Rolling G limits

[gerrychuck]

Has anyone ever seen any published info for rolling g limits on the 6? I wasn't aware of the fact that most aircraft have different load limits depending on whether the aircraft has any rolling moment or not. I was made aware of this by both my son (currently training on the T-38) and my hangar partner (F-18 pilot and 4 year Snowbird pilot). Sounds like for most aircraft the rolling limit will be about 2G less than with pulling G just in one plane. Any info out there?

 

[sailvi767]

Has anyone ever seen any published info for rolling g limits on the 6? I wasn't aware of the fact that most aircraft have different load limits depending on whether the aircraft has any rolling moment or not. I was made aware of this by both my son (currently training on the T-38) and my hangar partner (F-18 pilot and 4 year Snowbird pilot). Sounds like for most aircraft the rolling limit will be about 2G less than with pulling G just in one plane. Any info out there?

Your number sounds about right. Rolling G's are far more critical on a aircraft with hardpoints on the wings carrying large loads of fuel or bombs. I can't think of a reason to do rolling pulls in a RV. I would simply avoid them.

George

 

[B25Flyer]

The general rule for rolling Gs is a 1/3 reduction, or 4 Gs in the RVs. And that definitely applies to the RVs.

Doug Rozendaal
F-1 EVO

 

[sandifer]

Never heard of this being quantified for any aircraft type. The degree of extra stress beyond what the G meter indicates depends on airspeed as well as the degree of aileron deflection. Seems like too many variables exist to make any meaningful generalizations on limits once you're beyond maneuvering speed. Regarding 4G's, I would be much more hesitant to pull 4G's and apply full aileron at Vne than pull 5G at 140 mph and roll slowly. I can't think of any maneuver (even in the unlimited aerobatic world) where simultaneous hard pulling and fast rolling is required. It's usually one or the other. Barrel rolls, clovers, chandelles, wingovers, etc. involve rolling G, but there's no need for high G, and they typically involve very modest aileron deflection. Just be aware of it and use common sense. No need to consciously think too much about it as you're flying, IMO.

 

[gerrychuck]

Thanks for the replies. The context of the discussion was not regarding deliberately rolling and pulling in a planned manoeuvre, but rather the potential consequences of blowing a roll and doing a split s out the bottom; i.e make sure you roll wings level before pulling out of the dive rather than trying to do both at once.

 

[hendrik]

Don't know about the -6, but from analysing aerobatic videos of the -8 I know that it is capable of at least 7 rad/s^2 roll rate changes, which corresponds to about 2g at the tip of the spar. Nothing official, but food for thought.

 

[ronschreck]

Rolling G's happen easily in mock air combat. Something that's lots of fun in a RV!

 

[rvbuilder2002]

Regarding 4G's, I would be much more hesitant to pull 4G's and apply full aileron at Vne than pull 5G at 140 mph and roll slowly.

I agree, but would also like to point out that you should never be doing full aileron input at anything near VNE anyway. The maneuvering speed limitation is valid for aileron inputs in the same way it is for pitch inputs. If you are above maneuvering speed, you should be limiting your aileron input by an appropriate amount in the same way you would pitch inputs.

 

[Vac]

Asymmetric G Limits

Asymmetric G limits are often specified for fighters. Newer airplanes with either an "OWS" (overload warning system) or true fly-by-wire either warn you or preclude an over-G condition from occuring. Older airplanes (including the T-38) were/are dependent on the pilot observing limits (which is why they tend to end up bent!).

Depending on how the plane is designed, asymmetric conditions (application of simultaneous flight controls about two axis) may be accounted for in the specified G limits--i.e., it can be designed in. If not specified, a rule of thumb is to reduce specified limits by 20%--Doug's 1/3 ROT is just about perfect from a handling perspective. For typical recreational aerobatics, 4 G's is generally plenty. In a typical aerobatic RV loaded within designer's limits, asymetric application of controls isn't of much concern if you operate within this envelope.

The real discussion is loaded vs. unloaded rolling. This is sometimes refered to as "lift vector control." The lift vector extends straight up from your seated position in the cockpit and it's essentially "where the lift is pointed." The short answer here is if your intent is to apply maximum allowable G (assuming you're flying faster than corner velocity [maneuvering speed] and have 6 G's available), it's generally best to have the lift vector set where you want it prior to application of maximum allowable G. Energy bleed rates are high at 6 G's (if you have airspeed under control!), so you're not going to stay there very long.

Thread drift. One thing to keep in mind with any RV is the amount of aerodynamic G available at speeds above corner velocity. Due to the wide speed band (margin between stall and Vmax), the amount of instantaneous G you can develop with a stick snatch is impressive and more than enough to reak havoc, even if the wings or tail don't come off. Our RV-4 at 1375 lbs gross is capable of generating 9 G's (ultimate design load limit) at 165 MPH CAS. At 200 MPH (cruise descent in smooth air at an appropriate altitude), 13.2 G's are possible. During maneuvering flight (i.e., aerobatics, unusual attitudes, etc.), this phenominon can manifest itself if the velocity vector (where the airplane is going) is buried and the airplane is allowed to acellerate above corner. Any time you are above corner/maneuvering speed, an over-G is possible with improper application of the flight controls.

Cheers,

Vac

 

[sandifer]

I agree, but would also like to point out that you should never be doing full aileron input at anything near VNE anyway. The maneuvering speed limitation is valid for aileron inputs in the same way it is for pitch inputs. If you are above maneuvering speed, you should be limiting your aileron input by an appropriate amount in the same way you would pitch inputs.

It is true that maneuvering speed (Va) specifies the speed at which you can move any single flight control (aileron, elevator, or rudder) one time to its full deflection. Va does not apply to simultaneous control deflection. But the reality is that Va is normally limited by (and specifically set to) the speed at which full elevator cannot exceed the g-limit on the airframe. It's 115kt (132mph) in the RV-4...pulling the stick fully aft would produce 6G before stalling the wing. Common sense would tell us that (in reality) each individual flight control has its own Va, but that this information is not provided, given the FAA's definition of Va.

Applying full aileron (at 1G) a normal and reasonable margin above 115kt will not damage the wing of an RV-4. In theory, there would be an airspeed for which it would, but this is not known to those who do not have the capability to calculate it. Anyone who does aerobatics knows this, and in general, Va is not applied to aileron use in aerobatic airplanes...you just won't get a lot out of the airplane otherwise. That being said, I admit I would not apply full aileron at Vne in an RV, but I would not limit it to Va either. Yes, it's a gray area and anyone who limits full aileron use to Va and below cannot be criticized. Aerobatic airplanes will commonly see full aileron use well into the yellow arc in order to obtain max performance. I've never heard of or seen any damages or failures associated with this. I think it helps to be in an airplane stressed for acro to begin with. I would definitely be more mindful of Va and aileron use in normal category airplanes.

 

[rvbuilder2002]

Common sense would tell us that (in reality) each individual flight control has its own Va, but that this information is not provided, given the FAA's definition of Va.

I agree, but also why I shudder at people discussing full aileron input at high speed.

Applying full aileron (at 1G) a normal and reasonable margin above 115kt will not damage the wing of an RV-4. In theory, there would be an airspeed for which it would, but this is not known to those who do not have the capability to calculate it. Anyone who does aerobatics knows this, and in general, Va is not applied to aileron use in aerobatic airplanes...you just won't get a lot out of the airplane otherwise. That being said, I admit I would not apply full aileron at Vne in an RV, but I would not limit it to Va either.

I agree, but I don't think that "anyone who does aerobatics does know this". And what is a reasonable margin? I have been involved in static testing of RV wings, but I still don't know what a reasonable margin is for any given situation. So, not meaning to be critical, just wanting to clarify that VNE is by no means the limit.

In a forum venue such as VAF, there are thousands of people reading what is posted by people who sound credible, and then doing what they read. I just felt in this case that it was worth clarifying to the masses.

Now I just wish there were specific #'s that could be quoted for specific situations..... but there aren't.

 

[sandifer]

And what is a reasonable margin?

IMO, the recommended aileron roll entry speed range listed in the builders manual. I recall this information in my old RV-3 manual, at least.

This is good reading for folks fairly new to acro who would like to get started in their RV. Van makes no mention of Va as it relates to aileron deflection, not that it's not worth considering. He writes a "How to Roll" section, with the general "getting started" recommendation of 170mph IAS and "firm aileron input; half stick deflection or more". I take this to mean up to full aileron deflection. If full aileron would damage the wing at this speed, not only would this have been written very differently (given Van's caution and engineering expertise), but would have also become a widely-known issue, given the prevalence of the design, and the large numbers doing RV acro.

http://www.vansairforce.net/safety/AnAerobaticEpistle.pdf

My comment about folks doing acro was related to the fact that most take advantage of full (or nearly so) roll rate, even if above Va. But normal ops would not see rolls done near Vne. I just don't think this is much of a concern when doing aerobatics in the typical (and recommended) speed range, which will typically be above Va (at least in an RV).

 

[sailvi767]

I think there is some confusion on what is a rolling pull. A aileron roll is never a rolling pull if done properly. Its a unloaded maneuver so the speed of entry is not really relevant. A rolling pull is rolling the aircraft while pulling a significant amount of G at the same moment. Its not something you would normally encounter while flying a RV. Ron S mentions simulated air combat and that is one area where it could happen but for the most part it not common.
Most aircraft also have a slower roll rate while loaded up. They roll the fastest at zero G. Rolling pulls happen at times when a pilot realizes for whatever reason ground contact is possible. If they are in a angle of bank a inexperienced pilot might snatch the stick back while still banked and then try and roll wings level. He would more then likely have less altitude loss by first rolling wings level and then applying G force up to the airframe limits or even beyond if required to miss a obstacle.
I just don't see rolling pulls as a big issue in day to day RV flying. Perhaps a badly botched acro maneuver could lead to a rolling pull but again the proper response to a nose low high speed situation where ground contract might be a factor is to roll wings level and then apply G for the pullout.

George

 

[rvbuilder2002]

I take this to mean up to full aileron deflection. If full aileron would damage the wing at this speed, not only would this have been written very differently (given Van's caution and engineering expertise), but would have also become a widely-known issue, given the prevalence of the design, and the large numbers doing RV acro.

Conversations I have had with Van indicate he has concerns with peoples disregard for the amount of control input they use above Va, so this is not an assumption I personally would make.