[flyenforfun]

Just a quick question for you guys. I know little about aerobatics and I always come up with questions as I daydream about my future RV.

Our airspeed indicator ordered from vans has a blue tick mark. I know in twin engine planes this indicates Vyse which is best rate of climb single engine. On my airspeed indicator from Vans, I guess this is meant to indicate maneuvering speed?

If so, what is it about the design of the -8 that makes maneuvering speed so low? 123 knots?

Also, as maneuvering speed means full control deflection without causing damage to the airframe, so I assume aileron rolls to the stops must be performed under 123 kts?

 

[David-aviator]

Just a quick question for you guys. I know little about aerobatics and I always come up with questions as I daydream about my future RV.

Our airspeed indicator ordered from vans has a blue tick mark. I know in twin engine planes this indicates Vyse which is best rate of climb single engine. On my airspeed indicator from Vans, I guess this is meant to indicate maneuvering speed?

If so, what is it about the design of the -8 that makes maneuvering speed so low? 123 knots?

Also, as maneuvering speed means full control deflection without causing damage to the airframe, so I assume aileron rolls to the stops must be performed under 123 kts?

Some, perhaps not all, Vans airspeed indicators have the blue tick and maybe it is intended to be Va. But the official method of noting Va is via a placard, not a blue tick. The reason being, Va varies with gross weight. It is not a fixed number.

The 123kts could be Va at max gross weight. In older airplanes without a FHB reference to Va it can be determined by 1.7 times stall speed which equates to approximately a 3G load. But 123 kts is much more than 1.7 times stall in any RV so I don't know what the blue tick indicates. Good question for Vans to answer.

(With the -7 I built, the blue tick was very close to a best rate of climb speed with a fixed pitch prop.)

 

[sandifer]

Also, as maneuvering speed means full control deflection without causing damage to the airframe, so I assume aileron rolls to the stops must be performed under 123 kts?

Va has to be set at the max speed that any one of the three control surfaces (elevator, rudder, and ailerons) can be moved one time to full deflection without compromising the aicraft structure. In light airplanes, this speed is generally limited by the elevator's ability to stress the wings beyond the load limits (6G in an RV). Common sense would conclude that each of the flight control surfaces have a different "Va", but no aircraft designer lists different Va speeds for elevator, rudder, and ailerons.

It would be very conservative (and safe) to only use full aileron deflection at or below Va, but Van has not explicitly stated this is required. It's a gray area. I don't know of any aerobatic aircraft design (including RVs) where there is a known structural problem with applying full aileron above Va. And I don't know any aerobatic pilots who restrict full aileron deflection to Va and below. This is not a recommendation for doing this, just reality. I little common sense and feel for the aircraft is in order. I would not apply full aileron in an RV at Vne, but I would not limit it to Va either. That's my choice. You have to arrive at your own. Some comments on this subject in this thread:

http://www.vansairforce.com/community/showthread.php?t=80491

 

[scsmith]

Slight mis-understanding of Va

The previous posts are missing a subtle thing that helps understanding of Va.

Va is the speed where the wing will stall at the limit load factor. For the RV-8, this means it is the speed where the airplane at gross weight will stall just as it reaches 6 g's. (Any rule of thumb such as 1.7 x Vs needs to be adjusted for the limit load factor of the airplane. For a 3-g limit load, it would be roughly correct, but for an RV designed to 6 g's, it is 2.45 x Vs) Note also that the RV-8 design gross weight for the 6-g limit is 1600 lb, whereas the Vs is determined at the utility category gross weight, 1800 lbs. So there isn't a consistent rule of thumb to use a multiplier on Vs to get Va.

So, obviously, it is impossible to overstress the wings below Va, because the wing stalls before it gets to limit load. It is not a case of the elevator having sufficient control authority to exceed limit load.

Once Va is established, the requirement for the entire structure to be able to withstand full and abrupt control input of any control becomes a design criteria -- you analyze the strength of fin, rudder, stabilizer, elevator, ailerons, etc. to demonstrate that a full, abrupt control input at that speed will maintain a safety factor of at least 1.5 on structural failure of any component. Some components may in fact be much stronger.

But the obvious limit is that full, abrupt elevator input above Va WILL exceed the limit load on the wings, because the wings are capable of producing enough lift to exceed the limit load without stalling.

 

[sandifer]

So, obviously, it is impossible to overstress the wings below Va, because the wing stalls before it gets to limit load. It is not a case of the elevator having sufficient control authority to exceed limit load.

No misunderstanding. I think this is generally well understood. All I was saying was that Va is generally limited by factors associated with the elevator, and not the rudder or ailerons, since the rudder and ailerons are incapable of loading the wings in the manner that is considered when determining Va speed as you describe. I think in most airplanes, the speed at which you can fully deflect the elevator and produce a stall before exceeding the load limit is well below the speed at which you can fully deflect the rudder or ailerons and cause damage. All planes are different, though. An Airbus may have a relatively weaker rudder than a Cessna. But for most light GA/sport planes, I think the lowest common denominator for Va is the elevator surface.