VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics


Go Back   VAF Forums > Main > RV General Discussion/News
Register FAQ Members List Calendar Today's Posts

Reply
 
Thread Tools Search this Thread Display Modes
  #31  
Old 10-07-2010, 10:03 AM
WingsOnWheels WingsOnWheels is offline
 
Join Date: Mar 2009
Location: Plano, TX
Posts: 2,089
Default

Not getting into the aerobatic gross weight discussion, but back to the spar strength:

Everyone has to keep in mind a couple of points:

1) The RV-6 was designed with the spar to be built from plans or a kit. I'm sure Van accounted for quite a bit of "builder error" margin in the design and limits. The RV-7 spar is pre-made and thus under quality control so it can be built with less margin.

2) The RV-6 was a paper design without the use of computer stress modeling, so I'm sure Van also left some room for design error also. How would you like to design an entire aircraft and then figure out you underbuilt the main spar, better safe than sorry.
__________________
Colin P.
RV-6A #20603
Complete 5/10/19
PP SEL / A&P
I donate every year on my B-Day (in Dec), but donated early in Sep'19.
Reply With Quote
  #32  
Old 10-08-2010, 12:15 AM
jjconstant's Avatar
jjconstant jjconstant is offline
 
Join Date: Dec 2006
Location: Oakland CA
Posts: 771
Default aerobatic gross weight discussion:

http://www.vansairforce.com/communit...ead.php?t=4595

Jeremy
Reply With Quote
  #33  
Old 10-08-2010, 09:14 AM
JonJay's Avatar
JonJay JonJay is offline
 
Join Date: Jan 2007
Location: Battleground
Posts: 4,348
Default Thanks Jeremy, but

Quote:
Originally Posted by jjconstant View Post
This thread just added to the confusion that seems to exist in regard to aerobatic weight. Perhaps we need to start a different thread on the discussion. While it does relate to the original topic it is drifting a bit...
__________________
Smart People do Stupid things all the time. I know, I've seen me do'em.

RV6 - Builder/Flying
Bucker Jungmann
Fiat G.46 -(restoration in progress, if I have enough life left in me)
RV1 - Proud Pilot.
Reply With Quote
  #34  
Old 10-08-2010, 09:48 AM
Bill Dicus Bill Dicus is offline
 
Join Date: Jan 2005
Location: Shorewood, WI (Milwaukee area)
Posts: 1,066
Default 6 vs 7

Hope somebody with more knowledge will step in here and correct this if I'm wrong. I thought the -6 spar is built so that strength is nearly symmetrical (pos and neg) but the -8 has different spar caps so that neg "G" tolerance is less than positive. Thanks to Wendell Voltz I know the -8 flies really well inverted, but if it could really break at 4.5 neg I'd want to know and wouldn't really feel comfortable doing much neg "G" stuff. Would appreciate some good structural info on this! Thanks in advance for any hard info.
__________________
Bill Dicus
Shorewood (Milwaukee) Wisconsin
RV-8 N9669D Flying 12/4/14!
Flying Pitts S-2A, Piper Lance
Reply With Quote
  #35  
Old 10-08-2010, 10:30 AM
DGlaeser DGlaeser is offline
 
Join Date: May 2005
Location: Rochester Hills, MI
Posts: 879
Default Not just the wing

Quote:
Originally Posted by Bill Dicus View Post
Hope somebody with more knowledge will step in here and correct this if I'm wrong. I thought the -6 spar is built so that strength is nearly symmetrical (pos and neg) but the -8 has different spar caps so that neg "G" tolerance is less than positive. Thanks to Wendell Voltz I know the -8 flies really well inverted, but if it could really break at 4.5 neg I'd want to know and wouldn't really feel comfortable doing much neg "G" stuff. Would appreciate some good structural info on this! Thanks in advance for any hard info.
The +/- G ratings don't just apply to the wings, it's the whole airframe. The wing spar may well be +/- 6Gs but if something else important breaks, it's not much consolation Many aerobatic planes are rated at +6-3Gs.
Using the term 'comfortable' and 'neg G' in the same sentence is an oxymoron
__________________
Dennis Glaeser CFII
Rochester Hills, MI
RV-7A - Eggenfellner H6, GRT Sport ES, EIS4000, 300XL, SL30, TT Gemini, PMA6000, AK950L, GT320,
uAvionixEcho ADSB in/out with GRT Safe Fly GPS
Reply With Quote
  #36  
Old 10-08-2010, 12:24 PM
rvmills's Avatar
rvmills rvmills is offline
 
Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
Default

Quote:
Originally Posted by DGlaeser View Post
The +/- G ratings don't just apply to the wings, it's the whole airframe. The wing spar may well be +/- 6Gs but if something else important breaks, it's not much consolation Many aerobatic planes are rated at +6-3Gs.
Using the term 'comfortable' and 'neg G' in the same sentence is an oxymoron
Depends on who the user of the terms is, I guess.

I wouldn't use the two terms in the same breath, but Bill, being the studly Pitts pilot he is, can get away with it!

Its a good question though, and as a Super Six owner, I'm interested in the strength issues discussed herein. I'm into gentleman's acro, versus the gnarly stuff Bill does in his Pitts, but I have a heavy 6, and he has an 8, so the concerns are similar...how to stay away from the margins!!

Good discussion of g ratings applying to more than just the wings...concur...there's that whole tail thing too, right!

Cheers,
Bob
__________________
Bob Mills
RV-6 "Rocket Six" N49VM
Reno-Stead, NV (KRTS)
President/Sport 47/49, Sport Class Air Racing
President, Formation Flying Inc (FFI)
Flight Lead, Lightning Formation Airshows
Reply With Quote
  #37  
Old 10-08-2010, 12:25 PM
rvbuilder2002's Avatar
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
Default

A few facts as I know them...
All RV models up through the 6 had wing spars that were symmetrical top to bottom (same spar cap structure top and bottom) which should in theory make the wing capable of the same load factor + and -, but my understanding has always been that the 3, 4, and 6 were promoted as being good for +6/-3. I don't remember ever seeing anything published +6/-6 (though I may have forgotten).

All models after the 6 were designed with asymmetrical spar structure. In a positive G load condition the top spar caps are loaded in compression and the bottom in tension. A tension loaded member can take more load before failure than an equally sized member in compression. Making the bottom smaller provides weight reduction but reduces the allowable neg. load factor (who wants to push -6 g's anyway).

"I have been told by very good sources that the factory built wing was tested well beyond the limits."

Totally true. Because "limit" load is 6 G's (the load at which no damage or deformation occurs). This is the maximum intentional flight limit.
The Ultimate load is the load which the wing must be capable of holding one time for 3 seconds without a catastrophic failure. It is 150% of limit load so in the context of designing for aerobatics the wing must be tested to 9 G's.

The RV-6 wing did go beyond 9 G's with no failure. I do not feel that the amount it went beyond is important for this reason. The precision level of the testing on all wings tested after the RV-6 was improved. If you look at photo's in old RVator news letters you will see huge piles of 75 lb bags of sand on the wings. These bags are big. It is difficult to get a load distribution with a high level of precision. All wings from the RV-8 on, have been tested with lead shot bags loaded on cell stations that are only about 6" X 12". This allows for a very accurate load profile for any given test. There is an RV-7 test wing on display in the hangar at Van's. It also was tested to 9 G's without failure. Some of the skins are cut away to allow inspection of the interior for any evidence of damage. If I remember correctly, there isn't any.

So is the RV-6 wing stronger than the RV-7? I don't think anyone can really say that.

Does fuel need to be factored in when calculating gross weight for aerobatics? Ken Krueger, Vans head of engineering has told me it does.
Van has apparently in the past said that it didn't need to be for the RV-3. I think that is related to its wings originally being designed for the use of a fuselage fuel tank. I think a miscommunication happened somewhere, and a statement Van made may have been taken out of context. I don't believe Van ever meant to say you didn't need to consider fuel in aerobatic gross weight for all models. But I can't speak for him.

The issue is that fuel in the tanks does reduce the bending moment on the wing, but as Kevin pointed out it only effects the portion of the wing where the fuel is. In an RV, the portion of the wing outboard the tank would be loaded higher than intended. If the wings were originally test to 9 G's using the aerobatic gross weight value, then the wing is only proven to that load value. If you fly at a weight higher than that, but with all of the extra weight in the cabin area, it is true that you have downward fuel load (induced by G's) counteracting the upward bending moment of the wings. It is possible that it would make the bending moment at the root end of the wing no higher than it would have been at a lower weight. The problem is that the portion of the wing without fuel actually does have a higher bending moment on it; beyond what was tested.

Regardless of what you have heard, or where you have heard it... my opinion is that you are reducing your safety margin if you do aerobatics at higher than the published gross weight Period
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")

Last edited by rvbuilder2002 : 10-08-2010 at 12:30 PM.
Reply With Quote
  #38  
Old 10-08-2010, 01:40 PM
JonJay's Avatar
JonJay JonJay is offline
 
Join Date: Jan 2007
Location: Battleground
Posts: 4,348
Default How are aerobatic gross weights determined?

Vary good points from Scott via Ken.
Keeping in context, the 7 has a higher aerobatic weight than a 6 so some argue the 7 must be stronger.
How is it determined? As mentioned, a roll is an aerobatic manuever, but you dont need to pull any g's to do one. What are the limits that aerobatic gross imply?
I guess I can ask KK next time I see him for the full blown description but it would be nice for all that do aerobatics in our 6's that frequent here to know. What are we dealing with?
__________________
Smart People do Stupid things all the time. I know, I've seen me do'em.

RV6 - Builder/Flying
Bucker Jungmann
Fiat G.46 -(restoration in progress, if I have enough life left in me)
RV1 - Proud Pilot.
Reply With Quote
  #39  
Old 10-08-2010, 01:52 PM
n5lp's Avatar
n5lp n5lp is offline
fugio ergo sum
 
Join Date: Apr 2006
Location: Carlsbad, NM
Posts: 1,912
Default

Quote:
Originally Posted by rvbuilder2002 View Post
...my understanding has always been that the 3, 4, and 6 were promoted as being good for +6/-3. I don't remember ever seeing anything published +6/-6 (though I may have forgotten)...
To check my memory I looked again at my RV-6 builder's manual (1997 ish) again this morning. It does indeed refer to +6/-6 and also lists the baggage weight capacity as 100 pounds versus the 60 pounds listed on the Van's site today. I would be happy to take photos if anyone would like. There seem to have been changes over time, not in the airplane, but in the specs.
__________________
Larry Pardue
Carlsbad, NM

RV-6 N441LP Flying
Reply With Quote
  #40  
Old 10-08-2010, 02:47 PM
JonJay's Avatar
JonJay JonJay is offline
 
Join Date: Jan 2007
Location: Battleground
Posts: 4,348
Default Mine too

Quote:
Originally Posted by n5lp View Post
To check my memory I looked again at my RV-6 builder's manual (1997 ish) again this morning. It does indeed refer to +6/-6 and also lists the baggage weight capacity as 100 pounds versus the 60 pounds listed on the Van's site today. I would be happy to take photos if anyone would like. There seem to have been changes over time, not in the airplane, but in the specs.
It also states that the airframe is stressed for aerobatics up to a gross weight of 1375lbs. per the manual... "This means that is has a design strength of 6 positive and 6 negative G's (plus a 50% safety factor) at up to this weight." It goes on to state that the RV6 is effectively a single place aerobatic machine.
I would suggest that it is a single place aerobatic machine for many manuevers, but low G manuevers, like aileron rolls or conservative loops, could be done safely beyond the 1375 weight limit. However, as Scott mentions, it was never tested, or documented beyond 1375lbs. It also goes on to say that the RV is a pilot limited airplane. I believe they where very conservative in their testing to cover for the wide variance in pilot skill and training.
Someone loan their 6 to Van's for some desctructive testing and let's see how she really does....ha!
__________________
Smart People do Stupid things all the time. I know, I've seen me do'em.

RV6 - Builder/Flying
Bucker Jungmann
Fiat G.46 -(restoration in progress, if I have enough life left in me)
RV1 - Proud Pilot.
Reply With Quote
Reply



Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 08:54 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.