Tip weight calculation

[Wolfgang Nieuwkamp]

When completing my new plane, I wondered how to calculate the amount of lea for the tip weight box. The calculations became interesting, and i did write them down in the attached document (I could not place the pages into this post, maybe somebody can advise?)

The least amount is obtained with about 5 cm difference between in- and outboard wing (like in the original Nobler?)

Comments will be very appreciated.

Regards,

Wolfgang

[phil c]

Since you aren't doing anything far out with the design just put in 1 oz.  It is likely to be a bit too much, but that is what trimming flights are for.  If it drops the outboard tip on both inside and outside loops it's too much.  Adjust as needed

Phil c

[Peter Germann]

Comments will be very appreciated.
Wolfgang

Much to my surprise, Wolfgang's, post so far remains without response from the ranks of our (real) rocket scientists ot there.You may want to have a look not only at his quite unusual airplane (to be equipped with an automatic variable pitch prop), but at his attached Word doc. describing wingtip weight calculations, too...

Peter G

[Serge_Krauss]

I'm impressed. I'm very busy now, but I do intend to study the derivations. I'm also wondering what will activate the prop pitch change. Thanks much for sharing your efforts!

[Howard Rush]

Cool calculation.  I didn't see any real-wing adjustment to lift distribution, not that I haven't left it out myself.  It's not proportional to chord: it falls off at the tip.  The amount of tip weight needed to balance the lines also varies with flight condition.  I put some calculations on SSW, I think, awhile back.  However, I just put some weight on that looks about right and go fly.  I adjust it by trial and error afterward.

How does the plane fly?  It looks very interesting.  Does it oscillate in yaw in tight turns? 

Variable pitch is wonderful.  Can you control it with an Igor timer while holding RPM constant?

[Howard Rush]

Here's something on lift distribution: http://naca.central.cranfield.ac.uk/reports/1937/naca-report-572.pdf .  It looks difficult to use.  I'll look for something more recent.

[Howard Rush]

Much better is Schrenk's method: section 1.2 of http://www.ingaero.uniroma1.it/attachments/461_Lect12.pdf .  Or you could Google it to get it in the original German. I had seen this before, but forgot where.

I think all these sources divide lift distribution between "basic" and "additional" .  Basic is the lift distribution due to twist, so all we care about for stunt is the "additional" lift distribution.

[Bob Reeves]

Haven't looked at the doc file yet... A tip weight calculator is something I really wanted to add to LineIII. I posted a thread asking for help figuring out how to calculate tip weight and was hit with a horde of people telling me it couldn't be done. If this looks like something that will just give one a starting point I would like to add it to the LineIII calculations.

[Wolfgang Nieuwkamp]

Hello Howard,

in my actual Excel sheet I did the calculations in the analytical way. I will switch it to numerical, so I can include corrections for the Aspect Ratio, as proposed in Schrenk. Thanks for the link!

Regarding the variable pitch control, I use my own timer to keep the rpm constant (Controline mode CC Phoenix edge lite 50 A), and to adjust the pitch so that the line tension stays constant overhead and level. Flights are planned for the coming weekend.

Best regards,

Wolfgang

[Tim Wescott]

Regarding the variable pitch control, I use my own timer to keep the rpm constant (Controline mode CC Phoenix edge lite 50 A), and to adjust the pitch so that the line tension stays constant overhead and level. Flights are planned for the coming weekend.

Do you measure line tension, or deduce it from outward acceleration ala Igor?

Is it a commercial adjustable-pitch prop, or is it something you cooked up yourself?

It seems that, if there's a commercial adjustable-pitch prop out there, one could use some variation on the Igor timer (possibly with some sort of a universal timer* mediating things, if such a thing existed) to control the pitch while the motor ran at a constant speed.

* Like this one, if only you could find the guy who makes it: http://stunthanger.com/smf/index.php/board,98.0.html.

[Wolfgang Nieuwkamp]

Tim,

Correct, I deduce line tension by the outward acceleration.

The mechanics of the variable pitch prop, including the spinner, is all homemade. Five ball bearings….

Regards,

Wolfgang

[Igor Burger]

Many of us tried to make such calculators. None of the are perfect, but does not matter, the value of such calculator is not its result and function, but understanding why it is not doable.

... by other word understanding what is going on there :- )))))))))))))))))

[Wolfgang Nieuwkamp]

Much better is Schrenk's method: section 1.2 of http://www.ingaero.uniroma1.it/attachments/461_Lect12.pdf .  Or you could Google it to get it in the original German. I had seen this before, but forgot where./quote]

When correcting for limited aspect ratio, the calculated tip forces are slightly lower. Please look at the modified doc. attachment

[Wolfgang Nieuwkamp]

Here is the Excel sheet for trapezoidal wings (attached)
The green fields can be changed to play around.

Any questions?

Wolfgang