Rear Spar Edge Distance - Why so critical?

[MS19087]

I have read all of the threads on this topic and understand that it is very critical to maintain 5/8 edge distance - to better understand, I have a few questions . . .

1) What kind of loads does the rear spar see during flight (inc turbulence).
2) Are these loads different for an RV-9 (non -acro) vs an RV-6,7,8?
3) What would a failure at this point result in? (Besides the obvious of changing the incidence, leading to I assume an unstable flight condition and possible disabling flap movement.) What else?

Thanks for your insight . . .

[Andy Hill]

As a wild guess:
1. Quite a lot
2. Yes, but 4.3g v 6g and 9(A) has lighter parts to compensate, so ED still critical
3. Death

HTH?
Andy

[Bill Wightman]

Mark,

The rear spar reacts drag and torsional loads on the wing. Drag loads can be MUCH higher than you'd think during maneuvering flight. Torsional loads arise due to pitching moment (that's the wing twisting nose up or down) from the wing. Torsional loading is mostly the result of the aerodynamic pitching moment of the wing. Additionally, large torsional loads arise from trailing edge deflection (aileron and/or flap).

These loads are reacted by the mechanical couple between the main spar and the rear spar (also sometimes called the "drag spar"). The rear spar is a "pin" joint, intentionally designed to force all bending into the main spar. Its designed so that it cannot react any bending load due to lift. Its also a potential single point of failure, and so must be designed and built correctly.

The drag loads can be HUGE. Even though drag loads are quite low at the typical cruise condition (less than 200# for the airplane), those loads can build tremendously in accelerated flight. My detailed analysis of RV8 flight loads show drag forces on the airplane rising to well over 800 pounds in worst case: basically a 6g maneuver at or above V_a.

You should understand that its indicated airspeeds that drive physical forces on the airplane. There are other threads on VAF that talk about true airspeeds, as they relate to V_ne but that's another matter called flutter. So, there are really two areas of concern: structural limits and aero flutter limits. The rear spar attach is a structural concern.

An improperly designed or built bolted joint here can result in failure. If the drag spar attach fails, the wing would most likely separate due to torsional and rearward bending failure of the main spar.

[Bill Wightman]

I forgot to add that turbulence really doesn't play a direct role in loading the rear spar attach. It bounces the airplane up and down, and so the rapidly changing lift on the wing would correspondingly affect the drag load on the wing. However, to really abuse the rear spar attach, the wing needs to be loaded up and producing high drag numbers and high torsion forces. Think: going fast, pulling hard and simultaneously rolling sharply.

An example of an airplane limited by asymmetric loading is the T-37 (USAF primary jet trainer that can now be found in the bone yard) which had a symmetric g limit of 6.67 and a "rolling" limit of around 4 g's.

My advise: keep your 5/8 edge distance.

[Bob_pipedream]

I had to replace the rear spar on my bodged wing (not bodged by me I hasten to add) and it is actually not that badder job with some thought and lots of tea.

I also had to use expensive NAS bolts on my other RV6 due to not reading the Vans website and the new words about using a reamer (in my defence, I did read the instructions which had not mentioned anything about reaming).

I now feel a lot better about tight turns and rolls etc.

[Andy Hill]

The Vans letter re the RV-3 issues states:

The fourth failure also resulted from a sharp pull-up after a high speed pass. The rear spar attached failed, presumably because undersize bolts had been used, possibly in conjunction with marginal edge distance.

Andy

[Finley Atherton]

Obviously I am not advocating anyone fly with a compromised rear spar attach but I do know of an aircraft (low wing, not an RV) that flew for many hours with the rear spar attach bolt missing.
This aircraft routinely operated at max gross and in retrospect was trying to tell the pilot and maintenance staff that something was wrong. For instance, the aircraft would not fly straight and it was assumed the flaps were misaligned so they were adjusted to compensate. The penny finally dropped when the Air Speed Indicator stopped working and on investigation it was found that the up/down movement of the wing rib had cut through the line from the pitot to the ASI where it exits the fuselage.

Fin
9A