The key considerations are:
- VA, Maneuver Speed and the limitations associated with VA
- Normal limit load factors and rolling G limitations
- Fatigue life limitations
The airworthiness design regulations specify the structural design considerations for small airplanes. It varies over the years so make sure that you apply the version appropriate to the aircraft that you fly. FAA AC 21.19A- Airframe Guide for Certification of Part 23 Airplanes explains what VA, design maneuvering speed is – it is the definitive source for this!
One important point must be noted: the designer may choose a value of VA greater than the “corner speed” determined by VS √n.
ACA SL 445, Aerobatic Maneuvering and Aileron Application, provides excellent guidance on loading limitations for the Super Decathlon as well as some useful general information.
Many have seen the generic flight envelopes provided by the FAA as training material and it is rare for a pilot to be provided with the actual envelope for the aircraft they fly. You may be surprised. Let’s have a look at the Pitts S-2B flight envelope which is included in the Owner’s Manual.
This was developed during the design phase for certification. Consider these points:
- VA, design maneuvering speed, is the same as VNO (from VC as used by design engineers) at 154 mph.
- Above VNO, the negative limit load factor reduces from -3 to -1.47 at VNE. No-one tells you that! You may think that the wing is designed to take -3G and it is OK however you must remember that the loads are different on other parts of the structure. Negative G at VD (VNE is normally 90% of VD) will be the design case for the horizontal stabilizer.
- Can you apply full forward stick at VA?
- This diagram shows a stall speed of 66 mph however, after flight tests, the stall speed was determined to be 60 mph per the POH. With the limit load factor of 6, VS √n = 147 mph CAS so VA is greater than the corner speed. i.e. full back stick applied at VA will result in the limit load factor being exceeded!
You will find that a similar situation applies to most other Pitts models – one of the most common aerobatic types in this country so you must be aware of the above. VA does not provide the protection from exceeding the limit load factor that you may think!
With many of the other aerobatic types this is not the case and VA is indeed the same as VS √n. Perhaps you have a type which is certified to a later version of FAR 23 and you have an operating maneuvering speed, VO, rather than the design maneuvering speed, VA!
VO is what many pilots think VA is. “VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits.” If your aircraft has a VO then you will find that VA won’t be mentioned in your POH at all. Just for info, the VA that the designers used would’ve been a higher speed than the VO.
“The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons.” VA is the design maneuvering speed and there are several specific operational limits which arise from it.
- you must know what VA is when you are flying as it is not marked on the ASI and the placarded value is for normal category operations, not acrobatic category.
- up to VA you can apply a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane.
- above VA you must know what you can do. It is not good enough to simply think that you cannot do that! Read that ACA SL445 for guidance. The Decathlon is one of the very few types for which such guidance is provided.
The other interesting note about VA in AC23.19 is that the aircraft will, if VA = Vs.sqrt(n), stall in a nose-up pitching maneuver at, or near, limit load factor. Recall that VS is the power off stall speed. Power will decrease the stall speed.
This note below in the CAP 10 Flight Manual adds another consideration.
The stall line in that flight envelope above is really fictitious for practical aerobatic operations. For example, consider the Pitts S-2B with full power applied and moving the stick full back very quickly when at VA – you will significantly exceed the limit load factor of 6. Maneuver speed VA does not provide the protection to exceeding G limits that you might have expected! As AC23.19 states: “VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits,”
Metal Fatigue
This video is a good primer on the subject Airplane Design Tutorial 21 Structural Fatigue.
Different manufacturers treat the airframe fatigue life in different ways.
Consider the Zlin 242. An aluminium semi-monocoque structure typical of light aircraft. Aluminium aircraft will suffer from fatigue and it is just a matter of when.
Check out the detailed report at Service Letter No L120.