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What is neutral pitch?

Carl Froehlich

Well Known Member
Today I did an experiment.

The new RV-8 has 80 hours on it, and I have had opportunity to do all the tweaks. Part of this included setting the SkyView ADHARS so the HSI is on the horizon for level flight, and this setting has been confirmed over several long cross country runs. This was done solo with 25 pounds of balllast in the rear baggage compartment.

Today I jacked up the tail to compare HSI in level fight to the canopy rails (what i always assumed as “baseline level”). The result was with the plane, as measured on the canopy rails, is nose up by 1.4 degrees in level flight.

So my questions to all you AEs out there:
- Is this what others have found - as in typical for RV-8s?
- Is this indication the thrust line is not quite high enough?
- Anyone experiment with changing the engine thrust angle?

Carl
 
Section 15

Section 15. Page 13
Hi Warren, I'm not sure which version of Section 15 you have, but mine does not seem to have anything related to this question, that I can see. Can you give me a pointer to what you mean? Mine has the 4 diagrams of how CG affects pitch stability.

Carl - what does your elevator show? Do you see some "pitch up" in order to maintain level flight?

I'm at a similar place in my flight testing, but struggling to get some smooth air recently, even in the morning.

BTW, every time I re-read Section 15 I pick up something new - it's packed with info.
 
Some details to respond:

There is no stability issue. Not my question.

If anything during solo cruise the elevators are slight nose down (as in trailing edges low). The W&B shows that the plane is not tail heavy, so I assume the horizontal stabilizer (installed per the plans) is providing too much downward force. I had the same on my first RV (8A) and added a shim under the HS forward spar to increase the angle if incidence. It did put the elevators more in trail and I still had just enough pitch trim for solo landing. I did not however measure any gains in speed or cruise efficiency, so I decided not to do that with this plane.

My question is my measured neutral angle is 1.4 degrees nose up as compared to the longerons. This does not seem like much until you see the plane set at this angle. Is my observation the norm or not? It it is the norm than I assume the Van's instructions for setting neutral angle using the longerons is strictly associated with setting wing angle of incidence, and not that it is also neutral for in flight pitch angle.

Carl
 
Pitch Angle

For what it's worth, I always set my AHRS to zero degrees with reference to the canopy rail.

Zero pitch is zero pitch. Flying often requires positive pitch for aerobatic aircraft wings and is normal. The pitch angle during flight will change depending on airspeed and center of gravity.

I want my AHRS to read true deck angle, not some adjusted "level flight" picture.
 
Canopy rails are not necessarily level when the aircraft is in level flight. They might be, but I wouldn't assume that without checking it. It doesn't take much change in wing angle of incidence to change how the fuselage sits in level flight. Every airplane is different.

Some EFISs have a setup function where you could tweak the lubber line up or down to show level when the airplane is not climbing or descending.

Ed Holyoke
 
The OP was asking about the pitch angle of his airplane in level flight wasn't he? I am confused as to why people are responding regarding elevator angle, which is irrelevant, as is thrust line.

In order for the airplane to maintain level flight, I.e. lift =weight, the airplane has to maintain a given AOA at a given speed. This is the most basic element of flight dynamics. Nothing to do with the thrust line or elevator deflection. Go faster in level flight and you will be more nose down. Go slower and you will be more nose up. You will only be at 0 at one specific speed.

Don't confuse the AHRS 0 with flight path. AHRS shows the angle of some reference on the airplane with respect to the earth. Like the other poster suggested do a decel from cruise down to say approach speed, carefully maintaining altitude and the AHRS pitch indication will follow the pitch axis of the airplane, not the flight path. The fancier eris units can calculate the flight path and display it, but in this case we are talking about pitch attitude.

Forget the thrust line, elevator deflection and even CG. The CG has a small effect on the AOA in level flight but it is secondary.

The difference between stability and trim is one of the most universally poorly taught and poorly understood aspects of aerodynamics. What is taught to pilots in most ground school material is at best confusing and at worst dead wrong. I don't blame anyone for being confused.

I will bet anyone here a dozen donuts (brave man) that this 8 is within half a degree of pitch at a given speed of any other 8 in the world and any differences are far more likely to be due to airspeed measurement errors. The lift vs aoa relationship for a given wing design is pretty much bullet proof. Unless you have a massive debt in the wing or you installed it upside down, I expect you are fine.
 
It is quite common for aircraft to not have their "deck angle" at zero during cruise. For constant altitide, wings-level flight, that angle will be greater at lower speeds (for equal weight) and higher weights (for equal speeds). Remember the lift equation:
Lift = (1/2) x (density) x (velocity)² x (wing reference area) x (lift coefficient)

In normal flight regimes, lift coefficient is essentially directly proportional to angle of attack.

Even akrliners and transports will often have some small nose-up attitude relative to fuselage level.

Of course, then there is the B-52, which will have a pronounced nose-down deck angle at higher speeds and lower altitudes. It makes for interesting pictures.
 
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