the wings lift supporting the load of the fuselage under positive G's.
But this influence is only present where the additional weight is located, so on the portion of the wing that is just outboard of the fuel tank, the bending moment will have increased because of the additional fuel weight.
As already mentioned, if all of the additional weight is within the fuselage, then the bending moment on the wing attach point at any given G value will be increased.
I agree with all of this. But it makes me wonder, since Van's doesn't publish different aerobatic gross weights for different fuel loading, is it safe to say that the aerobatic gross is valid at *any* fuel loading?
 
Definitely! ;)


I agree with all of this. But it makes me wonder, since Van's doesn't publish different aerobatic gross weights for different fuel loading, is it safe to say that the aerobatic gross is valid at *any* fuel loading?

Vans has answered that in the past and the answer was “the limit is the limit” or words to that effect. They don’t want to own the grey area where builders/flyers like to venture.
 
I agree with all of this. But it makes me wonder, since Van's doesn't publish different aerobatic gross weights for different fuel loading, is it safe to say that the aerobatic gross is valid at *any* fuel loading?

No.
For the reason already mentioned.
Fuel is only carried in the inboard portion of the wing, so fuel weight only reduces the bending moment on that portion of the wing…. There would still be an increase in bending moment on the portion of the wing that is outboard of the fuel tank.
 
Aerobatic Gross

No.
For the reason already mentioned.
Fuel is only carried in the inboard portion of the wing, so fuel weight only reduces the bending moment on that portion of the wing…. There would still be an increase in bending moment on the portion of the wing that is outboard of the fuel tank.

Scott,

I think you misinterpreted the question. It appears to me that he is asking if the aerobatic gross is the same regardless of the fuel load…i.e. 1600 lbs on empty tanks AND 1600 lbs on full tanks, NOT 1600 lbs excluding fuel….

I’m assuming that he is also referring to fuel load in the stock tanks only…

Skylor
 
Scott,

I think you misinterpreted the question. It appears to me that he is asking if the aerobatic gross is the same regardless of the fuel load…i.e. 1600 lbs on empty tanks AND 1600 lbs on full tanks, NOT 1600 lbs excluding fuel….

I’m assuming that he is also referring to fuel load in the stock tanks only…

Skylor

After rereading it, I think you’re right. I miss interpreted it as a question regarding whether fuel load was considered as part of the Aerobatic gross weight limit. The aerobatic gross weight for all of the different models is listed as the weight limit with any amount of fuel that you could carry in the standard kit built tanks.
 
Threads like this are entertaining because I've been seeing them come up every few months for the last 11 years, with people trying to find the one loophole nobody has yet discovered for doing aerobatics at max gross :D


Maybe a different question:

If a builder said, "I've flight tested this specific experimental/amateur-built airplane performing the following aerobatic manoeuvres at max gross with full fuel and sandbags in the pax seat without observing inelastic deformation," would a DAR sign it off?

(and yes, I'm aware that flight testing doesn't begin until the DAR has already signed it off, but that's not really the point)

This does seem like a higher-g and higher-stakes version of the way RV-6 builders spent years writing 1650 lb or even 1800 lb max-gross on their paperwork even though the plans said 1600.

This is very much a wise-vs-legal discussion, like a lot of other aviation endless debates. The math and engineering is pretty clear and has been widely stated ad nauseam for at least as long as the expressed intent to fly barrel rolls above max aerobatic gross.

- mark
 
The aerobatic gross weight for all of the different models is listed as the weight limit with any amount of fuel that you could carry in the standard kit built tanks.
Yes, that is exactly what I was asking, thanks. Sorry if that was worded poorly. :)
 
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