Pitch attitude in cruise

[jan-hudec]

The aircraft (at least the base wheels variant) cruises with significant nose-down attitude. Even in climb at 120 knots it already flies with alpha = -2.50 and at a 140 knot cruise it flies with alpha = -3.54 (/orientation/alpha-deg is body-relative). That seems rather wrong; fuselage produces negative lift if flying at negative alpha and this just reduces efficiency, so aircraft are normally designed so they never fly at negative alpha (which means zero alpha when light, slightly positive when heavy).

I believe this is wrong and is caused by excessive airplane/wing@camber. The value means to YASim the percentage of maximum lift that is developed at zero angle of attack (relative to the camber line). If I am calculating it correctly, stall@aoa = 10 with @camber = 0.24 shift the zero-lift line by 3.16° and together with @incidence = 2.5 to total of 5.66°. That just seems excessive. After all, looking at the model the wing does not seem to be cambered much.

@bolan, would it be OK to just reduce that value or do you have a implying that the Twin Otter does in fact fly in such unusual nose-down attitude?

[Deleted user]

Yes, this aircraft has a negative AOA at zero lift. The Twin Otter uses a modified NACA 63A516 airfoil. This is a very high camber, high-lift airfoil.

The aircraft has an AOA of about -6 degree at zero lift. In a typical cruise speed and about 10000 ft, it is about -3 degree of nose down. Rocky Mountain Airways' tails and some tails in Nepal Himalayan areas fly above 20000 ft with about 0 degree AOA. If it has a positive AOA and small chamber wings, it can't do steep approach and can't land in TFFJ, Lukla and Everest Base safe.

I also have some notes when I modified the FDM. Some notes are from an expert who doesn't allow me to quote his name, but I can show more to you when I have a good internet. My internet works bad today, sorry about that.

Bo

On January 5, 2016 12:55:21 PM EST, Jan Hudec notifications@github.com wrote:

The aircraft (at least the base wheels variant) cruises with
significant nose-down attitude Even in climb at 120 knots it already
flies with alpha = -250 and at a 140 knot cruise it flies with alpha =
-354 (/orientation/alpha-deg is body-relative) That seems rather
wrong; fuselage produces negative lift if flying at negative alpha and
this just reduces efficiency, so aircraft are normally designed so they
never fly at negative alpha (which means zero alpha when light,
slightly positive when heavy)

I believe this is wrong and is caused by excessive
airplane/wing@camber The value means to YASim the percentage of
maximum lift that is developed at zero angle of attack (relative to the
camber line) If I am calculating it correctly, stall@aoa = 10 with
@camber = 024 shift the zero-lift line by 316° and together with
@incidence = 25 to total of 566° That just seems excessive After all,
looking at the model the wing does not seem to be cambered much

@bolan, would it be OK to just reduce that value or do you have a
implying that the Twin Otter does in fact fly in such unusual nose-down
attitude?

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[jan-hudec]

Thank you very much.