Will there be a noticeable difference in flight performance from glider wing ribs cut from a template and balsa knife (as accurate as hand and eye can be) or those cut using a CNC machine.
I'm assuming a slight error will exist but considering it's for an 8" chord 100" span thermal glider, will it greatly affect its flight performance ?

No discernable difference, in my opinion. A far larger performance loss can be caused by trimming errors, ie: incorrect CG, incidence errors, warped flying surfaces, etc.

I cut my ribs with a homemade CNC router and cannot tell any difference between CNC cut and hand cut, other than the fit!

DavidO,

It might depend also on what airfoil you are using. Some airfoils need to be built extremely accurately in order to get the performance they are designed for. If you aren't using one of these, and you build carefully, I would guess you would be fine.

banktoturn

Many thanks for both your replies.

I'll be using the AG35 and AG36 from the Mark Drela stable. I'm new to the world of airfoils which appears to be a minefield and looking at the accuracy of computer plotted airfoils thought this accuracy would have be maintained throughout finishing with CNC cutting.
I think I will just take my time in hand making accurate templates.
Looks like balsa bashing isn't a thing of the past after all !

Dickeybird - I've tried to reproduce a picture here of my brand new Cox 051 number 200 purchased December 1974. I normally place it over our fireplace which I think is an equivalent work of art compared to my wife's ceramic figures - its a bone of contention but there it stays !!
DavidO.

Hi David,

Ahh, vintage Cox engines, my favorites!

Dr. Drela seems to have a handle on glider stuff for sure. Is your wing D-tubed with film covering or fully sheeted? Seems like some of his airfoils take the covering sag characteristics into consideration.

Martin Hepperle's site talks about built up wing airfoils as well.
http://www.mh-aerotools.de/airfoils/index.htm

ps: Martin's site also has the best online Cox museum around too!

I assume that you'll be using the full sheeting back to where he recomends?

A quick check showed me that he has a DXF file available at....

http://www.charlesriverrc.org/articl...egrolite2m.htm

Load that up into some sort of CAD and scale it to your needs. Print it out and glue the paper to the plywood. Trim sand to split the line and make two. Cut the balsa ribs using a template and knife and then stack all the blanks between the templates. Sand the excess to match the ply templates and cut the spar slots while all the ribs are in the stack so the slots all line up. Trim carefully for a nice spar fit.

If it's tapers then make the templates and stack a whole lotta rectangles between the ply 'plates using long #8 bolts again to hold it all together. Carve and sand to form a block of ribs. Do two sets, one for each wing. For sharply tapered panels you'll want to sand the bevels off slightly afterwards.

Another trick, sand the wing sheeting before you put it on. That prevents the balsa sagging between the ribs under the pressure. With care you can keep any errors to less than 1%.

Don't forget the leading edge template for that part of the job. It's probably the most important task. I think Marc has some flat segment carving tricks to rough out a faceted shape that pretty much guarantees a decent leading edge shape once sanded smooth.

Brian,

Absolutely right. Charlesriverrc have pages of .pdf files on all of the techniques that are used in his gliders.

Best of all is to join the allegro-lite interest group on yahoo. Access is linked through charlesriverrc. The discussion on the post board gets pretty specific but the files and photos on the main site are first rate. And take a look through all of the reading matter they have - there is a wealth of ideas and techniques...

It would have to be one of the best glider interest sites I have found...

Its.....BRUCE ... but at least you aren't calling me "late for dinner" to quote the old song

But you also bring up a good point. Choosing Marc's airfoil pretty much puts you into the Allegro Lite family and bring's you a LOT of support along with entry into the discussion group.

I'm going to build a new glider of my own soon and it's also going to use the AG35 and 36 airfoils.

(imagine this said in very small voice) oh dear, sorry...I will try and remember.

Dickeybird - I'll go for the 'D' box and filmcovering - love seeing an open wing structure overhead. Had thought about using a geodetic structure a'la free flight days. This would reduce covering sag considerably and give greater rigidity, maybe with a weight penalty. MH-Aerotools is a fantastic site, should be compulsory reading for all modelers. I'm sure I could smell Nitro while reading it !

Bruce - Charles River R/C. Spent all day Wednesday printing and then doing a glue and stick job to produce the Mark Drela creation. Interesting to read how the design on some of the AG airfoils has taken into consideration covering sag. All those tips and guidance notes I shall follow to the letter. Good tip on sanding the sheeting before applying ! Leading edge sharpness....hmmm....I've seen a thread about this very topic - will have to read and inwardly digest that one.

Probligo - Have joined the allegro - lite interest group, should be an education in reading about other modelers experiences with this model.

Many thanks for the guidance in your replies.
DavidO

I may have misexplained this one. Marc describes methods to rough shape the leading edge using accuratley formed tangental facets to set up guides for the final smoothing from sanding that takes off the peaks without altering the desired smooth and properley radiused shape that is wanted to form the nose of the airfoil. It's worth the trouble as the first 1/4 of any airfoil is the most critical towards ensuring that the air flows as desired. Not that the other 3/4's isn't important but that first bit sets up the airflow for the rest. If it isn't right then you have no hope at all for the designed performace.