A load on loads.

[FlyXwire]

There's been a lot of good postings on the forum here lately concerning flight impressions drawn from wartime flying of captured scouts, and discussion about the maximum diving speeds of the Great War's combat planes. I thought I'd post an article I submitted on another forum pertaining to some research I did on the relationship between airframe load ratings and potential flight performance of the conflict's combat aircraft...........perhaps it may be of interest to some of you.

For a quick introduction into the subject of WWI load ratings (g-loading), let me begin by listing some background information:

To help create a historical baseline, Germany's Idflieg (Inspektion der Fliegertruppen) specified a static wing load rating of 4.5G's (4.5 times the plane's fully loaded weight), during the period 1915-1917.

Prior to the war it was recommended that an acceleration of 3G's could be expected during common aerobatic flight maneuvers, and a safety factor of 2 times was agreed upon for an ultimate load factor standard of 6G's (this was also accepted for military aircraft).

As the war progressed, and as additional data was deemed useful for aircraft development, mock combat tests were performed by the British using a number of aircraft types. In these test the SE5A fighter recorded load factors of up to 4G's (3.8G's while looping), and it was found that 3G's was quite common for typical manuevers. It was also noted that bumpy weather could cause accelerations of +/- .5G.

Finally, calculations were done of loads expected when pulling out a B.E.2 after a vertical dive of 1,000ft., with 9.5G's possible, and a figure of 11.5G's at terminal velocity determined.

In a later report conducted for the N.A.C.A. in 1924 titled: The Vertical, Longitudinal, and Lateral Accelerations Experienced by an S.E. 5A Airplane While Maneuvering (Report No.163). The test was conducted to determine the accelerations (G loadings) experienced by an SE5A flying six maneuvers; the loop, spin, roll, wing-over, skid, and slip. The G loads shown in the table below were the maximum recorded results:

Chart 1

The report's conclusion reads as follows:

"It may be concluded from these tests that the accelerations acting along the longitudinal and lateral axes are very small compared with the acceleration along the vertical axis. It is also shown that the normal acceleration experienced by an airplane such as the S.E. 5A. in ordinary maneuvering are no higher than for a training airplane of the JN4h type. .
It should be noted that the accelerations in a given maneuver and with a given airplane are dependent on the manner in which the pilot handles the controls. If he is rough the airplane will be heavily loaded, but if he is skilful the loadings will be small. In general, however, the pilot feels the accelerations and unconsciously keeps from reaching high values, except under the stress of a combat when the loads may be much higher than in ordinary maneuvering."

The greatest G-load conditions occur in flight during high speed/high angle of attack attitudes where maximal lift coefficient is realized, such as with vertical dives and recovery (pull-out), and during quick and violent combat maneuvers.

In 1924, Jimmy Doolittle flew exhaustive test in a Fokker PW-7 pursuit plane to determine G-loads realized at various speeds and for a number of flight manuevers. The report prepared for M.I.T. was titled: Accelerations In Flight, and some of the data generated is shown below for the loads encountered during pulling out of a dive from different airspeeds.

Chart 2

In the conclusion of Doolittle's report the following comments were made concerning the correlation of pilot handling, combat conditions, and the theoretical limits of the subject plane's airframe integrity:

"In the case of the pursuit airplane the speed range is great, and stabilizing and damping forces are reduced to a minimum consistent with easy handling in combat. The ability of the pilot to impose large dynamic loads therefore depends largely upon the ease with which he can move the elevators when traveling at speeds in excess of the maximum horizontal speed. If the elevators are perfectly balanced, there seems to be no reason why he can not impose loads very close to the theoretical values, and in the above tests, made with an airplane having elevators almost perfectly balanced, the actual loads obtained were but 3.5 per cent less than the theoreti- cal. This airplane was designed to support a dynamic load of 8.5. Actually it would probably support about 10, judging by a static test of an airplane exactly like this one except that the wings were fabric covered. It would follow that if the airplane were suddenly pulled out of a dive at a speed in excess of 185 M. P. H. (and which would frequently occur in actual combat with this airplane) the wings would fail. It was this consideration which caused the engineering division of the Air Service to require a factor of 12 at high angles of atttack for pursuit airplanes and to recommend against the use of balanced controls on that type."

It may be of interest to note that after landing, inspection of the wings of the PW-7 showed that the veneer covering of the upper wing, on the under surface, had split from the trailing edge to the rear spar, and from the trailing edge to a point back of the rear spar on top. On this particular type of wing design the typical strengthening acheived by drag bracing between the spars was replaced instead by the veneer wing covering itself........!!!

In 1918 the last stipulations for German aircraft loads were issued (the BLV), which only slightly increased the requirements from the lows of the 1915-17 period. As the Allied standard for G-loading had never decreased from the accepted 6G's rating, the modest increases of the BLV were still below those of the enemy's requirements, even at this late date. Although many German aircraft likely exceeded the BLV requirements, this was not mandated by authorities. It is interesting to note that the SPAD 7 enjoyed a static G-load coefficient of 7.9, which according to the "pull-out" chart above would safely allow a dive recovery from a speed just above 160mph. Obviously a rating of only 5G's yielded a much lower margin for safety, at any speed above 130mph!!!

Here are some pertinent figures listed in the 1918 BLV stipulations:

Chart 3

It is interesting to note also the lower requirements for inverted flight.

Finally, as additional information that may be of interest to our discussions here, let me quote the follow passage concerning the effects of G-loads on pilot and aircrew, from Doolittle's "Accelerations In Flight" report:

"From the results of these tests it is apparent that serious physical disorders do not result from extremely high accelerations of very short duration, but that accelerations of the order of 4.5 g., continued for any length of time, result in a complete loss of faculties. This loss of faculties is due to the fact that the blood is driven from the head, thus depriving the brain tissues of the necessary oxygen. To the pilot it seemed that sight was the only faculty that was lost. The flight surgeons at McCook Field are of the opinion that sight is the last faculty to be lost under these conditions, even though the pilot may be under the impression that he retains all the others. This opinion is based on the observation of men undergoing rebreather test. The acceleration which an individual can withstand for any length of time depends upon his blood pressure, the person with the higher blood pressure being able to withstand the higher acceleration. Upon the condition of the heart depends the ability of the individual to recover quickly from the effect of prolonged acceleration. If the heart is in good condition, there is no danger in undergoing such a strain unless the acceleration is continued for a period in excess of l0 or 12 minutes, after which death will result. The same is true of the rebreather test; unconsciousness will result from the deprivation of oxygen and death will result if this is continued for the same length of time."

I'm hopeful that the information I have presented here in this thread might promote further discussions on this forum concerning the limitations imposed on potential aerobatic performance of the Great War's combat aircraft.............both in it's affects on airframe and pilot performance.

[Barrett]

Excellent contribution: thanx a bunch!
FWIW, the major difference between Gs Then & Now is onset and sustainability. The best way to get rapid onset in the Great War machines was dive recovery: no matter how hard anybody pulled in the horizontal, airspeed bled off rapidly (which, in a dogfight, may or may not be A Bad Thing.) More likely, sustained slow flight was desireable, as in the D-VII's fabled ability to hang on the prop, sustaining high AoA while hosing the cockaded target overhead.

[Wind In The Wires]

Thanks for your research!

The G reading for a roll would seem to indicate that they were performing what is now known as a barrell roll rather than the straight ahead slow roll which shouldn't require more than 1.5G. The 2.6 G for a wingover is interesting and I wonder if it might be the G load for the pullup and pullout sections of the manuever.

Having flown aerobatics in a draggy biplane I believe that pulling more than 3.5G costs more in induced drag than is gained in vertical performance.

There is a lot more research that could be done in this area. For instance, did the manufacturers limit elevator authority to prevent the pilots from exceeding the capabilities of the aircraft? Anyone in reasonable shape and with some experience of G forces can pull 8G for a second or maybe two.

BTW, someone analyzed the Wright Flyer and it's immediate descendants for stability and control and G capability in a series of publications available from the Smithsonian. The 1903 Flyer was designed for 5.5G.

[Barrett]

Fascinatin' fact about the Flyer's stress limit. Looking at all those films this past month, I very much doubt W&O ever saw 2 G (even in '08) but you can't blame 'em for building in some velvet.

[Boom]

All a good discussion, except I'm really having trouble picturing a Bloater at 9.5 G.

cheers, Boom

[FlyXwire]

Wind In The Wires questioned whether the aircraft manufacturers limited elevator authority to prevent pilots from exceeding the stress-load capabilities of their aircraft..........this indeed was one of the interesting consequences of Doolittle's report:

It was this consideration which caused the engineering division of the Air Service to require a factor of 12 at high angles of attack for pursuit airplanes and to recommend against the use of balanced controls on that type.

I find it quite remarkable that the engineering division of the United States Air Service in the 1920's mandated that inherent control stiffness be built into it's pursuit planes in order to place limitations on a pilot's ability to apply input forces during high speed maneuvers! This is obviously quite a contrast from the servo-assisted, fly by wire control systems found on today's modern jet fighters. It has also been widely held that part of the Fokker D.VII's superlative peformance came from the fact that it possessed well balanced and harmonized flying controls that made it easy to fly to it's limits in combat by experts and novices alike, but would this not have promoted novices to exceed the stress-loading capabilities of the plane?

As Barrett inferred:

More likely, sustained slow flight was desireable, as in the D-VII's fabled ability to hang on the prop, sustaining high AoA while hosing the cockaded target overhead.

One of the interesting comments that Sir Gordon Taylor made from the excerpted Albatros D.III flight test that Grevan posted was the following:

Latrally it was quite light, but when I steepened the turn and tried to pull the machine around with the elevator it seemed very heavy, putting up a resistance to the turn. I could see immediately why the Albatros pilots kept out of the close duelling turns.

What would have been the consequence of a heavy elevator in the horizontal flight profile when the flight attitude transitioned to the vertical............and the dive..........and the pull-out? As Doolittle wrote:

In the case of the pursuit airplane the speed range is great, and stabilizing and damping forces are reduced to a minimum consistent with easy handling in combat. The ability of the pilot to impose large dynamic loads therefore depends largely upon the ease with which he can move the elevators when traveling at speeds in excess of the maximum horizontal speed.

Further on Gregvan quotes Cecil Lewis on his Albatros D.III flight impressions:

........but I am certain of one thing - to throw an Albatros about in the air was hard work and it would have made you sweat in a dogfight.

Without a doubt, the work load required to deflect a fighter's control surfaces directly affects the effectiveness of it's combat efficiency......especially in protracted dogfighting. Adding further to the possible degradation of potential performance was the erroneous belief during WWI that "bigger was better", and for example that more ailerons or bigger ailerons on a plane inherently meant faster rolling performance (the consequence was often increased drag, lift loss, and roll damping instead).

Finally, all discussion about aircraft performance must first derive from the pilot's ability to master the control of his plane. Of course aerial combat places even greater premiums on a pilot's knowledge and skill. Unharmonized and stiff flight controls can only exact performance penalties. Couple this to doubts about the airframe integrity of one's mount, and their should be no doubt that many pilots of the Great War never were able or willing to fly their aeroplanes to the limits of their performance!

[Barrett]

The whole subject of control harmonization is widely overlooked (except on The Forum&#33 beginning from 1914 onward. It was a revelation to sit in a reproduction Eindecker and feel how heavy the roll (wing warping) axis was and how much effort was required for so little deflection at the tips--about 3 inches I'd say. Jim Appleby who built & flew 2 of 'em said the effect was somewhat diminished under an air load but still needed muscle. He flew with his right arm on his thigh to avoid excessive pitch, and the rudder was equally sensitive. In short, a real bear to fly in a co-ordinated manner.
Then there's the Taube. They put a honking great wheel on that hummer for good reason: to provide enough mechanical advantage to move so much airfoil.

[FlyXwire]

Agreed Boom!

The 9.5g figure was a theoretical calculation for the pull-out forces to be encountered after a vertical 1,000 foot dive..............particularly unlikely to be sustained in RL.

[FlyXwire]

Great insights Barrett!

Thus the two grips and hand throttles on so many Great War control sticks...........

[Barrett]

Correct, FXW! In the Eindecker you gain a distinct advantage by grasping both sides of the grip and pushing-pulling in the desired direction. I.E., if you want to go left, pull with the left hand and push with the right. I've tried it one-handed both ways, and while you can get the same amount of camber change as two-handed, it definitely takes more effort.

'nother interesting thing about the Eindecker was the flying tail. I used to give tours to graduating F-15/16 classes from Luke, and they were inevitably astonished to see "their" type of control surface on a 1914 machine.