Interesting Read: Vne-TAS or IAS That is the question

[Vac]

I've been hesitant to contribute to this thread, but just a couple of points to ponder from an old fighter pilot...

If you carefully read both of these mishap reports, although flutter is discussed, it is apparent that in both cases the pilot likely removed the tail of the airplane using the stick (i.e., aircraft handling error). In the case of the Canadian mishap, a pilot was attempting BFM (basic fighter maneuvering, or 1 v 1), oversped the airplane, and applied excessive G resulting in structural failure. In the second example, it appears as though the pilot may have attempted a high speed split-S. Unless G and buffet (i.e. aerodynamic limit) are carefully managed, any split-S can result in dangerous speeds in RV-types.

Some airplanes are built stoutly and have high drag characteristics (e.g., Pitts) that allow more margin for handling error when attempting to max perform the airplane. Other airplanes are simply more stoutly built (e.g., Extra) and have more strength margin.

At Vne, my RV-4 is capable of generating over 14 G's. If I apply rapid G under those conditions, I can generate G far in excess of ultimate design load limit (9.0, assuming no construction error or fatigue damage) and cause structural failure. If I'm applying asymetric G (i.e., maneuvering about more than one axis simultaneously), G limits are further reduced (i.e., it takes even less G to remove the tail). The ability to "generate G" is a function of airplane and flight control design as well as "q" (dynamic pressure, or more simply, airspeed). Snatching the stick (i.e., an abrupt pull) can generate more than 9 G's in less than a second in the typical RV at speeds above 165-170 MPH CAS (typical cruise speed, still inside the green arc). Smooth application of flight controls and proper use of G/buffet is critical during maneuvering flight to avoid exceeding limits.

Vne is effectively a TAS limit, as has been discussed. If you have advanced flight displays, it may be indicated in the cockpit, otherwise a simple table can be generated to give you some very good IAS rules of thumb. The yellow arc does not even exist on turbine powered airplanes, so piston airplanes have a privilege to use up some of the structural design margin not available under more conservative certification/design rules. I'm very congnicent of airspeed during cruise descent, especially from "high" altitude (at or abov 8000'). Smokey's article referenced earlier in this thread has an excellent description of the importance of properly controlling airspeed during descent.

RV-types have three handling characteristics that need to be understood to avoid handling mishaps: low drag charactersitics that result in the aircraft accellerating rapidly when the velocity vector is below the horizon (i.e., the nose is down)--moreso if the lift vector is also below the horizon (i.e., the pilot's head is point down toward the ground)--especially for airplanes with fixed-pitch propellers; reduced static margin (i.e., pitch stability), particularly at aft CG and/or high pitch angles/power/reduced speed and limited stall warning.

It's important to understand maneuvering speed, symetric or asymetric G application and learning to live "inside the green arc" during maneuvering flight. It's very seldom that I exceed Vno (max structural cruising speed or the top of the green arc) under any conditions. During maneuvering flight, speed in excess of maneuvering speed only provides energy for going up, going down is best conducted at corner velocity (maneuvering speed) or less to make sure that you have maximum G allowable for maneuvering. Maneuvering speed is not a fixed value, it varies with gross weight and whether or not manuevering is about one or more axis.

RV's are outstanding sport airplanes. They are very well designed, engineered and have excellent handling characteristics. If operated within the envelope using proper handling techniques, they are some of the sweetest flying airplanes that are affordable for average folks like us. They are not, however, bullet-proof. Proper handling training is critical or proper experience that can be carefully applied during testing is necessry to learn how to fly them.

Story: It was my second flight in my new to me RV-4. My insructor was in a Christen Eagle and was chasing me for the flight (normal technique utilized for checking out in single seat airplanes in the military). At the conclusion of basic handling training, we set up for fighting wing (i.e., flying formation in a loose chase position, think "rat racing" or following another airplane around during maneuvering flight mantaining a defined area behind them). At one point during maneuvering (loose lag roll to the outside for folks that understand what that means), I noted excessive wind noise. A quick glance at my airspeed indicator showed I was well in excess of red line. My attitude at that point was about 10-15 degrees nose low and about 210 degrees of bank or so (i.e., inverted, nose low, right roll past vertical). As soon as I recognized the overspeed condition, I smoothly unloaded (reduced G to 1/4 or so) rolled up right, and carefully applied minimum G to get the nose tracking back up above the horizon to allow the airplane to slow back down. Our maneuvering floor and altitude at that point gave me the luxury of proper control application, assuming I didn't encounter any flutter as a result of the over-speed. That was my first introduction to RV acelleration characteristics and taught me that cross-checking airspeed is an integral part of maneuvering flight--even when maneuvering "eyes outside of the cockpit" it's necessary to know what your energy (airspeed) is at all times.

Part 3 of the transition training manual posted in the sticky at the top of this Safety section has an extensive discussion of limits, handling characteristics and maneuvering flight that may help shed some light on this topic for folks that might want more information.

Forgive the jargon!

Fly safe,

Vac

[David-aviator]

Excellent post Mike, thanks.

[grubbat]

Wondering what the weakest link is always a thought while I build. Cruising at 180mph true, one doesn't want a strong gust of wind to separate the tail or whatever the weakest link is on our build. If I knew that adding a gusset here, or a stiffener there, would increase the margins, I would probably do it just to give a peace of mind at 180mph. Granted, the tails came off in the already mentioned incidents under different circumstances, but the thoughts of increasing margins is ever present.

Originally Posted by BillL

From my research, not experience - read that twice.

1. As Paul Dye as pointed out, flight in smooth air, beyond the flutter speed can occur, but if flutter begins, it has a range of results. Physically, the result will depend on the damping of the system and strength of various components. The result could range from buzz to separation.

2. From literature, ground vibration test animation, it is all the above. wings, torsion/bending of aft fuse, HS, VS and rudder. Oh - and all the control surfaces. All will participate, like one side of a tuning fork won't naturally vibrate without the other.

3. Rudders.

Original 7 rudder is the same as the 8. Also referred to as the short 7 rudder.
The current 7 rudder uses 9 leading numbers. AKA - tall rudder.

The weight is without upper and lower fiberglass. No paint. With balance wt.
short:
Wt = 3472 grams
Aft CG = 3.18"
skin = .020"
area = 718 in2

tall:
Wt = 3855 grams
Aft CG = 4.03"
skin = .016"
area = 936 in2

Both use the same counterweight. 1.85 lb. or 840 grams.

Fiberglass caps and paint will push CG further aft.

This is just a comparison, I could not possible quantify what it means.
FWIW - the Lancair rudder is balanced before and after paint.

DO NOT assume that adding more weight is the thing to do. That would just lower the VS vibration frequency and with all the interactions - who knows if that is acceptable.

Does anyone have similar information for the 6 rudder with .016" skin?

[David Paule]

The problem with increasing the margins a bit is that our airplanes have multiple load cases and issues to consider. There are also very many parts. Since we do not have access to the test results and analysis we haven't a clue how to improve margins - except for this:

Build it to the plans and keep it as light as possible. Operate it strictly within the limitations that the factory suggests.

Dave

[iwannarv]

Good post Mike.

Correct me if I'm wrong here, but is the chance at these type of accidents greatly incresed by the pilot's reaction to get the nose up and slowed down right away (NOW?).

Taking your example - nose down, high bank, overspeed. Many pilot's reaction would be 'oh cra*', gotta get pointed up, resulting in a quick pull/roll, high'er' G move to get the nose up and bleed off airspeed, resulting in structural failure in the process.

Observing an expert trainee such as yourself, the better reaction would be to recognize that it may/is not best to get slowed down 'right now' by use of G force to get nose up. Rather, recognize that even if above VNE the airplane will more likely stay together by using as little input/stress load as possible to get to nose up attitude, although that very well may mean a fair amount more time spend above the red line?


Also on the debate on counter-balanced control surfaces... I notice the Nemesis NXT flying at 400mph does not have counter balanced tail surfaces unless there is some other mechanism of doing so.

[rbibb]

Another reaction that should be ingrained if in an overspeed situation in an RV with a fixed pitch propeller is to chop the throttle.

I've fallen out of some bother maneuvers and ended up nose down and even with the throttle pulled to idle used up a lot of altitude easing out of the dive while keeping the G low and the acceleration (speed increase) to a minimum.

Its the sudden jerk on the controls that will do you in here. Smooth control inputs are the order of the day.

[ChiefPilot]

Originally Posted by rbibb

Another reaction that should be ingrained if in an overspeed situation in an RV with a fixed pitch propeller is to chop the throttle.

I've fallen out of some bother maneuvers and ended up nose down and even with the throttle pulled to idle used up a lot of altitude easing out of the dive while keeping the G low and the acceleration (speed increase) to a minimum.

Its the sudden jerk on the controls that will do you in here. Smooth control inputs are the order of the day.

Agree, except for keeping the G low. The best way to slow the aircraft is to load up the wing. With the throttle at idle, a quick-but-smooth pull to >5G (assuming you are under aerobatic gross) will definitely chip away at any energy surplus while you're recovering to normal flight.

[rbibb]

By G low I meant under the limit. So yes a hard enough pull or push or whatever to load things up (and increase the drag doing so) is in order.

Its the panic hard pull that grossly over stresses things. Frankly for most I suspect if you pull smoothly to a point where you are still awake (assuming a positive G here) you will be OK.

I guess another thing Im saying is that conditioning oneself not to panic as the speed starts to increase and do things in a methodical way as opposed to a reactionary, panic driven way is key to survival.

Probably not a bad idea to jet start out easing the nose down from a slow airspeed and learn just how fast these things accelerate is a good place to start learning what is is all about. Reduce power, ease out of dive with a decent G pull.
Rinse, repeat.

[ChiefPilot]

Originally Posted by rbibb

Its the panic hard pull that grossly over stresses things. Frankly for most I suspect if you pull smoothly to a point where you are still awake (assuming a positive G here) you will be OK.

Right on - I think we're in complete agreement.

[iwannarv]

The smooth straight pull to 5g is not always the issue, but the bank angle as well. We all read the stories on here of the 'rolling g's'.