Mild aerobatics in the 9A?

numbers

You can have all those numbers ratteling around inside your head if you want, but as a pilot suppose you are flying many different type airplanes. Will you memorize the structural numbers for each plane you fly? The way to fly many different type airplanes and still keep your sanity is to comply with the POH. What your interested in is gross weight, CG limits, VNE, VLE, VFE, VNA, operating limitations, and placards (spins prohibited etc etc etc). Since you write your own POH for experimental aircraft, The structural numbers are really nice to add to your document but that dont fly the plane. Does anyone know of a DAR who will sign off an RV9 for aerobatics?

This has nothing to do with aerbatics, but did you know, they are still looking for Amelia Earhart.

kevinh said:

Yes. When I read this thread I sent an email to support@vans to confirm. And Van himself wrote back to say, yes - the fuel in the wing tanks doesn't count against the aerobatic gross weight - because it is so close to the center of lift. Of course, lighter is better, but worth considering if you are thinking about a six.

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OK, so if we can go 1375 without fuel, since it doesn't count, then add 32 gallons at 6 #'s/gallon, or 192#'s, yielding a gross weight of 1567#'s.

Based on Van's specs, normal gross weight is only 1500#'s!

What am I missing here??? I am trying to finalize my POH, and this has just become an important consideration

Fuel doesn't count against you as far as pulling G's during manuevers because it's stored in the wing which is producing the lift, so it doesn't add additional stress to the spars and other aircraft structure. It does, however, add total weight - and the wing still has to support all that weight somehow - so it will affect your stall speed and angle of attack, both of which will be critical items if you are planning on pushing the envelope.

A 6-G turn when empty of fuel is the same as a 6-G turn full of fuel, as far as the aircraft structure is concerned - but it's quite different as far as the angle of attack and lift reserve are concerned.

Or did I misread that entirely, and you were only asking about the weight issue as a math question? Max gross does not always mean full fuel - many airplanes cannot carry full fuel plus two plus bags - you have to leave a seat empty, or baggage light, if you want full fuel. That's simply to keep the wing loading down to keep the stall speed down to a reasonable number.

airguy said:

Fuel doesn't count against you as far as pulling G's during manuevers because it's stored in the wing which is producing the lift, so it doesn't add additional stress to the spars and other aircraft structure.

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This is only partially true. The fuel is not stored in the entire span of the wing in RV's so there is a variable load applied to the outer portion of the wing in direct relation ship to how much fuel is in the tank.

The only RV model that it is acceptable to not include fuel load when calculating aerobatic gross weight is an RV-3 with wing tanks.

Fuel weight must be considered as part of the total when calculating aerobatic gross weight on all other RV models that are approved for aerobatics.

If someone thinks they heard differently when talking to a person at Van's, they either misunderstood or received incorrect information.

F=ma?

airguy said:

A 6-G turn when empty of fuel is the same as a 6-G turn full of fuel, as far as the aircraft structure is concerned - but it's quite different as far as the angle of attack and lift reserve are concerned.

.

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Im trying to think this thru, and if I use 1400 lbs for the mass with no fuel, and 6g for the acceleration, I get a force of 8400. If I then add 36 galloons of fuel at 6lbs per gallon, I get a new mass of 1616 lbs. I then pull 6g with 1616 lbs and I get a force of 9696. Thats quite a bit more force than 8400, and since the aluminum will bend when force is applied what is to keep my plane from bending when a greater force is applied? Does 8400=9696? Im a bit rusty at this so I forgot what the units would be but I think if yur using lbs for mass and ft/sec^ for accelration you would get ft/lbs of force. If youve ever used a torque wrench on an AN4 with 160 inch/lbs, imagine how much it would be at 9696 ft/lbs. Of course if you divide that by 2 you get 4848 for each wing providing the force is equally distributed. But thats not likely. To me it brings up the question of how many ft/lbs does it take to fail the wing on a RV9? I dont know the answer.

I realize my math is not so right on and the real numbers using 32ft/sec/sec= 1g you get a different number, but you also would get different units.

RVbuilder2002 you're correct - I wondered if anyone would catch the bit about full-span fuel loading.

Sportpilot - your numbers aren't wrong - that's the correct amount of total lift the wing must produce to turn the aircraft, since the lift must move the total amount of weight. The key difference here is WHERE that amount of force is applied. If the fuel is in the wing tanks (and lets simplify here and say it's spread along the WHOLE wing, like the RV3 RVBuilder2002 mentioned), and each wing is producing 4848 pounds of lift, then we now have a force budget to work with. The fuel in the wing is supported by the wing itself - the tank is integral to the wing, attached to the spars, same as the wing skin which is actually transmitting the lift to the spars. As the wing tries to move upward in reaction to the lift force, it carries the fuel upward with it - accelerating the fuel at 6 G's. So subtract that amount of force from your total force budget (total lift available) and you'll get the remaining available force that can be transmitted to the fuselage. This remaining amount of force is the amount of force required to move the fuselage at 6 G's - and it will always be the same, regardless of fuel quantity, since no fuel is carried in the fuselage (on this hypothetical aircraft for this exercise).

Let's assume 6G is the max allowable for this aircraft - that's based on the BENDING MOMENT loads imposed on the carry-thru spar that transmits the lift force from the wing to the fuselage. At 6G, xxxx pounds of upward force are transmitted thru the spar - and it can't carry any more. The fuel in the wing is already being accelerated by the wing force BEFORE that force gets to the fuselage - so the bending moment of the carry-thru is not changed by the amount of force required to accelerate the fuel.

Put another way - the wing has to support the amount of force required to accelerate the fuselage at 6G's (which exerts a bending moment on the carry-thru spar) PLUS it's own weight (with no bending moment) PLUS fuel weight (with no bending moment). Structural limitations of the carry-thru spar are not affected by the fuel load for FULL-SPAN wing tanks. This exercise is only partially correct for the other models with less than full-span tanks, as you'll be imposing a new bending load location at the point of the wing where the outboard edge of the fuel tank exists.

If you could build a true, pure flying wing, with all the weight of the aircraft spread completely evenly across all the surface of the wing, there would be essentially no limit to the G forces it could pull - until you got the point where the lift force is so great it rips the wing skin off the spar.

n2prise said:

This thread really grew fast! Lots of statements both ways. I for one, did not like doing the spin recovery in a 152 when I was a student pilot in 1992..........

........As for Bob Hoover and the AeroCommander Shrike, when he finished his routine with both engines off from way up there, he always did three loops (very lazy), three rolls (also lazy), and did a dead-stick spot landing at show center. When a guy like that survives WW2 combat, does the stuff he did at Edwards AFB after the war with Chuck Yeager, and as an aging air show pilot, makes the maneuvers look easy - - it is obvious he understands the physics (G-loads) of safe flight.

Nuff said......fly safe!

Jerry K. Thorne
East Ridge, TN
RV-9A #90622 Slow-build N2PZ

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Jerry,

The 152 (150) can be VERY scary if a spin recovery is not done correctly. A long time ago a young CFI was told by his boss to get me signed off for spin recovery in pursuit of a CFI rating. I asked the young guy if he was going to demonstrate the manuever and he said no, you've done enough of them with your training. I said OK, lets do it.

Stall the airplane, kick some left rudder, full aft stick and sit there and watch it wind up for several turns. Pretty routine. But not for long. The military trainers I had been broken in on required definite forward stick action to get the nose down, the little Cessna does not. In fact, such stick action in a Cessna will result in a near vertical dive with the ASI through the red line very quick. I must have used 3000' vertical down getting back to level flight very gently, trying not to jerk the wings off. I asked the CFI if he wanted to see another one and he said, no that will do.

Point here, just RELAX back pressure with a Cessna, it will resume flight nicely without going through Mach 2.

Bob Hover did get away with a lot of neat flying stuff over the years and made it look effortless, but not without some pain now and then. On at least one occasion, he broke his back in several places crash landing a F-100 that did not flare without power.

Mild aerobatics the with a -9? A good pilot can get away with it indefinitely. Any airplane will stand 2 G's all day. The problem with doing seemingly innocent little manuevers is they can get out of hand quick if the pilot is not spun up and paying attention. It happened to me in the -7A not long ago doing a simple roll. Problem was I went into it with the nose not high enough and found myself inverted looking a lot of trees and continuing the roll did not make the trees go away, resulting in a lot more speed than necessary or smart, and took a few unspecified G's to get rid of the tree picture. Yea, forward stick would have gotten rid of the trees inverted, but the engine sure would have quit as there are no inverted systems installed. It was one of those, "why did I do that really dumb thing"?

Point two here, don't do anything inverted half cocked and not thinking about what is going on. Steep turns, wing overs, lazy eights, chandelles, have at it, but keep the G load positive and smooth. Leave the "stick in the corner" stuff for the professional stunt pilot, who on occassion, as confident as can be, does not have a maneuver turn out as planned. Several times a year these guys make a smokey hole in the ground in front of a crowd somewhere on the planet.

As you said, Nuff said......fly safe!

dd
Troy, Missouri
RV-7A N707DD
Subby H6

airguy said:

If you could build a true, pure flying wing, .

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Ok I think Im closing in on the concept. So it is only the weight of the fuselage and tail that is trying to bend the wings, because the wings themselves arent trying to bend the spar because they are producing lift. And all these years when the FAA says Gross weight, I thought it meant Gross weight. I wonder why the FAA cant figure this out. They keep telling me that my speed limits include the wing. Now im wondering what do the equations for structural failure look like if an equation must embrace the sum of all the forces equal zero. That would mean the wing would produce negative numbers? Does it matter if you encounter gust loads? I saw a boeing 720 that got destroyed by gust loads while flying straight and level. What does gorss weight mean?

Just to clarify one point.

airguy said:

RVbuilder2002 you're correct - I wondered if anyone would catch the bit about full-span fuel loading.

If the fuel is in the wing tanks (and lets simplify here and say it's spread along the WHOLE wing, like the RV3 RVBuilder2002 mentioned),

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I just wanted to clarify that the RV-3 doesn't have full span fuel tanks. My understanding is that because the wing structure was orig. designed for flight loads using a single fuselage tank, that with wing tanks of very similar capacity to the orig fuselage tank, the fuel weight is not enough to be critical for the load imposed on the outboard wing structure at aerobatic G loading. An RV-3 with a fuselage tank, the fuel weight must be figured into the aerobatic gross weight since all fuel is in the fuselage and inducing a bending moment at the wing to fuselage attach point for any given G load.

Great explanation airguy

rvbuilder2002 said:

The only RV model that it is acceptable to not include fuel load when calculating aerobatic gross weight is an RV-3 with wing tanks.

.

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Scott:

What is the source of this statement? According to Randy Lervold's site, the following is supposedly Van's response:


See the whole story here:

http://www.romeolima.com/RV3works/Info/info.htm.

Please advise as there are many pilots out here doing acro in RV's.

chuck said:

Scott:

What is the source of this statement? According to Randy Lervold's site, the following is supposedly Van's response:

-There has been some confusion on computing Aerobatic gross weight. As confirmed with Van's aircraft on 12/16/05, for any RV, including the RV-3, which has wing tanks, Aerobatic gross weight does NOT include fuel in the wing tanks. Said differently, the pilot may compute (and not exceed) the published Aerobatic Gross Weight figure for his/her model, then add fuel on top of it up to the Normal/Utility category gross weight of the aircraft. The reason for this is that fuel essentially becomes part of the wing structure. Van's is quick to point out however the detrimental effects of higher weights on aircraft performance when operating in the Aerobatic category and encourages pilots to excercise good judgement and caution.​

See the whole story here:

http://www.romeolima.com/RV3works/Info/info.htm.

Please advise as there are many pilots out here doing acro in RV's.

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I personaly know Randy, but I believe he either misunderstood some info provided or someone provided incorrect information.

Ken K. (head of engineering at Van's) and and I were talking about this today.

The word gross generaly means all inclusive. Maximum Aerobatic gross weight is just that... Max. total weight for aerobatics.
If this were not the case then Van's specifying an aerobatic gross weight for RV's would be pointless.
Such as the following example...

RV-8 has an aerobatic gross weight of 1550 lbs. The fuel capacity is 252 lbs (42 gal / 6lbs per gal). If we need not figure fuel into the aerobatic gross weight, we can just fill the tanks and go yank and bank as long as we are 1550 or below before adding fuel.
This is false.
1550 lbs + 252 lbs is 1802 lbs. This is 2 pounds more than the max allowable gross weight of 1800 lbs. If this is OK then specifying the aerobatic gross weight is pointless.

The reason for publishing the aerobatic gross weight is this is the weight designed for at a limit load of 6 G's, and this is the weight the wings were tested to to prove they actually performed as designed.

As stated previously, the only exception to this is the RV-3 with wing tanks.

rvbuilder2002 said:

This is false.
1550 lbs + 252 lbs is 1802 lbs. This is 2 pounds more than the max allowable gross weight of 1800 lbs. If this is OK then specifying the aerobatic gross weight is pointless.

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I understand your example but if you do the same for the RV3 I calculate that acro gross of 1050 + 180lb of fuel is 80lb over the 1150 gross....

So now I'm confused, one lister claims that Van himself said that ALL acrobatic RV's acro weight does not include fuel and another says that Ken K says this is only true for 3's with wing tanks.

Is there someone on the list (Randy & Scott?) who can get someone at Van's to explain the Acro Gross Weight for the 3,4,6,7 & 8. A PDF with Vans Aircraft letterhead would be prefered Seems pretty important that Vans aircraft be precise on this operating limitation.

This thread is hijacked enough at this point, perhaps the answer could be put in a more aptly named thread

So the plane is everything ??????

rvbuilder2002 said:

Don't you think the guys at Van's that designed the airplane know what the airplane is capable of?
(P.S. Not meaning this as a flame job directed at you Cam, just tired reading all the second guessing the designer posts in this thread)

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Do I think that the guys at Vans know? Yes, of course. I'm not second guessing them at all. I'm second guessing the guys on this list that keep wanting to promote the idea that the plane is what makes you safe. So you guys keep beating your chest, I'm going flying.

Know the best way to increase what you can carry at aerobatic gross weight? Go on a diet. Every 6 lbs is an extra gallon of fuel.

So far, I've lost 2 gallons. I've got another 2 or 3 gallons to go...

N916K said:

Do I think that the guys at Vans know? Yes, of course. I'm not second guessing them at all. I'm second guessing the guys on this list that keep wanting to promote the idea that the plane is what makes you safe. So you guys keep beating your chest, I'm going flying.

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[ edit - I post this just to correct the record. I think it would be dumb to attempt acro in the 9. ]

Here's the reply I recieved from Van - he says fuel in the wing has little effect on the aerobatic gross:

Subject: Re: For max aerobatic gross weight, does fuel count
From: "Dick VanGrunsven"
Date: Tue, 2 May 2006 11:32:40 -0700
To: Kevin Hester

Kevin,

The aerobatic gross weight of the RVs (with wing tanks) is
basically the zero fuel wt. Weight of Fuel in the wings has little
effect on the spar bending loads caused by G-forces. That said, it
is still a good idea to keep fuel to a minimum during aerobatics.
More fuel in the wings affects rolling inertia and increases stall
speed. The lightest airplane is the most aerobatic.

Dick VanGrunsven


> > Hi,
> >
> > I'm just about to start doing aerobatics in my RV-7A. In the process
> > I've been reading lots of web articles. In the process I found the
> > attached posting. I had always assumed that fuel in the wing tanks
> > was included in the weight for purposes of the max aerobatic weight,
> > but here someone says Van said that was not his intent. Can you clear
> > this up for me? Is the attached post an accurate description of the
> > Van's position?
> >
> > Kevin
> >
> > Encl: the posting that confused me
> >
> > (from
> > http://www.vansairforce.com/community/showthread.php?t=4595&highlight=
> > aerobatic+gross+weight+fuel)
> > ...

sportpilot said:

Ok I think Im closing in on the concept. So it is only the weight of the fuselage and tail that is trying to bend the wings, because the wings themselves arent trying to bend the spar because they are producing lift. And all these years when the FAA says Gross weight, I thought it meant Gross weight. I wonder why the FAA cant figure this out. They keep telling me that my speed limits include the wing. Now im wondering what do the equations for structural failure look like if an equation must embrace the sum of all the forces equal zero. That would mean the wing would produce negative numbers? Does it matter if you encounter gust loads? I saw a boeing 720 that got destroyed by gust loads while flying straight and level. What does gorss weight mean?

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I thought we were talking strictly about accelerative forces - of course you still have to consider drag/airspeed on the wing, you don't get to ignore that. Gust loads do matter, of course - if you have a gust that exceeds the structural integrity you're SOL. That's the reason for building in a 50% excess load factor above what the aircraft is rated for. Rated G's and absolute structural G's are not the same - they take into account likely gust loads.

airguy said:

Rated G's and absolute structural G's are not the same - they take into account likely gust loads.

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Thanks airguy, for guiding me thru this thread. I come away with new knowledge because I did not know you could ignore the fuel at aerobatic gross. I do understand that a design limit of 6g carries an ultimate failure of 9g. As long as you dont bend anything beyond 6g you should be safe if you comply with the aerobatic gross weight limit. I dont think it means you can extrapolate that to the RV9 with the idea if you dont bend it beyond 3.5g you will be safe. If your going to bend an RV9 around your on your own just like any other normal catagory plane. thanks

Uhhhh - no. I don't intend to try bending my 9 around anything. If I wanted acro I would choose a 7. The conversation was hypothetical (I thought).

RV9798 said:

Every plane has a mission:

A 9 is for crosscountry. If you want to do acro.....don't build a 9. If you know what you are doing (Acro) you can do in any plane. Just don't blame the designer if it goes wrong.

Fly safe

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I wrote this in 2006. Since then I flew 250 hrs on the 9 and I did a few "Recoveries from unusual attitude" with this cool machine. The manoevers looks like lazy barrel rolls, wing overs and large looping patterns. I think the forces of a mid day turbulence on the plains of Idaho or Arizona is much worse ("G" wise) than the recovery from a roll or a loop. I practice to make this a non ("G") event. Good knowledge and skill.....so I don't panic and screw up when it really matters.

thallock said:

So, Dave, I guess you're saying I was irresponsible when I took that 2 hour aerobaticas intro in the 152 aerobat? The only difference between a 152 and a 152 aerobat is that the aerobat has quick release pins for the door, so you can get out if the tail falls off.

Cheers,
Tracy.

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The 152 AEROBAT also has about 10 pounds of additional stucture at the tail attach points.

rvbuilder2002 said:

(the reason that FAR 23 certification requirements for aerobatics approval requires a +6/-3 G. limit load rating, among other things, isn't so that people flying the airplanes can go out and do 6 G. loops all day. )

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How does a brand new Citabria get away with being 5g rated and legal for aerobatics?