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Tip - RV-4 Centre of Gravity Observations

islandmonkey

Well Known Member
I spent yesterday knocking together a generic RV-4 weight and balance spreadsheet. I derived the sheet from the one available from my website for the 3. After I finished the sheet I started playing around with the numbers and I found that it is possible with a certain set of parameters for the c of g to migrate outside the rear limit with fuel usage. There are simple safeguards however to prevent this. Removing any and all weight from the rear baggage area helps but the c of g must be checked against fuel usage so it stays inside the required envelope. Using the optional front baggage area does help a lot to keep c of g within range. I am talking here of rear seat loads of 250 to 300 lbs which are very unlikely but still have to be considered.

Normal operation with one on board, leaves the c of g well forward and near the front limit with no issues on fuel usage bringing the c of g outside the envelope.

It is almost impossible to do aerobatics in the 4 with two people on board the Aerobatic Gross Weight of 1375lbs sees to that, but with a light pax a short aero sortie may be possible.

I am going to use the 4 as a single seater mostly with an option of the occasional second seat for pax weighing no more than 170 lbs. Used as a single seater and provided I have no more than 100lbs of luggage divided between the rear seat and the baggage area, I will not have to give c of g a thought. When pax are involved, the c of g calculations will be done before the flight to ensure c of g remains in the required envelope.

Here is a link to the generic spreadsheet. Use it at your own risk:

RV-4 Weight and Balance
 
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200lb pax, everyday stuff.

I have a very nice Excel spread sheet for my -4 and I play with it often that way I always know what is possible before I give a ride, even if it’s on the spur of the moment out at the airport. I have had a 250 lb. pax in the back of my -4 and you are correct that there is a minimum fuel number to keep the CG from going out to the aft.

Flying the RV-4 at its most aft limit is not for beginners, work your way up to it slowly, it is a whole different animal loaded aft, be careful out there!
 
I have lost several friends to aft CG induced accidents. I don't see how you could squeeze a 250-300 pounder in the rear seat of an RV4 or why you would want to. A basic -4 is a wonderful single seater and will carry two but with attendant control issues as you approach rear CG. You can confirm this by loading to aft CG and then pulling 2-2.5 Gs. Far better to build a side by side or RV8 if you want to haul large folks.
 
I have lost several friends to aft CG induced accidents. I don't see how you could squeeze a 250-300 pounder in the rear seat of an RV4 or why you would want to. A basic -4 is a wonderful single seater and will carry two but with attendant control issues as you approach rear CG. You can confirm this by loading to aft CG and then pulling 2-2.5 Gs. Far better to build a side by side or RV8 if you want to haul large folks.

No need to judge. Just remember, every -4 is going to be different, do the math on your airplane, I have done the math on mine and know what is possible and have safely demonstrated it.
 
Thanks but...

Hi Thanks for doing this spreadsheet it saves me doing it. One point though, your statement that is impossible to do aeros 2 up is apparently in error.

Empty Aircraft 990 68.21464646 67532.5
Forward Baggage 0 56.5 0
FUEL 88 70 6160
Pilot 176 82.5 14520
Pax 121 107 12947
Aft Baggage 0 130 0



TOTAL 1375 101159.5
CG 73.57054545 CG = MOMENT/WEIGHT

Thats my weight and my wife, so it seems if I only had 88lbs fuel on board I could turn her world upside down......... makes a change from her doing it to me!!:)
 
I have lost several friends to aft CG induced accidents. I don't see how you could squeeze a 250-300 pounder in the rear seat of an RV4 or why you would want to. A basic -4 is a wonderful single seater and will carry two but with attendant control issues as you approach rear CG. You can confirm this by loading to aft CG and then pulling 2-2.5 Gs. Far better to build a side by side or RV8 if you want to haul large folks.

For me the 250lb pax will not be entering my airplane. Sorry big guys. Others here like Russ have different criteria and they seem not to mind carrying a large heavy mass in the rear. However Russ does it with knowledge that the c of g WILL move outside the envelope if one is not very careful.

I started this thread basically to give RV-4 owners and builders a heads up on a potential safety issue that could lead to someone having a really bad day. The 4 can be loaded so that it is legal at the start of a flight:


Loading Schedule 250lb pax (No way Jose)

Load Weight Arm Moment
Empty Aircraft 990 68,21464646 67532,5
Forward Baggage 0 56,5 0
FUEL 120 70 8400
Pilot 150 82,5 12375
Pax 250 107 26750
Aft Baggage 30 130 3900



TOTAL 1540 118957,5
CG 77,24512987 CG = MOMENT/WEIGHT


Maximum Gross Weight 1550 lbs
Aerobatic Gross Weight 1375 lbs

CG RANGE
Forward Limit 68,7 inches aft of DATUM
Aft Limit 77,4 inches aft of DATUM
Aerobatic Aft Limit 75,9 inches aft of DATUM

At the end of the flight with 50lbs of fuel remaining (8 US gallons approx):


Typical Loading Schedule (End of flight with 250lb pax)

Load Weight Arm Moment
Empty Aircraft 990 68,21464646 67532,5
Forward Baggage 0 56,5 0
FUEL 50 70 3500
Pilot 150 82,5 12375
Pax 250 107 26750
Aft Baggage 30 130 3900



TOTAL 1470 114057,5
CG 77,59013605 CG = MOMENT/WEIGHT


Maximum Gross Weight 1550 lbs
Aerobatic Gross Weight 1375 lbs

CG RANGE
Forward Limit 68,7 inches aft of DATUM
Aft Limit 77,4 inches aft of DATUM
Aerobatic Aft Limit 75,9 inches aft of DATUM

What other people choose to do when loading their RV-4 is entirely up to them. But be warned the 4 will bite you in the bum if you ignore this. I agree that a side by side or an 8 is better for large pax, especially side by side where the pax is located almost on the c of g.

Maybe Andy Hill can give his opinion on the 8 with heavy pax installed in the rear?
 
Hi Thanks for doing this spreadsheet it saves me doing it. One point though, your statement that is impossible to do aeros 2 up is apparently in error.

Empty Aircraft 990 68.21464646 67532.5
Forward Baggage 0 56.5 0
FUEL 88 70 6160
Pilot 176 82.5 14520
Pax 121 107 12947
Aft Baggage 0 130 0



TOTAL 1375 101159.5
CG 73.57054545 CG = MOMENT/WEIGHT

Thats my weight and my wife, so it seems if I only had 88lbs fuel on board I could turn her world upside down......... makes a change from her doing it to me!!:)

That means you will take off with just over 14 US gallons of fuel on board. Hopefully no one blocks the runway while your hanging your wife upside down Steve.

Or you could take off with more fuel and then time the fuel usage so that eventually you are inside the weight limit. Anyway I have edited my first post to say aeros with two up are almost impossible.
 
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Actually to Quote Van.

That means you will take off with just over 14 US gallons of fuel on board. Hopefully no one blocks the runway while your hanging your wife upside down Steve.

Or you could take off with more fuel and then time the fuel usage so that eventually you are inside the weight limit. Anyway I have edited my first post to say aeros with two up are almost impossible.

You need not count the fuel in the wings towards your aerobatic gross. either way I never pull more then 3gs, I commit two up acro frequently. YMMV;)

P.S. by the way never with a 250#er in the back!!!
 
You need not count the fuel in the wings towards your aerobatic gross. either way I never pull more then 3gs, I commit two up acro frequently. YMMV;)

P.S. by the way never with a 250#er in the back!!!

I personally, am not so sure on this one Russ. I know the fuel weight is concentrated outside of the wing root and therefore the effective downforce is eliminated. But the weight does have to be carried by something. So until I see a more comprehensive statement from Van that fuel weight can be eliminated, I am going to include fuel weight in my calculations for aerobatic gross weight. My spreadsheet tells me that I am under Aerobatic Gross Weight with just me aboard and full tanks with 40lbs of ballast in the baggage area to bring the plane into the c of g envelope.
 
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Morning

Hey

Things are getting better, I weighed myself this morning 165 lbs so thats 10lbs more fuel......... now how do I get that woman on a diet? Actually I could loos 10 lbs......... going at this rate we could have full tanks on takeoff!

I hope you are feeling better Anthony.

I had a good flight in the Rv9 yesterday, there was a Kittyhawk at the airfield. When I was coming downwind they decided to do some aeros as it left so I orbited at the end of the downwind leg. As he started his aeros I turned base so had a great view of him looping and rolling....... the best seat in the house!!

It made me think about the skill of the pilots in WW2, how would you shoot down something that was moving like that, when chasing it. Much better to bounce them when they are not expecting it!
 
Don't consider the CG issue as just for acro. On my XC trips with a passenger and baggage, I can easily exceed aft CG as fuel burns off so need to time my trips based on that not when I'm down to 1 hr of reserve (8 gal-my minimum). I've landed with aft CG and while it is not difficult, it is different and care must be taken not to land tail wheel first.

Russ, can you point out where Van made the comment on not needing to add in the fuel towards the aerobatic gross? Not doubting you but I would find it interesting to see the entire statement in context with where he said it.
 
Don't consider the CG issue as just for acro. On my XC trips with a passenger and baggage, I can easily exceed aft CG as fuel burns off so need to time my trips based on that not when I'm down to 1 hr of reserve (8 gal-my minimum). I've landed with aft CG and while it is not difficult, it is different and care must be taken not to land tail wheel first.

Russ, can you point out where Van made the comment on not needing to add in the fuel towards the aerobatic gross? Not doubting you but I would find it interesting to see the entire statement in context with where he said it.

I don?t think he has ever published it but a number of people on this forum have said he told them this and one had it in an e-mail and pasted it in this forum a number of years ago, might be able to find it using the search.
 
I don?t think he has ever published it but a number of people on this forum have said he told them this and one had it in an e-mail and pasted it in this forum a number of years ago, might be able to find it using the search.

I have been rummaging around without success trying to find this statement by Van. I have an idea it might have been in context with flying overweight RV-10's but I am probably wrong on this one. Oh by the way I have no axe to grind here on the subject of overweight 10's so please do not flame me on this one.

If anyone can find this statement by Van or has an idea where it might be located then I would be very grateful for the information. The statement I think says fuel need not be calculated into Aerobatic Gross Weight.

Thanks in advance.

Tony
 
I have been rummaging around without success trying to find this statement by Van. I have an idea it might have been in context with flying overweight RV-10's but I am probably wrong on this one.
...
If anyone can find this statement by Van or has an idea where it might be located then I would be very grateful for the information. The statement I think says fuel need not be calculated into Aerobatic Gross Weight.

Um - I think that long ago post was from me. I asked Van for his comment and he seemed to confirm what I had heard from others:

http://www.vansairforce.com/community/showthread.php?p=84684#post84684
 
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Hey

Things are getting better, I weighed myself this morning 165 lbs so thats 10lbs more fuel......... now how do I get that woman on a diet? Actually I could loos 10 lbs......... going at this rate we could have full tanks on takeoff!

I hope you are feeling better Anthony.

I had a good flight in the Rv9 yesterday, there was a Kittyhawk at the airfield. When I was coming downwind they decided to do some aeros as it left so I orbited at the end of the downwind leg. As he started his aeros I turned base so had a great view of him looping and rolling....... the best seat in the house!!

It made me think about the skill of the pilots in WW2, how would you shoot down something that was moving like that, when chasing it. Much better to bounce them when they are not expecting it!

Hi Steve,

At the moment I am feeling pretty lousy, having just finished Radiation Therapy. The doc said I would get to feel worse before I started to get better. Let's see what happens in the next couple of weeks. By the way over the last three months my weight has gone from 210lbs down to 154lbs. I would not recommend the diet plan however. I have also been amusing myself lately making plans for building this 4 of mine hence the number of posts on various 4 related issues by me recently.

I, along with my good friend Trevor Archer, briefly helped out Peter Teichman back in 2005 and 2007 display his Kittyhawk at Hahnweide near Stuttgart. That guy does a great display. The P-40 is an excellent aircraft:

p402.jpg


Kittyhawk



Tony
 
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Dont do it!!

Lets forget about design safety margins and call the absolute failure limit 6g.

The wing fails due to exceeding the Bending Moment (probably very near the root).

Moment = sum of forces x distance

max aero gross weight for an RV-8 is 1600lb

Each wing must carry half the weight in steady level flight (forget other contributions to lift for this analysis) so each wing carries 800 lbs in steady level flight.

now pull 6g's - each wing now must provide 4800lbs of lift.

If the tanks are empty most of weight is in the fuselage (forget wing weight for this). The wing bending moment will be calculated as the integral of the lift distribution function over the span the wing - but for simplicity lets collapse the lift force to the 75% span location.

Therefore the max bending moment for a single wing = 4800lb*7.5ft = 36000ft-lbf

Now distribute some of the weight to fuel. Lets add full fuel or 21 gals each wing. Our total weight is still 1600lbs so each wing still needs to provide 4800lbs of lift at 6g's but:

fuel weight = 21gal * 6.02 lb/gal = 126lb * 6'gs = 758 lbs

The fuel tanks on an RV8 are 56" long so lets say the fuel moment is concentrated at the half point (since the tanks are more or less of equal volume as a function of span) = 2.3ft


The bending moment = 4800lb * 7.5ft minus 758lbs * 2.3 ft

= 34257 ft-lbf which is less than the limit bending moment of 36000 ft-lbf at 6'gs.

now lets say we load the fuselage to 1600 AND add full tanks because we believe we can ignore fuel weight.

Total weight now equals = 1600 + 253 = 1852 divide by 2 for load on each wing = 926 lbf times 6 gs = 5558 lbf

Bending moment = (5558lbf * 7.5 ft) - (758lbf * 2.3 ft) = 41685 ft-lbf - 1743 =39942 ft-lbf - you just exceeded the ultimate bending limit and became a lawn dart in your neighbors front yard - congratulations!


Assumptions needing to be answered to make this valid:

1. Spar weak point is inboard of the 1/2 fuel tank location
2. 75% span is a valid approximation for the wing lift point. The closer the lift point is to the spar weak point the greater influence the fuel will have.
3. Assumes wing spar is the limiting factor (horizontal stab, engine mount, elevator hinge, etc...)

Van didn't say you could ignore fuel weight he said it had little effect on bending moment and he's right... look at the first analysis at full fuel the fuel contribution to reduced bending moment was only 4.8% (1743/36000).
 
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No need.

No need to thrash it out and no need for all the math, you won?t know the reasons for Van?s saying this is what it is, I strongly suspect they know the answers and if they say this is how you figure gross aerobatic weight then I see no reason to argue with them about it.
 
Weight loss

Chin up and stiff upper lip and all that Anthony!!

I had a similar weight loss program when I had a kidney stone when I was 21. Obviously not as serious as your problem but very persistent pain, lost 14lbs in a week. I could not hold anything down and it went on for 3 months before a sugar puff appeared one early morning call....... that was painful.

Good news is that several of my flying buddies have recently recovered from various forms of cancer and I am sure you will too, so as I say Chin up!!

BTW. not trying to continue this argument about the weight of fuel but I have read the thread and find it to be difficult to interpret at best. A clear statement is what is needed. I understand about the weight of the fuel being near the wing root but on one hand we have a statement of max aerobatic weight being 1375lbs, then a strangely worded statement from Dick which frankly I do not understand.

Perhaps someone from Vans can chime in her and make a definative statement that the weight of fuel can be discounted or not........please.



Best
 
Chin up and stiff upper lip and all that Anthony!!

--snip--

BTW. not trying to continue this argument about the weight of fuel but I have read the thread and find it to be difficult to interpret at best. A clear statement is what is needed. I understand about the weight of the fuel being near the wing root but on one hand we have a statement of max aerobatic weight being 1375lbs, then a strangely worded statement from Dick which frankly I do not understand.

Perhaps someone from Vans can chime in her and make a definative statement that the weight of fuel can be discounted or not........please.



Best

Thanks for those words Steve, they help a lot.

I too unlike Russ says am interested in the reasons and the math behind this. Obviously we do need a more definitive statement from Dick or someone at Vans to clarify his comments to Kevin Horton. If there are any aeronautical engineers out there that would like to elaborate and then sum up in laymans words, I would love to hear from them.

Kevin, do you have any more to add?
 
You just heard it...

Ken you also said in a previous post:

Assumptions needing to be answered to make this valid:

1. Spar weak point is inboard of the 1/2 fuel tank location
2. 75% span is a valid approximation for the wing lift point. The closer the lift point is to the spar weak point the greater influence the fuel will have.
3. Assumes wing spar is the limiting factor (horizontal stab, engine mount, elevator hinge, etc...)

Van didn't say you could ignore fuel weight he said it had little effect on bending moment and he's right... look at the first analysis at full fuel the fuel contribution to reduced bending moment was only 4.8% (1743/36000).

Thanks Ken for the qualified aeronautical engineers point of view.

The three points listed by Ken need to be addressed by Vans. I am not so sure we will get a statement out of them on this. Lets see if anyone at Vans is looking at this thread. If not, I will fire off an email and let you know the answers.
 
What Ken says.

I think what Ken says pretty much clears it up.

'Van did not say you could exclude fuel weight' Unfortunately the comments after muddied the water.

It reminds me of the straw that broke the camels back. 4.8% might not be much but might be that straw.
 
I think what Ken says pretty much clears it up.

'Van did not say you could exclude fuel weight' Unfortunately the comments after muddied the water.

It reminds me of the straw that broke the camels back. 4.8% might not be much but might be that straw.

"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"


Van’s statements are clear to me, he made two separate statements and they do not contradict each other, the first statement says you do not need to include fuel in your aerobatic gross calculation, the second statement says the airplane will perform better lighter. So you’re not going to break it if you put fuel in the wing tanks that bring the total weight above the aerobatic gross because fuel is not counted, butt lighter is better because the airplane will fly better, more fun, lighter controls and more power to weight.

What I’m saying about hashing this out or doing all the math etc. is simple, all you have is what the designer tells you, if he says it is adequate to do what he says then it is, there is no way for you to prove it or disprove it without load testing the structure, you’re not going to so there is little point arguing about it. I happen to know that they do load test their designs at Van’s; they have all the data and the engineering data, so I trust Van’s statement. It looks like Randy came to the same conclusion after talking to Van.
 
Russ - Van did not qualify his statement by stating this applies above max aero gross weight. If you stay within the max aero Gross weight then you can in fact ignore the fuel contribution as I showed. If you are already at max gross and then load the fuel you will exceed the limit.

Math is math - Do what you will...don't do it over my house please.

out
 
Interpretaion

Russ

I am sorry but the first line of what Van is aid to have said makes absolutely no sense to me, I simply do not understand what he is trying to convey.The second line makes sense. Ken interprets it to mean that you cannot exclude fuel, you the opposite?

Consider this, you and your passenger put the weight of your RV4 at 1375, the 1375 documented aero max, with empty tanks. You then put fuel in her putting her over 1375. Something goes wrong the aeroplane breaks up, you survive, your passenger is killed and someone on the ground is injured by the debris.

At least two people are coming looking for compensation. The insurance company realise that the aeroplane was above the aerobatic gross becuase the fuel had not been factored in. Do you think they would pay out?

So, then the lawyers get involved...... I have a law degree and have been in a few battles with lawyers mostly in criminal but sometimes in civil court so I know how they work. In the UK the AIB would be doing an investigation and there would be a possibility they would prosecute.

Now, you are relying on a statement on your defence which at best is difficult to interpret. Meanwhile the people who are suing your estate, or trying to put you in jail just have to point at the published document which says aerobatic weight 1375!! They would argue that that is the weight that should rely court should rely on.

Vans could themselves become party to the litigation as they probably have the deepest pockets. This is why I think that they should clear this up once and for all so everyone knows with certainty what to factor in or not.

Best regards
 
Russ

I am sorry but the first line of what Van is aid to have said makes absolutely no sense to me, I simply do not understand what he is trying to convey.The second line makes sense. Ken interprets it to mean that you cannot exclude fuel, you the opposite?

Consider this, you and your passenger put the weight of your RV4 at 1375, the 1375 documented aero max, with empty tanks. You then put fuel in her putting her over 1375. Something goes wrong the aeroplane breaks up, you survive, your passenger is killed and someone on the ground is injured by the debris.

At least two people are coming looking for compensation. The insurance company realise that the aeroplane was above the aerobatic gross becuase the fuel had not been factored in. Do you think they would pay out?

So, then the lawyers get involved...... I have a law degree and have been in a few battles with lawyers mostly in criminal but sometimes in civil court so I know how they work. In the UK the AIB would be doing an investigation and there would be a possibility they would prosecute.

Now, you are relying on a statement on your defence which at best is difficult to interpret. Meanwhile the people who are suing your estate, or trying to put you in jail just have to point at the published document which says aerobatic weight 1375!! They would argue that that is the weight that should rely court should rely on.

Vans could themselves become party to the litigation as they probably have the deepest pockets. This is why I think that they should clear this up once and for all so everyone knows with certainty what to factor in or not.

Best regards

Did you follow the links through and read Randy's conversation with Van? This coupled with Vans reply to Kevin makes it very clear to me.

http://www.vansairforce.com/community/showthread.php?t=4595&highlight=

I don't worry about it to much anyway because I'm not out trying to pull 6g anyway at any point and have never pulled more then about 3 for any reason. YMMV.
 
Hmmmm

Russ I am not questioning what you do and am sure you fly within limits.

However, everything in that post is hearsay, I am not saying its not true, but it is hearsay as against documented figures.

Now lets just consider the RV 4, with two people on board at aerobatic weight, if we put full fuel in it we would exceed max gross weight.

1375 plus 192 is 1567, is this OK or can we exceed gross because we can exclude the fuel? Or is van really saying the 4 is aerobatic at its gross weight.

This is what he says on his web page

The RV-3B, RV-4, RV-6/6A, RV-7/7A, and RV-8/8A 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).

I would hope everyone can see the conflict we have here?
 
Russ I am not questioning what you do and am sure you fly within limits.

However, everything in that post is hearsay, I am not saying its not true, but it is hearsay as against documented figures.

Now lets just consider the RV 4, with two people on board at aerobatic weight, if we put full fuel in it we would exceed max gross weight.

1375 plus 192 is 1567, is this OK or can we exceed gross because we can exclude the fuel? Or is van really saying the 4 is aerobatic at its gross weight.

This is what he says on his web page

The RV-3B, RV-4, RV-6/6A, RV-7/7A, and RV-8/8A 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).

I would hope everyone can see the conflict we have here?

No worries, I don?t see you or anyone questioning me but rather comments coming from Van?s. As to Randy, I knew him when he lived here in Vancouver WA and he seemed to be in tight with the guys at Van?s so it seems less like hearsay to me then it might to others. I do see the conflict and it makes for a good discussion at the very least.
 
Questioning

Thanks Russ, I am sure that Randy can be relied on as well, the question here is simply what do people rely on for deciding how they can load thier Rv's for aeros. However, even if Randy was my best mate I would still want to verify the facts and have them documented to ensure I was both physically and legally safe.

We have documented weights from Vans, even on the newer types they impose restrictions which would keep the aeroplane below gross when loading fuel, Then we have a discussion between Vans and an owner which says something completely different.

Vans need to clear this one up and document the true position if they have not already done so, it cold prevent them and an owner having a lot of trouble.

Actually this has made me sound like I am documentation mad, truth is I hate all the documentation that goes with getting an aeroplane off the ground, but here I can see a reason for it.

Best regards
 
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