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Tip tanks and wing root stresses

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Dennis Glaeser

Joined: 10 Jan 2006
Posts: 53
Location: Rochester Hills, Michigan

Posted: Wed Mar 29, 2006 7:56 pm Post subject: Tip tanks and wing root stresses

Konrad,



 	  Quote: 	
		  From a purely structural standpoint, you would get the lowest in-flight wing

root stresses by burning the main (inboard) fuel first.
	




However typically, other considerations (i.e. unporting, pressure drops from

long lines, ...) drive the operational procedures that should be used.



	Dennis Glaeser




That brings up another question that I have regarding Wing Spar Root stress

(not

necessarily in the RV-Series though):



Let me just say that in this scenario one would be able to switch between

all tanks,

.. not draining one into the other.



Would it be best to burn most fuel of the Mains before switching to the

Tips?

Or would one first burn most of the Tips before going to the Mains?



Which way would be better in a fully switchable 4-Tank System?



Thanks,

do not archive

Randy Richter

Joined: 09 Jan 2006
Posts: 5
Location: Reston, VA

Posted: Wed Mar 29, 2006 8:25 pm Post subject: Tip tanks and wing root stresses

I gotta chime in here. I've flown KC-135s and C-130s, both of which are

about 130' wide. You can put something like 62,000 lbs of fuel across the

wings of the tanker and 44,000 in the wing of the Herc. For these types of

acft, wing fuel distribution, loading and burn sequence is a factor. They're

limited to 2.5g load factor for this and other reasons.



When you're talking a roughly 10' wing with 120 lbs of fuel in each side

(chock full), I'm thinking that sweating the burn sequence is a waste of

brain cells. If you've got tips, burn mains down till they can take the tip

quantity, then drain. Don't over-complicate your plumbing.



This is all the thought I'll devote to this subject.



Randy



--

_________________
Randy Richter
-7AQB Emp started

davidfenster(at)comcast.n
Guest

Posted: Wed Mar 29, 2006 9:40 pm Post subject: Tip tanks and wing root stresses

Randall - where were you in the 135?   Anyway...



So you must remember a spin in a 37 was a wild ride "Low, Left, and Light".

Low entry (Non-aggressive stall)

Left entry (Had to do with the rotation of the engine)

Light weight (yes, LIGHT weight)



I have spun (in my younger and dumber days) civil AC.  In comparison to 

the 37, they were more of a spiral.  Let loose of pro spin controls and 

the thing stops.  Unlike the 37 - if you do not provide anti-spin 

controls in a definite sequence, the spin will continue or flip over to 

inverted.  With your head banging around the canopy.  My point - if you 

think about spinning an aircraft w/o knowing how to get out (not just 

letting go of the stick) you are asking for trouble.  Yes - in most 

circumstances, letting go of pro-spin inputs will drop you out of the 

spin.  But - does anyone really know the inputs to get you out  in an RV 

if the crap hits the fan?



In the military we had BOLDFACE.  Procedures which were committed to 

memory.  Spin recovery was one of them.



I would like to see a discussion of pro-spin inputs to get out.



I'll start:



Throttles--Idle

Rudder and Ailerons--Neutral

Stick--Abruptly full aft and hold

Rudder--Abruptly apply full Rudder opposite spin direction (opposite 

turn needle) and hold

Stick--Abruptly full forward one turn after applying rudder

Controls--Neutral after spinning stops and recover from dive



The above is the 37 boldface.  It is meant as a start.  Chances are it 

would work, but what do I know...



Let's start the flaming.

Randall Richter wrote:

 	  Quote: 	
		   



 I gotta chime in here. I've flown KC-135s and C-130s, both of which are

 about 130' wide. You can put something like 62,000 lbs of fuel across the

 wings of the tanker and 44,000 in the wing of the Herc. For these types of

 acft, wing fuel distribution, loading and burn sequence is a factor. They're

 limited to 2.5g load factor for this and other reasons.



 When you're talking a roughly 10' wing with 120 lbs of fuel in each side

 (chock full), I'm thinking that sweating the burn sequence is a waste of

 brain cells. If you've got tips, burn mains down till they can take the tip

 quantity, then drain. Don't over-complicate your plumbing.



 This is all the thought I'll devote to this subject.



 Randy

klwerner(at)comcast.net
Guest

Posted: Wed Mar 29, 2006 10:06 pm Post subject: Tip tanks and wing root stresses

Dear Randy,



Thank you for your input on fuel management!



Konrad

do not archive (again)

  ---

aerobubba(at)earthlink.ne
Guest

Posted: Thu Mar 30, 2006 2:13 am Post subject: Tip tanks and wing root stresses

Subject: RE: Tip tanks and wing root stresses

 

 

 	  Quote: 	
		  .I gotta chime in here. I've flown KC-135s and C-130s, both of which are

about 130' wide...   ...when you're talking a roughly 10' wing with 120

lbs of fuel in each side
	


 	  Quote: 	
		  (chock full), I'm thinking that sweating the burn sequence is a waste of

brain cells...

 
	


Let's see, the Boeings I've flown burn the center first, as do the

Airbusses.  On the other hand, the 45' Westwind I've flown burns...  ah...

the center first.  Hmmm.  Well, at least the 36' Cessna 310 I've flown

burns the...  ah.. inboards first.

 

 	  Quote: 	
		  This is all the thought I'll devote to this subject.

 
	


How unfortunate.



For those who are interested, consider two chairs, a yardstick, and a roll

of quarters.  Place the yardstick across the backs of the chairs.  Now

place the roll of quarters on the middle of the yardstick and watch the

yardstick flex and fall between the chairs.  Replace the yardstick, unroll

the quarters, and distribute them along the span of the yardstick.  Watch

in amazement as the yardstick carries the load.



Now consider that the RV series was not designed with tip tanks.  



The size of the aircraft is not relevant.  Whatever it's size, it was

designed to do a particular job.  Distributing the fuel towards the tips

decreases bending loads on the spar, which has a positive impact on both

the instantaneous load it can carry as well as it's fatigue life.



Operate your aircraft however you see fit, but physics is physics...



Glen Matejcek

aerobubba(at)earthlink.net

khorton01(at)rogers.com
Guest

Posted: Thu Mar 30, 2006 3:33 am Post subject: Tip tanks and wing root stresses

On 30 Mar 2006, at 05:05, Glen Matejcek wrote:



 	  Quote: 	
		   

 <aerobubba(at)earthlink.net>



 Now consider that the RV series was not designed with tip tanks.



 The size of the aircraft is not relevant.  Whatever it's size, it was

 designed to do a particular job.  Distributing the fuel towards the  

 tips

 decreases bending loads on the spar, which has a positive impact on  

 both

 the instantaneous load it can carry as well as it's fatigue life.



 Operate your aircraft however you see fit, but physics is physics...


	


I certainly agree that burning the inboard fuel first will reduce  

wing bending loads.  That could be a significant issue if you wanted  

to do aerobatics with fuel in the tip tanks.  But everyone seems to  

agree that aerobatics should only be done when the tip tanks are empty.



The tip tanks will only be filled for cross country flight, when the  

load factors are low.  In this case, the wing bending stress would  

only seem to be a concern if you got into heavy turbulence.   If you  

get into heavy turbulence with fuel in the tip tanks, you should slow  

down, well below Va, to reduce the loads on the tip tank to wing  

attachment.



I don't see wing bending loads as a major issue, considering how  

people will fly the aircraft when there is fuel in the tip tanks.  If  

the wing is designed to take 6gs without tip tanks, surely it can  

easily handle a typical cruise flight load spectrum with fuel in the  

tips.  What am I missing?



Kevin Horton         RV-8 (finishing kit)

Ottawa, Canada

http://www.kilohotel.com/rv8

dennis.glaeser(at)eds.com
Guest

Posted: Thu Mar 30, 2006 6:52 am Post subject: Tip tanks and wing root stresses

You aren't missing a thing (as usual!)  The change in root stresses that

tip tanks in an RV would produce (or reduce) are miniscule in the big

picture. I think Konrad was looking for more of a theoretical answer to

enhance his understanding of how wing loading works.



Dennis Glaeser

RV-7A Fuselage



------------------------------------------

I don't see wing bending loads as a major issue, considering how people

will fly the aircraft when there is fuel in the tip tanks. If the wing

is designed to take 6gs without tip tanks, surely it can easily handle a

typical cruise flight load spectrum with fuel in the tips. 

What am I missing? 



Kevin Horton 

RV-8 (finishing kit)

rv7(at)b4.ca
Guest

Posted: Thu Mar 30, 2006 8:22 am Post subject: Tip tanks and wing root stresses

On 2:05:17 2006-03-30 "Glen Matejcek" <aerobubba(at)earthlink.net> wrote:

 	  Quote: 	
		   Now consider that the RV series was not designed with tip tanks.


	


This is irrelevant.  The RV was designed to carry the fuel as (effectively)

a roll of quarters placed at the center of the span, since it was designed

with only inboard tanks.



Adding fuel further out merely decreases the load on the wing during

cruise.  Burning the fuel from the wingtip first, you just return to the

stock configuration, which is fuel centered near the fuselage.  So at no

point are you worse off than having just fuel in the inboard tanks.



 	  Quote: 	
		   physics is physics...


	


Funny, that's what I was thinking... 



-Rob

ceengland(at)bellsouth.ne
Guest

Posted: Thu Mar 30, 2006 11:34 am Post subject: Tip tanks and wing root stresses

Kevin Horton wrote:

 	  Quote: 	

On 30 Mar 2006, at 05:05, Glen Matejcek wrote:

>

><aerobubba(at)earthlink.net>

>

>Now consider that the RV series was not designed with tip tanks.

>

>The size of the aircraft is not relevant.  Whatever it's size, it was

>designed to do a particular job.  Distributing the fuel towards the  

>tips

>decreases bending loads on the spar, which has a positive impact on  

>both

>the instantaneous load it can carry as well as it's fatigue life.

>

>Operate your aircraft however you see fit, but physics is physics...

>    

>

I certainly agree that burning the inboard fuel first will reduce  

wing bending loads.  That could be a significant issue if you wanted  

to do aerobatics with fuel in the tip tanks.  But everyone seems to  

agree that aerobatics should only be done when the tip tanks are empty.

The tip tanks will only be filled for cross country flight, when the  

load factors are low.  In this case, the wing bending stress would  

only seem to be a concern if you got into heavy turbulence.   If you  

get into heavy turbulence with fuel in the tip tanks, you should slow  

down, well below Va, to reduce the loads on the tip tank to wing  

attachment.

I don't see wing bending loads as a major issue, considering how  

people will fly the aircraft when there is fuel in the tip tanks.  If  

the wing is designed to take 6gs without tip tanks, surely it can  

easily handle a typical cruise flight load spectrum with fuel in the  

tips.  What am I missing?

Kevin Horton         RV-8 (finishing kit)

Ottawa, Canada

http://www.kilohotel.com/rv8

I don't see bending loads as an issue; I see roll & yaw authority when 

low & slow in the pattern as the issue. In cruise configuration maybe it 

makes sense to reduce control sensitivity a bit, but I want 'normal' 

handling in the pattern. My concern is breaking a wing against the 

ground after an inadvertent almost developed spin, not overloading it. 

Aren't handling issues why most wing mounted aux tank systems say no acro?

I know Kevin knows about this, but for those who want to burn the 

inboard tanks 1st, try this. Apply the roll of quarters trick to 

control. Spread 'em out, tape them down, twirl the stick around your 

body, & stop it as quickly as possible.  Then try the same trick with 

the whole roll at the outer end.

jhstarn(at)verizon.net
Guest

Posted: Thu Mar 30, 2006 2:54 pm Post subject: Tip tanks and wing root stresses

I see your point, not an injenear just an old plumber. AND Kevin knows a 

whole more about this stuff than me.

Take a 10' stick of 1/2" PVC (the older the better) and tape the quarters on 

the end next to your hand & "snap" the pipe like a whip. It "may" break. 

Take hold of the same pipe on the other end, snap it in the same motion.

OH yea, before you snap it the second time, clear the area of people, dogs & 

breakable things. Borrow a full face helmet, wear gloves & and a thick 

jacket. The second snap will result not on a simple break but PVC shrapnel. 

Been there, tried that. Now think of the PVC pipe as a wing. As I said I'm 

no injenear but I do live in the real world where stupid tricks have been 

done just to see how things work. KABONG  Do Not Archive



---

ronlee(at)pcisys.net
Guest

Posted: Thu Mar 30, 2006 4:08 pm Post subject: Tip tanks and wing root stresses

If having weight at the end of the wing is so great and beneficial then

why didn't Van's design it that way?  It seems to me that any benefit

is just unneeded gravy.



The biggie is that it causes no harm.



Ron Lee



do not archive

rv7(at)b4.ca
Guest

Posted: Thu Mar 30, 2006 4:36 pm Post subject: Tip tanks and wing root stresses

On 16:06:53 2006-03-30 Ron Lee <ronlee(at)pcisys.net> wrote:

 	  Quote: 	
		   If having weight at the end of the wing is so great and beneficial

 then why didn't Van's design it that way?


	


It's only great for a cross-country cruiser.  For a light, sporty aircraft

like an RV, the extra weight out there robs you of roll rate and increases

your moment of inertia in spins.  It's also more complex to build, and more

plumbing to install.  I guess Van wanted to keep it simple.



-Rob

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