Normal / Ultimate Ratings w/ Wood

[Zipper730]

Generally, as I understand it, most metals have a safety factor of 1.5. What does wood have? I'm just curious because the Mosquito was rated for 8g ultimate load.

 

[tyrodtom]

What's your definition of safety factor ?

If you're defining safety factor as the ratio between the elastic point of a material, and it's fracture point, there is no general safety factor.
It depends on the specific metal and the alloys in it.

Or are you talking of safety factor as a part of a design ?

 

[Zipper730]

What's your definition of safety factor ?

Say an aircraft has a normal rated load of 6.5g, so the ultimate load is 9.75g which, as I grasp it, is when the wings come off. I'm curious if this figure applied to wooden designs, or just aluminum and other?

 

[pbehn]

Generally, as I understand it, most metals have a safety factor of 1.5. What does wood have? I'm just curious because the Mosquito was rated for 8g ultimate load.

You cannot compare wood and metal in a simple way. A Mosquito wasn't made of wood, it was a wooden structure using various properties of various woods and adhesives to produce a structure that was comparable to a metal aircraft. However aircraft aren't just made of metal, they are metal structures, also using the properties of various metals in various ways to produce a metal structure. As an explanation of sorts, the Mk 1 Hurricane originally had fabric and dope surfaced wings, these were replaced by metal surfaced wings later. Metal skin weighs more than dope and fabric but the metal wing was lighter and stronger because they were able to do away with a lot of the internal reinforcement the fabric wings needed. As a material wood has a similar strength per weight to steel but completely different properties as far as rigidity, elongation and directional strength.

 

[tomo pauk]

Hornet & Sea Hornet - 10G ultimate load factor. Vampire - 12G.
link

 

[pbehn]

Generally, as I understand it, most metals have a safety factor of 1.5. What does wood have? I'm just curious because the Mosquito was rated for 8g ultimate load.

8g just means it can withstand a load of 8 times gravity, that of course must state the condition of the plane a fully loaded PR Mosquito with wing tanks would not perform an 8g turn. You need to find where your safety factor comes from and what it says, it is quite possible that it works another way, that in order to guarantee safety at 8g in all conditions of service and manufacture you must specify strength of materials and design for 12g.

 

[pbehn]

Hornet & Sea Hornet - 10G ultimate load factor. Vampire - 12G.
link

According to Wiki the Hornet used Alclad bonded to wood in the wing surfaces.

Fuselage construction was identical to the earlier Mosquito: a balsa wood "pith" sandwiched between plywood sheets which were laid in diagonal panels. Aerolite formaldehyde cement was the bonding agent.[21] The fuselage halves were built on large concrete or wood patterns and equipment was fitted in each half; they were then joined along the top and bottom centre lines using wooden reinforcing strips. The entire fuselage was then tightly wrapped in fine aviation fabric which was doped in place. The tailfin which had the trademark gracefully-curved de Havilland shape, was an integral part of the rear fuselage. On late F.1s and further models of production aircraft, a fin fillet was added to the base of the unit.[22] The horizontal tail unit was an all-metal structure, again featuring the distinctive de Havilland shape,[14] which was later repeated on the Chipmunk and Beaver.
Construction was of mixed balsa/plywood similar to the Mosquito but the Hornet differed in incorporating stressed Alclad lower-wing skins bonded to the wooden upper wing structure using the new adhesive Redux.[23] The two wing spars were redesigned to withstand a higher load factor of 10 versus 8.[14] Apart from the revised structure, the Hornet's wings were a synthesis of aerodynamic knowledge that had been gathered since the design of the Mosquito, being much thinner in cross-section, and with a laminar flow profile similar to the P-51 Mustang and Hawker Tempest. The control surfaces consisted of hydraulically-operated split flaps extending from the wing root to outboard of the engine nacelles; as on the Mosquito, the rear of the nacelle was part of the flap structure. Outboard, the Alclad-covered ailerons extended close to the clipped wing tips and gave excellent roll control.[12][22

 

[swampyankee]

Generally, as I understand it, most metals have a safety factor of 1.5. What does wood have? I'm just curious because the Mosquito was rated for 8g ultimate load.

"Safety factor" is a property of the structure, not of the material making it up. While most metals have a range where they will plastically deform, that is have a permanent change in shape, before they start to come apart[1], many materials, including wood[2] and composites[3] will not. However, metals will also fail without plastic deformation in many cases, including if there is a severe stress concentration or multi-axial loading.

The load-elongation curve of most metals will have an elastic range, where elongation is directly proportional to load and where the specimen will return to its original length if the load is removed, followed by a plastic range, where some elongation will remain after the load is removed. If the loading is continued, there will be a point where the metal will fail; this is called the ultimate strength. Where it is relative to the limiting elastic load depends on a number of factors, but may be well under 1.5.

---
1 Yield strength is where they object will return to its original shape. Ultimate is where it comes apart.
2 Yes, I know wood may bend permanently under load, but it doesn't tend to do so under a short term load.
3 See 2

 

[pbehn]

Zipper730
1. There are specific standards of airframe structures set by the customer - for US, the AAF and BUNAV
2. The most common threads above touch on three aspects of the design standards. a.) Limit and Ultimate Loads for which individual components critical to flight safety are analyzed for the material Stress due to the applied loads, b.) The standard and published material Properties of Materials which include Young's Modulus Tensile Strength in PSI at separate temperatures, Density, Coefficient of Thermal Expansion, Hardness, etc, c.)
3.) Approved methods by the Contractor are framed for calculation of stability of structures under compression (Pinned, Fixed, etc) and shear, methods for computing moments of inertia for complex structures for bending (and buckling for compression stability analysis) analysis.

As an aside the stated Tensile strength of the materials listed are developed by Bureau of Standards via Testing performed by ASTM and if you look at the Strain vs Applied Load/sq (Tensile Strength) plot the point 'selected' for Tensile Strength is usually (for aluminum) a defined point on the plot where it deviates from straight line and enters the region of Permanent Strain - further along the plot (now a non linear curve) is an area where the strain value peaks without fail in permanent deformation. That is the usual Ultimate Allowable Stress. For 2024/7075 Aluminum Ultimate Stress is set in the tables ~ 1.5 to 1.7 times the Stress at Yield. Whatever the Tables state is what you use.

Next - the airframe structures group is given the allowable Load factors (E.g. 3G positive for many bomber/transport, 8 G positive, etc for fighter aircraft)

Next for Limit and Ultimate Stress calculations the Angle of Attack Loads are usually for CL max in a diving pullout a design Gross Weight. Each major assembly is broken down and analyzed piece by piece as they fit into assembled parts, shear panels, Longerons, bulkheads, etc, to determine if the designed parts as received from airframe design will be within set boundary condition safety factors. Usually design team and structures team are working together to make sure the designs 'make sense' with respect to approach, weight and load paths. The Weights Control Group keeps track of every part weight as calculated and/or actually measured.

Next - the airframes are subjected to planned load testing on wings, empennage and landing gear.

Next - life happens and the mission adaptations grow, increasing gross weight and thereby REDUCE allowable load factors directly proportional to the increase in mission weight.

Summary - every nation had different notions of Airframe Structures Standards - and some applied slightly different philosophies for Yield and Ultimate for comparative materials. For US the standard was 8/12 for England 7/11 for fighters. I was very surprised to see the Zero as low as 6G Limit but that is in keeping with reducing weight to a minimum. If that was for combat mission with full fuel and ammo (no externals) it started out where the P-51D finished - but a lot more nimble than the P-51D when it increased mission gross weight, fully loaded internally, to 10,200 pounds vs NA-73/P-51 at 8000 pounds and 8G/12G. The XP-51F was all about taking the P-51B capability back to NA-73 original weights.

Every Design Gross Weight is for a projected mission envelope and usually the extreme mission within that envelope is selected for analysis.

Not my work of course, but Drgondog Determining aircraft strength

 

[pbehn]

From my side sometimes things are lost in wording. In my training and experience there were three important points on a stress strain curve. The "yield point" which is where, as others have said elastic deformation changes to plastic. Then there is what I would call the maximum stress, this is the load which has the highest value on the stress/strain (load extension) curve. Next is where the material breaks, for me that is the ultimate (end) tensile strength. On the stress strain curve the stress or load at which a material breaks is lower than the maximum and may be lower than the yield value. This is the difference between engineering stress (calculated against original test area) and true stress (calculated against the actual cross sectional area at the time the load is recorded). I have highlighted in bold from Drgondogs post "an area where the strain value peaks without fail in permanent deformation." This peak is where the material under test starts to "neck down" so the cross sectional area of the original gauge length under test becomes irrelevant. In two nations divided by a single language, Drgondogs "Ultimate allowable stress" is my "Maximum stress" and Swampyankees "Ultimate strength" is the same as mine, except he is referring to true stress and I would be referring to engineering stress (or strength). It is all very simple, here is a wiki article. Stress–strain curve - Wikipedia

 

[pbehn]

When it comes to wood it is a whole different game and science. The Youngs modulus, Poisson's ratio and shear modulus are completely different so it is a different world of design, testing and engineering. Despite all this I am reliably assured that the Mosquito did fly and was quite successful at flying.
https://www.conradfp.com/pdf/ch4-Mechanical-Properties-of-Wood.pdf
Woods elastic modulus and Poisson ratio | Sonelastic®
Young's Modulus (Modulus of Elasticity) of Wood
Poisson's ratio - Wikipedia.

 

[ThomasP]

Some ultimate g loads, all except A6M2 from UK documents. A6M2 ultimate load from Jiro Horicoshi.

A6M2 Zero______11g_____5,335 lbs
Hurricane IIC____11.2g___ 7,400 lbs
Spitfire IX_______10g_____ 7,240 lbs
Typhoon IB_____ 11.5g__ 10,500 lbs
Mustang________12g_____ 7,850 lbs (model unspecified)
Airacobra_______12g_____ 7,400 lbs (model unspecified)
Tomahawk______12g_____ 6,840 lbs (model unspecified)
Martlet V_______ 11g_____ 7,000 lbs
Hellcat_________ 13.5g__ 11,000 lbs (model unspecified)

Whirlwind_______10g____ 10,200 lbs
Mosquito F II_____8g____ 18,500 lbs
Lightning_______ 10.5g__ 15,500 lbs (model unspecified)

Wellington II_____11g____ 24,500 lbs
Boston__________ 6g____ 19,750 lbs (model unspecified)

edit: added Wellington and Boston

 

[pbehn]

Some ultimate g loads, all except A6M2 from UK documents. A6M2 ultimate load from Jiro Horicoshi.

A6M2 Zero______11g_____5,335 lbs
Hurricane IIC____11.2g___ 7,400 lbs
Spitfire IX_______10g_____ 7,240 lbs
Typhoon IB_____ 11.5g__ 10,500 lbs
Mustang________12g_____ 7,850 lbs (model unspecified)
Airacobra_______12g_____ 7,400 lbs (model unspecified)
Tomahawk______12g_____ 6,840 lbs (model unspecified)
Martlet V_______ 11g_____ 7,000 lbs
Hellcat_________ 13.5g__ 11,000 lbs (model unspecified)

Whirlwind_______10g____10,200 lbs
Mosquito F II_____ 8g___ 18,500 lbs
Lightning________10.5g__15,500 lbs (model unspecified)

Please refer to my post #9 quote originally from Drgondog "Every Design Gross Weight is for a projected mission envelope and usually the extreme mission within that envelope is selected for analysis." Only one plane on your list has a two man crew and radar as standard with an internal space that can be used for bombs or fuel as required. Alternatively, consider putting in an extra man with radar equipment, 4 belt fed cannon and a lot more fuel and see how they get on with high g manoeuvres, they are completely different fighting machines.

 

[ThomasP]

Hey pbehn,

Sorry, but my post was not addressing your post. I was simply posting some data on ultimate g loads that people might find useful relative to the different aircraft types/structures&materials. It might give a feel for the advantages/disadvantages of the different structures. I have not been able to find much data on the different g loads for 'normal safe' operational load factors. But if the safety factor is 1.5 for US airframes then the values above should allow a ballpark figure to be determined.

For example:

The designed operational g load for the P-51 would be 8g somewhere around 7800 lbs?
The designed operational g load for the P-51H was 7.33g at 9450 lbs, so this would make the ultimate g load 11 g at 9450 lbs?

The designed operational g load for the A6M2 was 7g clean, but only 6g with the drop tank. (Jiro Horicoshi)
The ultimate g load for the Wellington X was 4.2g at 36,500 lbs. (UK documents)

re: "Only one plane on your list has a two man crew and radar as standard with an internal space that can be used for bombs or fuel as required. Alternatively, consider putting in an extra man with radar equipment, 4 belt fed cannon and a lot more fuel and see how they get on with high g manoeuvres, they are completely different fighting machines."

As far as I am aware none of the planes I listed came with radar as standard (or do you mean the NF II variant of the Mosquito?).

NOTE: Added Wellington II and Boston to the ultimate g load list in my post#12.
______Changed "maximum allowable (ultimate) g load for the Wellington X" to "ultimate g load for the Wellington X" as that is specifically how it is listed in the document. Not sure why it is listed that way as opposed to how the Wellington II was listed.

 

[cvairwerks]

Couple of quick definitions to stay within design norms...
Max Load...The designed maximum loading of the structure for every predicted configuration and operation that will result in no structural damage and will meet fatigue life requirements
Ultimate load...The point where exceeding this point can produce structural damage and will reduce fatigue life.
Depending on the design criteria, in combination with structural testing, there will be a margin between Max and Ultimate. There are a huge number of factors that come into play when designing and building an aircraft. It's why the Stress Group is so important at a manufacturer. Long years ago, I was in my office when the go ahead was given to load an F-16 wing to failure. Quite surprising at how much loading beyond ultimate it took to fail it. Even more impressive is that this wing was on an iron bird that had been used for fatigue life testing and had gone to either 2 or 3 times design life prior to the load to failure. When it failed, the shock loading thru the hangar floor bounced my desk around a bit.

 

[pbehn]

Hey pbehn,

Sorry, but my post was not addressing your post. I was simply posting some data on ultimate g loads that people might find useful relative to the different aircraft types/structures&materials. It might give a feel for the advantages/disadvantages of the different structures. I have not been able to find much data on the different g loads for 'normal safe' operational load factors. But if the safety factor is 1.5 for US airframes then the values above should allow a ballpark figure to be determined.

For example:

1. The designed operational g load for the P-51 would be 8g somewhere around 7800 lbs?
The designed operational g load for the P-51H was 7.33g at 9450 lbs, so this would make the ultimate g load 11 g at 9450 lbs?

The designed operational g load for the A6M2 was 7g clean, but only 6g with the drop tank. (Jiro Horicoshi)
The ultimate g load for the Wellington X was 4.2g at 36,500 lbs. (UK documents)

re: "Only one plane on your list has a two man crew and radar as standard with an internal space that can be used for bombs or fuel as required. Alternatively, consider putting in an extra man with radar equipment, 4 belt fed cannon and a lot more fuel and see how they get on with high g manoeuvres, they are completely different fighting machines."

2. As far as I am aware none of the planes I listed came with radar as standard (or do you mean the NF II variant of the Mosquito?).

NOTE: Added Wellington II and Boston to the ultimate g load list in my post#12.
3 ______Changed "maximum allowable (ultimate) g load for the Wellington X" to "ultimate g load for the Wellington X" as that is specifically how it is listed in the document. Not sure why it is listed that way as opposed to how the Wellington II was listed.

As per my previous post, copied from Drgondog. The design g load rating represents a typical or worst case scenario, it is a calculation, intended to ensure the plane is competitive in the "trim" it will meet its opponent. For the point 1. in bold the P-51 was a fine machine, but with 2 external 100 gal fuel tanks and the rear fuselage tank full it wasn't cleared for any turns at all, let alone 8 g. AFAIK the P-51 was only cleared for use of full 100 gal external tanks as a war time necessity, it cant legally be done in peace time. The weight of a P-51 on maximum power was changing at 15lbs a minute, on fuel alone, they also burned oil and fired guns.

2 Yes, I was referring to the Mosquito MkII which was a fighter but mainly a night fighter, having been designed as a light bomber putting a radar set, fuel and cannon in place of bombs doesn't affect its loading. The heaviest Mosquitos on take off were the recon versions, so on take off they were the heaviest and when landing they were among the lightest, any calculation of their g limit changes as they consume fuel and oil at max continuous for 2 Merlin engines.

3. G loads for bombers obviously depend on the bomb load, a Lancaster with a Grand Slam slung underneath was close to its limits in level flight. Parking a Lancaster with an unused Tallboy or Grand slam would cause permanent damage to the wing spars. In the air pilots were advised to keep manoeuvres to less than 2 g, however once the Grand Slam was dropped the pilot was in control of the hottest Lanc that ever went on a mission, only one turret and a 5 man crew no radios and with the latest uprated engines. In the course of a mission it went from dangerously overloaded to lighter than any other Lancaster in service, how do you make the "g" calculation, same for the Wellington.

 

[Zipper730]

Some ultimate g loads, all except A6M2 from UK documents. A6M2 ultimate load from Jiro Horicoshi.

A6M2 Zero______11g_____5,335 lbs
Hurricane IIC____11.2g___ 7,400 lbs
Spitfire IX_______10g_____ 7,240 lbs
Typhoon IB_____ 11.5g__ 10,500 lbs
Mustang________12g_____ 7,850 lbs (model unspecified)
Airacobra_______12g_____ 7,400 lbs (model unspecified)
Tomahawk______12g_____ 6,840 lbs (model unspecified)
Martlet V_______ 11g_____ 7,000 lbs
Hellcat_________ 13.5g__ 11,000 lbs (model unspecified)

Whirlwind_______10g____ 10,200 lbs
Mosquito F II_____8g____ 18,500 lbs
Lightning_______ 10.5g__ 15,500 lbs (model unspecified)

Wellington II_____11g____ 24,500 lbs
Boston__________ 6g____ 19,750 lbs (model unspecified)

edit: added Wellington and Boston

Wait... the Wellington II could take 11g?

 

[ThomasP]

Apparently. The Air Ministry had tests done on the airframe structure to see if the geodetic construction was as good as Barnes Wallace thought, and found that it had an ultimate load of positive 11g at the design weight. It should be noted, however, that this does not mean that it could carry a bomb load and keep that bomb load in the bomb bay if it pulled a positive 11g maneuver - the bomb shackles and connectors/fasteners could not handle the load. It just means that the fuselage/tail/wing/etc would not fail catastrophically at a lower positive g load. Also, it should be noted that the testing was done using the normal static loading method, where weights were added and/or mechanical forces (static and/or dynamic) were applied along the airframe to simulate the effects of high g maneuvers and g shock loads (ie drop tests/etc). I do not know what all the details of the parameters for the test were.

 

[Zipper730]

Apparently. The Air Ministry had tests done on the airframe structure to see if the geodetic construction was as good as Barnes Wallace thought, and found that it had an ultimate load of positive 11g at the design weight.

Which was 24500 lb.? While I could be wrong, doing a rudimentary search on Google, the Mk.II's MTOW was around 33000 lb. provided everything's accurate.

It should be noted, however, that this does not mean that it could carry a bomb load and keep that bomb load in the bomb bay if it pulled a positive 11g maneuver - the bomb shackles and connectors/fasteners could not handle the load. It just means that the fuselage/tail/wing/etc would not fail catastrophically at a lower positive g load.

Still, pretty impressive. I'm glad the Germans didn't try geodetic construction on their Heinkel 177 (IIRC, some Zeppelins did use it)-- they'd have probably succeeded in meeting the speed and range benchmarks with some to spare lol.

Also, it should be noted that the testing was done using the normal static loading method, where weights were added and/or mechanical forces (static and/or dynamic) were applied along the airframe to simulate the effects of high g maneuvers and g shock loads (ie drop tests/etc).

How does that compare to tests in actual flight conditions?

 

[ThomasP]

In some ways the ground testing was quite good, and by using the relatively gross results obtained the more experienced engineers could identify various more obvious problems and predict other less obvious or potential problems. But, it was somewhat primitive by today's standards. Usually, new design airframes (or sub sections) were built and ground tested to various degrees for load capability, then at some later date during test flying the airframes were put through successively higher g maneuvers until it was noticed that there were problems. The encountered problems would be addressed and then the flight tests would resume, with the process being repeated until the airframe met the specifications (or sometimes until it was decided to relax the specifications).

FEA (Finite Element Analysis) was an extremely laborious process (remember at the time as they only had basic mechanical adding/multiplying machines and slide rules - no programmable or otherwise more capable calculators, or computers) and FEA was limited in its use. Dynamic loadings were somewhat difficult and expensive to perform (ie they had to build specialized machines, jig, fixtures, etc) that could mechanically simulate the flexing/bending/twisting of the wings and fuselage, and hence were somewhat rudimentary in results. While analysis using dynamic simulation such as is done with computers today was not possible.

Basically, the engineers used the ground static and dynamic load tests to get the design into the ballpark (having found out some things that definitely needed to be changed) and then made their best (usually educated) guesses as to what other things needed to be changed based on past experience and intuition. The eventual flight tests would show what they had gotten right/wrong and what they missed entirely.

The first part of the video linked below shows a modern version of the test performed on a wing:

"
View: https://www.youtube.com/watch?v=E2KoJNDR3OA"

While the testing in the video is in some ways vastly more sophisticated than was used in the WWII period, the basic idea was the same.