IO-360 -> Maximum safe HP?

[cjensen]

Here's a good definition of boundary layer-

Boundary Layer

Boundry layer.
The boundary layer is a very thin layer of air flowing over the surface of an aircraft wing, or airfoil, (as well as other surfaces of the aircraft). The molecules directly touching the surface of the wing are virtually motionless. Each layer of molecules within the boundary layer moves faster than the layer that is closer to the surface of the wing. At the top of the boundary layer, the molecules move at the same speed as the molecules outside the boundary layer. This speed is called the free-stream velocity. The actual speed at which the molecules move depends upon the shape of the wing, the viscosity, or stickiness, of the air, and its compressibility (how much it can be compacted).

Further, boundary layers may be either laminar (layered), or turbulent (disordered). As the boundary layer moves toward the center of the wing, it begins to lose speed due to skin friction drag. At its transition point, the boundary layer changes from laminar, where the velocity changes uniformly as one moves away from the object's surface, to turbulent, where the velocity is characterized by unsteady (changing with time) swirling flows inside the boundary layer.

The flow outside of the boundary layer reacts to the shape of the edge of the boundary layer just as it would to the physical surface of an object. So the boundary layer gives any object an "effective" shape that is usually slightly different from the physical shape. The boundary layer may also lift off or separate from the body, creating an effective shape much different from the physical shape of the object and causing a dramatic decrease in lift and increase in drag. When this happens, the airfoil has stalled.

As well as the development of airfoil stall, the details of the flow within the boundary layer are very important for many problems in aerodynamics, including the skin friction drag of an object and the heat transfer that occurs in high-speed flight.

Spike is right about airstream. It's all the relative air moving past the airframe.

[4kilo]

I normally try not to get into major technical discussions on these boards. However, the number of misconceptions, oversimplifications and outright misinformation being presented in this thread are driving me nuts!

First off, because I was trained as an aerospace engineer, some of the terminology used here really bothers me. Low speed flows (less than Mach .5 or so), are INCOMPRESSIBLE. If you are trying to think things through by imagining the air being 'compressed' by the wing or pitot tube of your RV, you are messing yourself up. Pressures and speeds may change, but a free air flow at low Mach numbers is INCOMPRESSIBLE.

Airframe airspeed limitations are driven by many factors, some of which are more important for some types of aircraft than others. For instance, Mach limitations, very important for high flying jets, are pretty much insignificant for RV's. For certificated aircraft, a maximum dive speed (Vd) is determined during the certification process. This speed must protect the airframe from structural damage in all conditions, so for some aircraft the actual numbers may vary based on altitude or other factors. A safety factor is then applied to arrive at either; Vne for reciprocating airplanes (90% of Vd), or Vmo for turbine airplanes (80% of Vd). For experimental aircraft there is not the same level of regulatory guidance for Vne, so we must come up with our own maximum speeds, with the major concerns for RV class airplanes being dynamic pressure and aero-elasticity (flutter).

Dynamic pressure is really pretty straight forward. Remember that your airspeed indicator is nothing more than a calibrated dynamic pressure gage (it measures the difference between pitot pressure and static pressure, which is dynamic pressure, and instead of presenting this pressure in lbs per square inch, just reads in knots or miles per hour). The amount of force which can be generated by different parts of the airframe is proportional to the dynamic pressure (remember Bernouli's equation?), so by limiting the dynamic pressure, you are limiting the amount of force which can be generated, weather by pulling G's, penetrating gusts, or whatever. If you keep the indicated airspeed below the limit, then you cannot exceed the dynamic pressure limits for the airframe, no matter what your true airspeed. (One of CJ's more colorful earlier explanations was actually quite accurate).

Flutter, on the other hand, is much more complicated. The onset and effect of flutter will change depending on flexibility of the airframe, balance of the surfaces being considered, natural resonant frequencies of the structure, angle of attack, dynamic pressure, and yes, true airspeed. It IS NOT solely dependant on true airspeed. If you need to prove this to yourself, imagine an RV operating at an altitude where 300 knots true airspeed results in an indicated airspeed of 1 knot. That dynamic pressure would not cause your eyelashes to flutter, much less any part of an RV (disregarding for the purpose of this thought experiment the fact that it also would not provide enough lift to fly). The airplane I fly at work has a Vmo of 330 kts at sea level, but at cruise altitudes we regularly fly at more than 500 kts true.

So if flutter onset is determined by both dynamic pressure (indicated airspeed) and true airspeed, how can we determine where it will start? This is actually quite difficult. For most testing of experimental aircraft, we just use the old expedient of empirical testing (trying things out to see how they work). Unfortunately, some flutter modes are extremely destructive. In fact, the time from the onset of flutter to complete disintegration of an aircraft can often be measured in tenths of a second. This is the reason that Van's no longer advocates the "slap the stick" method of flutter testing. Since more complex methods of flutter testing are usually beyond the means of the RV community, how are we going to set limits?

Van has taken the simple expedient of stating that the maximum speed should be limited by true airspeed. Although this is obviously incorrect, it is true that it is incorrect on the side of being too conservative. That means that from the standpoint of liability, Van has made any mistake on the side of being too safe. I am sure that makes his lawyers happy, but it doesn't help us out much.

I have to guess that flutter on an RV will probably not be a consideration until a fairly high altitude is reached. If you set the true airspeed reached at Vne (indicated) at this altitude as your limit, you should be quite safe. The problem is that I have no way of knowing what this altitude is. Van may know, or he may not, but since he says true airspeed for Vne from sea level up, any faster than that and you are REALLY test flying.

Pat

[Spike]

I normally try not to get into major technical discussions on these boards. However, the number of misconceptions, oversimplifications and outright misinformation being presented in this thread are driving me nuts!

Thanx Pat!! I was hoping something like this would happen to set us straight!

[cjensen]

Thanks for the write up Pat! Where were you three days ago!?

[jim_geo]

Very good read thanks.

[captain_john]

Now, that is deep! If I were to say that I understood it on this first read, I would be lying!

Pat, thanks for chiming in! We armchair en gah-nears obviously don't have a clue, but are willing to learn.

Soooo, if I do 231 Mph, I should live to talk about it? It isn't gonna come apart at the seams on me?

CJ

[Dan A]

CJ , I've had mine up to about 253 mph and it was solid as a rock. It wasn't intentional but the asi was reading about 220 kt. ( I really shouldn't be making diving low passes without retarding the throttle! )
Dan