Van's Air Force
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4 Aerobatics
- Thread starter Jetlinkin
- Start date
Have a look at this preliminary report of fatal accident with RV4 doing aerobatics in Australia with 2 people aboard. Overloaded and out of c of g envelope.
http://www.atsb.gov.au/publications/investigation_reports/2007/AAIR/pdf/aair200701033_prelim.pdf
Be careful.
John
http://www.atsb.gov.au/publications/investigation_reports/2007/AAIR/pdf/aair200701033_prelim.pdf
Be careful.
John
A 1000# 4 would leave 375# for pax. Who is doing the aerobatics is probably an important disclaimer. No the AL doesn't know who is holding the stick but a pilot that knows what he's doing can easily perform a whole series of aerobatics and never pull 3g.
A hamfist can mess up his first cuban 8 and rip the wings off by doing an attempted high onset 12g pull trying to teach himself.
If you say "Never" now the only way to learn -4 aerobatic handling...... is by yourself. No instruction available "IN" the -4. You said "Never"
A hamfist can mess up his first cuban 8 and rip the wings off by doing an attempted high onset 12g pull trying to teach himself.
If you say "Never" now the only way to learn -4 aerobatic handling...... is by yourself. No instruction available "IN" the -4. You said "Never"
The acro gross weight of the -4 is 1375 lbs, so if you have 2 people in it, you will most likely be over that weight. Also, with an aft CG, it is easy to pull too many G's because the stick forces are very light. With just pilot, it is very balanced
(assuming O-320 and lightweight prop).
BTW, I am a IAH based, Jetlink pilot. Are you (based on your user name)? PM me
I have understood it that the max weight doing aerobatics is calculated by extracting the weight of the fuel. The fuel tanks are lifting bodies (the wings), so there will be no added stress on the main spar with more fuel. Maybe even the opposite will happen, more fuel in the tanks may decrease stresses on the main spar due to g forces working in the opposite direction relative the main spar compared to more weigth in the cockpit.
375# for 2 pax puts you over? How many big Macs do you guys eat per day?
Bill Wightman
Well Known Member
Not true. Fuel in the RV airplanes is carried inboard on the wing. The fuel weight absolutely DOES influence wing spar shear and bending loads. Only fuel carried right on the tip is "flying" weight, and then you have the issue of secondary loads on the tips.
As for the "aerobatic" gross weight limit: Van is simply covering his rear end here. The RV4 is approved by Vans for a 6-g limit load (not ultimate) but then the approved limit drops to utility category (I think) at 1375 lbs.
Fuel weight MUST be factored into your overall gross and applied against the limits Van publishes for operation of the airplane.
-Bill
kevinh
Well Known Member
Actually, for aerobatics I don't think you are correct. Here's the reply I received from the man himself when I sent an email to support@vans:
Reposted here in an attempt to save Van's from getting another pile of emails.
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smokyray
Well Known Member
Feeling stressed...
I flew my 170HP/wood prop RV4 for 11 years and 1400 hours, half of that in some sort of unusual attitude other than level flight. My builders manual in 92' showed aerobatic gross weight as printed in the excerpt above from Van. How did I convert that to usable numbers I could apply in the cockpit without getting out a slide rule or scales every time I flew?
First I figured out what fuel weghed and how much my gas gauges showed vs what was in the tank. Then I calculated my aft CG limitation from my initial weigh-in. (I have found 50 lbs tailwheel weight in level flight attitude empty is a good round number for a decent RV4 CG)
What I came up with was a fuel level indication where I could and couldn't do aerobatics up to design limits. Do you ever need design limits? I have never seen more than 3.5 G's doing the entire sportsman routine, even in competition. Only once did I ever approach 6 G's and it was to avoid a mid-air collision when a hapless pilot attempted to fly through my formation while staring at his "map in lap". Nuff said.
I made a rule to never do over the top acro with any passenger over 120 lbs,( cute girls only) never do dual acro with more than 1/2 fuel indicated ( I competed with 1/4 showing on each side) and only do spins solo. 180 C/S birds have more wiggle room in this arena, but weigh more overall. Again, every airplane is different.
These rules kept me happy, safe and fun. Your mileage may vary.
Rob Ray
RV4. F16 formerly...
HR2 Currently
I flew my 170HP/wood prop RV4 for 11 years and 1400 hours, half of that in some sort of unusual attitude other than level flight. My builders manual in 92' showed aerobatic gross weight as printed in the excerpt above from Van. How did I convert that to usable numbers I could apply in the cockpit without getting out a slide rule or scales every time I flew?
First I figured out what fuel weghed and how much my gas gauges showed vs what was in the tank. Then I calculated my aft CG limitation from my initial weigh-in. (I have found 50 lbs tailwheel weight in level flight attitude empty is a good round number for a decent RV4 CG)
What I came up with was a fuel level indication where I could and couldn't do aerobatics up to design limits. Do you ever need design limits? I have never seen more than 3.5 G's doing the entire sportsman routine, even in competition. Only once did I ever approach 6 G's and it was to avoid a mid-air collision when a hapless pilot attempted to fly through my formation while staring at his "map in lap". Nuff said.
I made a rule to never do over the top acro with any passenger over 120 lbs,( cute girls only) never do dual acro with more than 1/2 fuel indicated ( I competed with 1/4 showing on each side) and only do spins solo. 180 C/S birds have more wiggle room in this arena, but weigh more overall. Again, every airplane is different.
These rules kept me happy, safe and fun. Your mileage may vary.
Rob Ray
RV4. F16 formerly...
HR2 Currently
gmcjetpilot
Well Known Member
RV-4's can legally (in limits) do two up Acro
I don't think its fair to say you can't make the Acro weight/CG limit in a RV-4 with two UP. It depends on the people and planes weight (and CG).
From Dans Data base.
Aerobatic Gross Weight: 1375 pounds
Aerobatic CG Range: 68.7" to 75.9" aft of the datum
Normal/Nominal Empty about 1000lbs?
Normal/Nominal CG about 65.25?
Arm/CG
Fuel 68.4"
Pilot 81.2"
Passenger 105.57"
Baggage 128.87"
Fuel in the wing does have a beneficial affect on wing bending. With positive g's the wing is bending UP but the fuel tends to bend the wing down and reduce shear (but not necessarily torsion). In large jets, fuel weight is used in design. If Van says fuel does not count towards acro weight, I would agree with that. I would not fly with full fuel either. If fuel weight is not credited towards total Acro weight, you can still consider if for CG, which should help move the CG fwd slightly.
If you look at the numbers above you can have 180 lb pilot, 120 lb passenger and still include 10 gal of fuel. Now if you want to disregard the 10 gal of fuel (60 lbs) and it to the 120 lbs passenger for 180 lbs, you will still be in CG. So two up 180 lbs is doable if you ignore the fuel weight (but not CG).
From my data base and Dan's, staying under or around 1,000 lbs is not impossible or even hard. Out of 31 RV-4's 15 where less than 1005 lbs. 10 RV-4's where under 975 lbs.
As far only having 10 gal aboard, you can takeoff with more since you burn some on the way to the Acro area. 20 min of Acro is all I need in one session. So 10 gal is plenty to do some Acro and land with reserves.
CG is another issue. What ever you do, don't exceed the aft CG limit. A tandem like the RV-4 has a CG challenge. Everything (or almost) makes the CG go aft.
I did lots of dual ACRO in the RV-4 in limits with two up. I can say the stick forces get real, real, light, so be careful. Like the poster above, my dual acro was typically with lighter weight females in the back, in the 120lb or less range. Clearly Acro with a good-ol BUBBA (ie big-boy) +200 lb'er in the back is not going to happen with a RV-4. I was OK with that.
-If you build a 1,200 lb or even 1,100 lb, RV-4, as Mel said FORGET IT
-Fat people + fat RV's means a SOLO acro only. This applies to all RV's
-Build it LIGHT & lose personal weight (comment about losing weight directed @ myself)
I don't think its fair to say you can't make the Acro weight/CG limit in a RV-4 with two UP. It depends on the people and planes weight (and CG).
From Dans Data base.
Aerobatic Gross Weight: 1375 pounds
Aerobatic CG Range: 68.7" to 75.9" aft of the datum
Normal/Nominal Empty about 1000lbs?
Normal/Nominal CG about 65.25?
Arm/CG
Fuel 68.4"
Pilot 81.2"
Passenger 105.57"
Baggage 128.87"
Fuel in the wing does have a beneficial affect on wing bending. With positive g's the wing is bending UP but the fuel tends to bend the wing down and reduce shear (but not necessarily torsion). In large jets, fuel weight is used in design. If Van says fuel does not count towards acro weight, I would agree with that. I would not fly with full fuel either. If fuel weight is not credited towards total Acro weight, you can still consider if for CG, which should help move the CG fwd slightly.
If you look at the numbers above you can have 180 lb pilot, 120 lb passenger and still include 10 gal of fuel. Now if you want to disregard the 10 gal of fuel (60 lbs) and it to the 120 lbs passenger for 180 lbs, you will still be in CG. So two up 180 lbs is doable if you ignore the fuel weight (but not CG).
From my data base and Dan's, staying under or around 1,000 lbs is not impossible or even hard. Out of 31 RV-4's 15 where less than 1005 lbs. 10 RV-4's where under 975 lbs.
As far only having 10 gal aboard, you can takeoff with more since you burn some on the way to the Acro area. 20 min of Acro is all I need in one session. So 10 gal is plenty to do some Acro and land with reserves.
CG is another issue. What ever you do, don't exceed the aft CG limit. A tandem like the RV-4 has a CG challenge. Everything (or almost) makes the CG go aft.
I did lots of dual ACRO in the RV-4 in limits with two up. I can say the stick forces get real, real, light, so be careful. Like the poster above, my dual acro was typically with lighter weight females in the back, in the 120lb or less range. Clearly Acro with a good-ol BUBBA (ie big-boy) +200 lb'er in the back is not going to happen with a RV-4. I was OK with that.
-If you build a 1,200 lb or even 1,100 lb, RV-4, as Mel said FORGET IT
-Fat people + fat RV's means a SOLO acro only. This applies to all RV's
-Build it LIGHT & lose personal weight (comment about losing weight directed @ myself)
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George,
You are right of course. When I made that statement, I was speaking of more common size people and CG issues.
I have been in the back seat of a -4 when the pilot almost lost it on flare. I grabbed the stick and throttle and managed to "power" out of it. (I flew the last leg home from the front seat!)
You are right of course. When I made that statement, I was speaking of more common size people and CG issues.
I have been in the back seat of a -4 when the pilot almost lost it on flare. I grabbed the stick and throttle and managed to "power" out of it. (I flew the last leg home from the front seat!)
Bill Wightman
Well Known Member
This is incredible...
The above quote posted in a previous reply within this thread is said to be from Dick himself.
Unfortunately, who ever stated this is dead wrong. Any competent aero engineer understands the need to detail both mass distribution as well as spanwise lift distribution in order to build the basis for calculating structural stresses. Weight carried by the wing itself is - without any question - a factor in spar stress. This business of "flying weight" is the product of misguided intuition. This is basic aero engineering knowlege.
Ask yourself the question: what certified airplane allows the pilot to deduct fuel weight when measuring compliance with structural loading issues? The answer: none. And this is simply because aero engineers know where the fuel is loaded; they run their stress models using that information and the end result is passed on to you in a properly developed flight manual.
It is true that fuel carried outboard on the wing reduces bending and shear loads - on that portion of wing that's inboard of the load. But what if we move the fuel load to mid-span? How about inboard tanks like all the RV's have? What if all the fuel is carried in a fuselage tank? You see, fuel carried inboard drives stresses higher on every inch of wing outboard of the fuel load - like your RV.
The only reason I'm expounding on this here is that it absolutely is a safety issue. More than one RV has shed wings in flight. If you insist on deducting the fuel from the "aerobatic" weight limit, go ahead and put 4g's on your bird at a relatively high weight. While you're doing it, take a look at your leading edge wing skins just outboard of the fuel cap. You just might rethink your decision.
Respectfully,
Bill Wightman
The above quote posted in a previous reply within this thread is said to be from Dick himself.
Unfortunately, who ever stated this is dead wrong. Any competent aero engineer understands the need to detail both mass distribution as well as spanwise lift distribution in order to build the basis for calculating structural stresses. Weight carried by the wing itself is - without any question - a factor in spar stress. This business of "flying weight" is the product of misguided intuition. This is basic aero engineering knowlege.
Ask yourself the question: what certified airplane allows the pilot to deduct fuel weight when measuring compliance with structural loading issues? The answer: none. And this is simply because aero engineers know where the fuel is loaded; they run their stress models using that information and the end result is passed on to you in a properly developed flight manual.
It is true that fuel carried outboard on the wing reduces bending and shear loads - on that portion of wing that's inboard of the load. But what if we move the fuel load to mid-span? How about inboard tanks like all the RV's have? What if all the fuel is carried in a fuselage tank? You see, fuel carried inboard drives stresses higher on every inch of wing outboard of the fuel load - like your RV.
The only reason I'm expounding on this here is that it absolutely is a safety issue. More than one RV has shed wings in flight. If you insist on deducting the fuel from the "aerobatic" weight limit, go ahead and put 4g's on your bird at a relatively high weight. While you're doing it, take a look at your leading edge wing skins just outboard of the fuel cap. You just might rethink your decision.
Respectfully,
Bill Wightman
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pierre smith
Well Known Member
Remember the -8..?
Bill,
IIRC, the -8 that shed its wings broke just outside of the fuel cap and I read that that's where Van's said the wings would break if overloaded. The concern stemmed from the fact that the newer 7/8 spars were so much shorter than the -4's and 6's and to the untrained eye they just couldn't be as strong.
BTW, kindly post a link to your terminal tool,
Thanks,
Bill,
IIRC, the -8 that shed its wings broke just outside of the fuel cap and I read that that's where Van's said the wings would break if overloaded. The concern stemmed from the fact that the newer 7/8 spars were so much shorter than the -4's and 6's and to the untrained eye they just couldn't be as strong.
BTW, kindly post a link to your terminal tool,
Thanks,
With all respect, I have to disagree on this one. There is no problem designing the aircraft so that the statement "The aerobatic gross weight of the RVs (with wing tanks) is basically the zero fuel wt" is true. Besides, wings are flying weight as per def, they just happen to be built much stronger than needed to carry only their own weight.
The extra weight of the fuel has to be caried somehow, and this will increase the total loading on the wing, but extra loading is not automatically the same as bending forces on the spar. So, if the spar outside of the fuel tank is strong enough to carry the extra weight of the fuel, and the spar inside the tank is strong enough to carry the weight minus the fuel, there should be no problem.
Anyway, the point is that if the wing is designed as the ref say it is, then I don't see any reason to doubt it. On the other hand that particular design feature is not exactly official in the right sense of the word.
Bill Wightman
Well Known Member
SvingenB,
I can see you have your mind made up. Unfortunately, I can't teach a basic loads/stress lesson within the confines of this forum.
Suffice it to say that my 20 years as a practicing aero engineer should be enough weight for you to take serious note of my warning. If you add mass (fuel) to the structure the resultant loads have to go somewhere. There's no free lunch in this business. That translates into increased shear/bending outboard of the applied load (ie outboard of the fuel tank). Get your hands on a basic aero loads text or take the question to a practicing engineer (aero engineer, not civil engineer).
On the other hand I do recognise that it is truly your own decision as to how you operate your experimental aircraft.
I wish you many good years of fun and SAFE sport flying. Remember, the ground never misses and gravity never takes a day off.
Pierre,
Yes the 8 spar did fail just outboard of the fuel tank. The "old" RV8 spar unfortunately was designed with three structural discontinuities all bunched fairly close together. I know - I did a very detailed forensic study of it. The three disco's (sharp changes in cross-sectional moment, or structural changes with span) were 1. Fuel tank/outboard skin juncture 2. Wing spar shear web stiffener termination 3. Wing spar cap thickness step-down.
Aero structures are elastic and must be designed to smoothly distribute moment, cross sectional area, and mechanical breaks with span. Its a bad idea to allow them to get grouped together. Do that and you get a wing that tends to "fold" at one point rather than to bend smoothly with span. Again, note how your wings show deformation under load. On my 4 it was very concentrated just outboard of the fuel cap, a sight that never sat well with me. But the airplane has proven safe over the years with many thousands of RV4 hours flown without incident. Kudos to Van on that.
Now couple the above discontinuities with the large amount of span carried outboard (ie lift generated outboard) on a rectangular planform and you get big bending loads inboard on the wing (SvingenB you listening here?) After the crash and investigation, the spar was redesigned by extending the anti-buckling plate outboard, and restricting the "old" spars to reduced gross weight. Wonder if any of us feel frisky enough to subtract the fuel weight off these airplanes when thinking about doing some acro....
About the Tool:
I don't want to use this board as a way of waving my flag in everybody's face about the Terminal Tool. I believe the tool will live or die based on the market's perception of value and cost.
I've made repeated attempts to contact DR but still have no reply. (Doug please call/email or pm me anytimek) I am interested in being a paying advertiser here. But to answer your question, the website is
http://www.theterminaltool.com
Thanks for your interest!
Bill
I can see you have your mind made up. Unfortunately, I can't teach a basic loads/stress lesson within the confines of this forum.
Suffice it to say that my 20 years as a practicing aero engineer should be enough weight for you to take serious note of my warning. If you add mass (fuel) to the structure the resultant loads have to go somewhere. There's no free lunch in this business. That translates into increased shear/bending outboard of the applied load (ie outboard of the fuel tank). Get your hands on a basic aero loads text or take the question to a practicing engineer (aero engineer, not civil engineer).
On the other hand I do recognise that it is truly your own decision as to how you operate your experimental aircraft.
I wish you many good years of fun and SAFE sport flying. Remember, the ground never misses and gravity never takes a day off.
Pierre,
Yes the 8 spar did fail just outboard of the fuel tank. The "old" RV8 spar unfortunately was designed with three structural discontinuities all bunched fairly close together. I know - I did a very detailed forensic study of it. The three disco's (sharp changes in cross-sectional moment, or structural changes with span) were 1. Fuel tank/outboard skin juncture 2. Wing spar shear web stiffener termination 3. Wing spar cap thickness step-down.
Aero structures are elastic and must be designed to smoothly distribute moment, cross sectional area, and mechanical breaks with span. Its a bad idea to allow them to get grouped together. Do that and you get a wing that tends to "fold" at one point rather than to bend smoothly with span. Again, note how your wings show deformation under load. On my 4 it was very concentrated just outboard of the fuel cap, a sight that never sat well with me. But the airplane has proven safe over the years with many thousands of RV4 hours flown without incident. Kudos to Van on that.
Now couple the above discontinuities with the large amount of span carried outboard (ie lift generated outboard) on a rectangular planform and you get big bending loads inboard on the wing (SvingenB you listening here?) After the crash and investigation, the spar was redesigned by extending the anti-buckling plate outboard, and restricting the "old" spars to reduced gross weight. Wonder if any of us feel frisky enough to subtract the fuel weight off these airplanes when thinking about doing some acro....
About the Tool:
I don't want to use this board as a way of waving my flag in everybody's face about the Terminal Tool. I believe the tool will live or die based on the market's perception of value and cost.
I've made repeated attempts to contact DR but still have no reply. (Doug please call/email or pm me anytimek) I am interested in being a paying advertiser here. But to answer your question, the website is
http://www.theterminaltool.com
Thanks for your interest!
Bill
The early 3's rear spar attch point and the 1st flying 8 are easy to research. Both of those issues have been addressed. The main spar in the -4 and -6 are very different (more parts heavier); not saying better, just saying different.
I'm unaware of an inflight overload failure of a -4 or -6 spar. If someone knows of this ever happening could they chime in.
I could agree with you if it was specifide that RV's were designed this way, but they obviously were not, because a specific aerobatic qross weight (which is different from the normal gross weight) is is specified for all of the two seat models that are approved for aerobatics.
Couple of corrections...
I agree with the general thrust of Bill's posts in trying to convince people to consider carefully before exceeding the manufacturers recommended aerobatic gross weight.
The reason for my post though is to make a couple of minor corrections.
In the truest sense for designing a structure like a wing with the best weight to strength ratio, you would have no discontinuities along the span. In the perfect world the structure would very in a manor that was linear to the change in load along the span. But, just because a wing is not that way, doesn't mean it is not strong enough in the area where the "discos" occur. It might be that the wing is actually carrying extra weight because it has extra structure that is not needed (but may be that way for simplification of the structure, etc.). In the case of the orig. RV-8 wing, it was proven by additional testing that it met the design requirements even with the discos.
The original (old) RV-8 wing was never given a reduced gross weight. Instead, the new wing was given a higher gross weight. One of the reasons for the redesign was that the RV-8 wing became the wing that is also used on the RV-7 (with very minor differences).
Lastly, I think the wing deformation that Bill has seen under load is largely the result of a skin thickness change as much as anything else. The fuel tank skin is .032" thick and the outboard leading edge skin is .025" thick. This seems like a small difference but it is a change in thickness of 25%, which in skin stiffness is a lot. In a perfect design world we might use skins that varied in thickness over the entire span, but that wouldn't be very practical.
I agree with the general thrust of Bill's posts in trying to convince people to consider carefully before exceeding the manufacturers recommended aerobatic gross weight.
The reason for my post though is to make a couple of minor corrections.
In the truest sense for designing a structure like a wing with the best weight to strength ratio, you would have no discontinuities along the span. In the perfect world the structure would very in a manor that was linear to the change in load along the span. But, just because a wing is not that way, doesn't mean it is not strong enough in the area where the "discos" occur. It might be that the wing is actually carrying extra weight because it has extra structure that is not needed (but may be that way for simplification of the structure, etc.). In the case of the orig. RV-8 wing, it was proven by additional testing that it met the design requirements even with the discos.
The original (old) RV-8 wing was never given a reduced gross weight. Instead, the new wing was given a higher gross weight. One of the reasons for the redesign was that the RV-8 wing became the wing that is also used on the RV-7 (with very minor differences).
Lastly, I think the wing deformation that Bill has seen under load is largely the result of a skin thickness change as much as anything else. The fuel tank skin is .032" thick and the outboard leading edge skin is .025" thick. This seems like a small difference but it is a change in thickness of 25%, which in skin stiffness is a lot. In a perfect design world we might use skins that varied in thickness over the entire span, but that wouldn't be very practical.
Well, I haven't studied the details of the wing loading and stresses on the -4, I haven't even thought about it that much (have been too preoccupied learning the basics of riveting and priming, checking out alternative engines etc ). But as I understood it, it was designed and/or tested by Van so that the max aerobatics weight is minus the fuel, like the mail say it is. I have also read this before in here somewhere. I still don't see why this couldn't be the case, because it is not irrelevant where and how the load is distributed.
Bill Wightman
Well Known Member
Exactly right, Scott. I'm not saying the 8 wing is deficient. I did say that the first-gen structure showed some deficient design practices, in my opinion. My forensic study of the original 8 spar showed a huge drop in cross sectional moment very close to the outboard fuel tank rib. The new spar helped to alleviate this by extending the shear web stiffener plate. I just thought (again my opinion) the original design was flawed. The wing that I'm doing right now has a very smooth stress distribution. To get this required parabolically shaped spar caps. No big deal in the CNC age though.
As for the 25% drop in skin stiffness, you again are right on. But all the buckling was nearly confined to about a 1-foot section of skin. Now, mind you, I was looking at this on both wings with MY fanny in the airplane. It was just a bit disconcerting.
Thks for the correction on the old spar design limits. I thought they'd been post-mortem reduced to 1550. Now I see the new ones were built for the 7 and 8 and carry a 1600 lb aerobatic category limit.
Anyway, you hit the nail right on the head as to the real reason for my commentary. I just don't want to see any of us here take something like gross weight / aerobatic flight in a cavalier way.
Y'all fly safe out there!!
Just to make it 100% clear, I am not advocating that people should just load down their aircrafts and fly aerobatics, possibly crossing some CG or structural limit. The only info about the -4 and this fuel issue I have read is on this board, and the mail is supposedly from Van, but we cannot be sure unless this info is made public by Van's. So untill Vans make such info public available, I will understand the weight limit of the -4 for aerobatics to be the printed weight with fuel.
But - to keep the discussion going , I am not completely lost in stresses and strain. Loading fuel in the wings is completely different from loading fuel in the fuselage regarding stress and bending.
A simple example just to show what I mean, the numbers may be inaccurate, but that doesn't really matter: The fuel tank covers about 1/10 of the wing total surface, but at a section with the highest lift - close to fuselage and fwd of main spar, so the tanks probably lift closer to 1/5 of the total. At 1g, the wing lifts 600 kg = 5900 N. 10 - 20% of this (the tank sections) is approx 620 to 1240 N of aerodynamic force upward. With full tanks, the force from the fuel is 90 kg = 890 N down.
For simplicity we can say that the g forces from the fuel in the tanks equals the lift created from the tanks, but in opposite direction. So, when looking at the net forces on the wing, they will be pointing up, except on the tanks where they are zero. Still, the tanks contribute to the structural rigidity of the wing. When calculating the bending moment of the spar at the fuselage, the lift from the tank sections can simply be disregarded. If the tanks were in the fuselage, the bending of the spar would be larger because the lifting force would not longer have the opposing force from the fuel.
This way, the wing tanks causes less bending on the spar, but an uneven distribution of forces causing shear and torsion, basically the weight of the fuel. But, this uneven distribution of forces is the normal distribution (with fuel in the tanks), and since the -4 is approved for +6g with at least one person and full tanks considering light built, it is more than certain that the wing is designed with this shear and torsion in mind, I mean the opposite would be unthinkable in any circumstance. The shear and torsion may not be the weakest link in the wing, it is highly unlikely that it is, since they are a function of the the fuel loading only. The bending moment on the spar may very well be. If this is so, well ?
But
- to keep the discussion going , I am not completely lost in stresses and strain. Loading fuel in the wings is completely different from loading fuel in the fuselage regarding stress and bending.A simple example just to show what I mean, the numbers may be inaccurate, but that doesn't really matter: The fuel tank covers about 1/10 of the wing total surface, but at a section with the highest lift - close to fuselage and fwd of main spar, so the tanks probably lift closer to 1/5 of the total. At 1g, the wing lifts 600 kg = 5900 N. 10 - 20% of this (the tank sections) is approx 620 to 1240 N of aerodynamic force upward. With full tanks, the force from the fuel is 90 kg = 890 N down.
For simplicity we can say that the g forces from the fuel in the tanks equals the lift created from the tanks, but in opposite direction. So, when looking at the net forces on the wing, they will be pointing up, except on the tanks where they are zero. Still, the tanks contribute to the structural rigidity of the wing. When calculating the bending moment of the spar at the fuselage, the lift from the tank sections can simply be disregarded. If the tanks were in the fuselage, the bending of the spar would be larger because the lifting force would not longer have the opposing force from the fuel.
This way, the wing tanks causes less bending on the spar, but an uneven distribution of forces causing shear and torsion, basically the weight of the fuel. But, this uneven distribution of forces is the normal distribution (with fuel in the tanks), and since the -4 is approved for +6g with at least one person and full tanks considering light built, it is more than certain that the wing is designed with this shear and torsion in mind, I mean the opposite would be unthinkable in any circumstance. The shear and torsion may not be the weakest link in the wing, it is highly unlikely that it is, since they are a function of the the fuel loading only. The bending moment on the spar may very well be. If this is so, well ?
kevinh
Well Known Member
I'm sure nothing else was intended, but just to be clear: That email was from Van himself - he sent it to me when I asked him to confirm something I read here. Not hearsay or a fabrication
Alas, I don't know enough about aeronautical engineering to speak to the facts that led him to this statement. He has apparently told Randy Leverold (sp?) the same thing.
Bill Wightman
Well Known Member
Intuitive thinking...
SvingenB - I know completely what you are saying and thinking. Your post thoroughly explains the idea that the load imposed by the fuel is offset basically by the lift of the wing above it. Hence we intuitively think of it as "flying weight".
The problem is, the wing can't choose to distribute its lift where its most needed.
Bottom line is in order to make the added lift needed to offset our fuel weight, the entire wing must now be operated at a higher CL. Remember, its basically a rigid structure.
The CL distribution is basically a function of the shape of the wing, angle of attack and airfoil. The RV's rectangular planform carries significant area - and therefore lift - outboard. If we make the simplifying assumption that the wing must lift the gross weight of the airplane, then we've added lift (and thus bending/shear) across the entire wing, and the fact the fuel might be "flying weight" says nothing about what's happening elsewhere in the structure.
I sure hope this helps
SvingenB - I know completely what you are saying and thinking. Your post thoroughly explains the idea that the load imposed by the fuel is offset basically by the lift of the wing above it. Hence we intuitively think of it as "flying weight".
The problem is, the wing can't choose to distribute its lift where its most needed.
Bottom line is in order to make the added lift needed to offset our fuel weight, the entire wing must now be operated at a higher CL. Remember, its basically a rigid structure.
The CL distribution is basically a function of the shape of the wing, angle of attack and airfoil. The RV's rectangular planform carries significant area - and therefore lift - outboard. If we make the simplifying assumption that the wing must lift the gross weight of the airplane, then we've added lift (and thus bending/shear) across the entire wing, and the fact the fuel might be "flying weight" says nothing about what's happening elsewhere in the structure.
I sure hope this helps
