VTVC is it calculated properly

[Fokapilot]

Hi Gents,
I'm busy analysing a model of the Polish homebuilt aircraft from the early 70-ies, J1 Don Quixote. It has some known stability problems.

When checking the VTVC, Flow5 calculates a value which is very different from my hand calculations. So, I made a dummy rectangular tail of the same area and positioned at the same Aerodynamic Centre. And the VTVC as calculated by Flow5 equals to my hand calculations.

The Flow5 calculates the Mean Aerodynamic Chord as per program below, but it doesn't show its position.
I calculated the 1/4 distance from the wing MC to the 1/4 VT MC as 593.5mm for both cases.

Is there a bug or am I doing anything wrong?

The input file is attached as well.

VTVC test.zip

[Flying-Geek]

@techwinder

I believe this is a bug caused by how the moment arm for the tail volume coefficient is calculated here and here.
The 25% point of the root chord is used instead of the 25% point of the surface's mean aerodynamic chords. It applies to both the vertical and horizontal tail.

Stefan

[Fokapilot]

Thank you for the quick answer.
Is this values being used anywhere in the calculations or is just displayed for general info?

[Flying-Geek]

I'll need to check that tomorrow in a free minute if this is actually correct, so take my answer with some salt:
But I think it's only displayed as general info here. I don't see where the tail volume coefficient could be used elsewhere, as it is generally only useful as an estimation and a good first guess for tail sizing, since it's only statistics at the end of the day.

With that in mind: If the vertical tail volume coefficient of the J1 is only about 0.029, lateral stability problems wouldn't surprise me with that tall, narrow front fuselage section counteracting the tail. Guidevalues from Raymer are 0.02 for gliders, 0.04 for homebuilt or general aviation single engine etc. and 0.06 for seaplanes ("flying boats"). So even with a more "normal" forward fuselage that would be on the lower end, but the tail it is of course also in the propeller slipstream, which one would need to account for.

Stefan

[Fokapilot]

If any of the Tail Volume Coefficients are not being used, what about the wing to tail distance: is this being used correctly in the program?

Yes, of course it is a bit low and this why I'm analysing it. However, the plane was designed in the late 60-ies when many of the data was not available. He also built this plane in his flat, on the second floor! Longer fuselage wouldn't fit into his flat.

Where is this Raymer data from? Could you share the source please.

Cheers
Andrew

[Flying-Geek]

In other calculations the point of application of the forces is calculated in a more sophisticated way. The moment arm for the tail volume coefficient is simply calculated on the fly in the function calculating the tail volume coefficients.

The design is certainly still impressive, and there were no catastrophic longitudinal stability problems from what I could find in a quick search.

I have Raymers aircraft design book at hand. I believe you might be able to find a scan of the tail volume coefficient table online if you search for it. Checking other literature (Gudmundsson, Torenbeek etc.) might also be a good idea, as some of the numbers vary, Raymer is simply the one I had next to my desk right now.

Stefan

[Fokapilot]

Thanks.
I actually have his book "Simplified Aircraft Design for Homebuilders" and Gudmundsson as well. Thank you for reminding me.

[Flying-Geek]

I only have the more extensive version "Aircraft Design: A Conceptual Approach", but I would be surprised if the values for the tail volume coefficient were any different or omitted.

[Flying-Geek]

Oh, and keep in mind that Gudmundsson refers to the mean aerodynamic chord as the mean geometric chord for some reason, opposed to pretty much all other literature.

[Fokapilot]

Ok I will try to get this book as well. Well, I'm aware that even 0.029 is a bit small just need to find a better one.

Would you consider please updating the calculations of both Coefficients in the next version. It is a bit painful to calculate them by hand :).