# XFLR5 Question: Zero Moment angle

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#### Aerowerx

##### Well-Known Member
A question for everyone familiar with XFLR5...

What I typically do is create a design and adjust the location of the wing and/or masses to get a moment of 0.0 at an AoA of 0.0 degrees with a certain airspeed. This would be using a static analysis.

After this I do a dynamic stability analysis.

My question: Why does the speed and AoA turn out different for the dynamic stability? I would think that if the plane is statically "trimmed" at a certain air speed, then the dynamic stability should have the same trim. That is, after the oscillations die out the AoA and airspeed should be the same as in the static analysis. But what it does is end up with a "speed to balance weight" and "zero moment AoA" that is sometimes quite different.

Sometimes the zero moment AoA ends up so far off that there is a negative lift, at which the analysis gives up.

Anyone else see this? What am I doing wrong?

#### StarJar

##### Well-Known Member
I don't, or I would help.
My only question, and probably a really dumb one is; Why do you start with 0.0 aoa. In aero terms, of coarse, it has little to do with '0', as in a starting point. It might as well be 3 or 2.43 or any 'starting point'.
I'm very foggy on xflr5 stability analysis. When I get frustrated I switch to xplane, although it's probably technically less accurate.

#### Aerowerx

##### Well-Known Member
It's not 0.0 AoA. It is the angle of the entire structure to the free stream air flow. In other words, the pitch angle of the aircraft.

OOOPS!:emb: Just looked at my OP. I did say AoA didn't I? That was a typo. Should say "pitch angle"

#### arj1

##### Well-Known Member
Aerowerx, Could that be because the Zero Moment AoA is at -xx degrees and in the negative CL range? Have you tried different airfoils with the analysis (cambered and symmetric)? Alex

#### Aerowerx

##### Well-Known Member
Aerowerx, Could that be because the Zero Moment AoA is at -xx degrees and in the negative CL range? Have you tried different airfoils with the analysis (cambered and symmetric)? Alex

The analysis stops if it gets a negative CL, so no. And see the correction in post #4.

#### Aerowerx

##### Well-Known Member
I've put some more thought into my problem, and think I am looking at it from the wrong angle (yes, that's a pun).

The dynamic analysis calculates the required airspeed for the wing to support the total weight, simultaneously looking for the AoA that creates zero pitching moment.

What I have to do, then, is find the CL for the total weight at my desired cruise speed, then adjust the wing to get that value in level flight, then adjust for zero moment in the static analysis.

The dynamic analysis should then come up with the same speed.

#### StarJar

##### Well-Known Member
Sounds good. I might try xflr5 again, lol.

#### Aerowerx

##### Well-Known Member
Problem solved!

This is what I did...

My target cruise speed is 100 mph. The wing is 120 square feet and gross weight is 800 pounds. At 100 mph this needs a CL of about 0.2.

First, since my wing has a tremendous amount of twist (based on the PRANDTL-D), I scaled down the twist by 50%. Then in a static analysis I adjusted the wing incidence to get the required CL.

When I then did a dynamic analysis the model trimmed out at 100 mph. Adding 10 degrees of self-trimming flaps dropped the speed down to 83 mph with no change in pitching moment. CL had jumped from 0.2 to 0.378.

With flaps up, the coefficient of drag is 0.007! With flaps down it is still a respectable 0.014. But I am skeptical of this since I read somewhere that XFLR5 is not too accurate when it comes to predicting drag, not to mention the added drag of landing gear, engine cooling, etc. This is encouraging, though, since if I actually build this thing I would like to use a 35 hp half VW engine.

##### Well-Known Member
Supporting Member
It's been a long time since I have used those analysis functions in XFLR5, but that sounds about right. That generally follows the procedure one would use to set the wing and tail incidence for cruise flight in a conventional configuration.

I am curious about the big reduction in twist from the theoretical baseline. Can you please elaborate a little there?

#### Aerowerx

##### Well-Known Member
I am curious about the big reduction in twist from the theoretical baseline. Can you please elaborate a little there?
It is actually not a "theoretical baseline". I am using the PRANDTL-D twist as a starting point, and have found that the important thing is the general shape of the twist distribution.

The PRANDTL-D has a total twist of over 10 degrees and is a nonlinear twist. So all the airfoil sections are operating at different CL and CD values. By keeping the same twist distribution but changing the absolute value you can operate at a lower CL (that is higher cruise speed) and get a lower CD (which means less horsepower required for that cruise speed).

I am sure there is a limit to this, though. And you have to be careful about the twist at the tip, which I modified from the scaled value---you want to keep the proverse yaw.