Sagitta airfoil suggestions, opinions

[scififlyer]

I have plans for a Sagitta 600 (2M) and would like some learned opinions on what airfoils to use with a poly wing in RES competition.

First thing to consider is I have a Mac computer without a Windows emulator so using a wing analysis program isn't an option for me.

The Drela sections are attractive, but I can't find evidence of their use in a heavier-than-a-feather 2M. There are some suggested sections in Model Aviation however.

Some F3B type sections maybe? There is a modified RG14-S7003 transitional wing suggestion, along with some sexy-looking Quabeck laminar sections that might do well without flaps/camber changing in a 2M.

Of course, should I even bother with a transitional wing in a 2M poly? It's no Allegro-lite, the Sagitta. Maybe a 7037 or the stock 3021 is as good as the design can use?

Your thoughts matter. I'm rather ambitious about getting a great Sagitta 600 up in the air.

[ICTHRMLS]

As a purist and former Sagitta 900 owner I can attest to the original airfoil being really sweet for a wide range of thermal activities (eppler 205, I think). I might be missing the gist of your thread but my question to you would be why mess with a good thing?

Sign me,

Stuck In The Past

[Ollie]

Any of the airfoils that you have mentioned will not be reproduced accurately with the Sagitta wing construction because of covering sag. The E205 is very well suited to the Sagita planform aspect ratio and twist (none). To do much better with another airfoil you have to redesign the whole wing in almost every respect. For example, a thinner airfoil will result in a weaker wing that bends more on launch unless the wing spar is made stiffer and stronger. Also, a thinner airfoil with thicker spar caps won't have enough room for the joiner tube.

Stick with the stock Sagitta or build an Allegro Lite. The Allegro is not limited to a light wing loading. There is ample provision for ballast. Its wing is strong enough to take a full pedal zoom launch, gaining at least 100 feet more altitude than the Sagitta. That launch height will have far more benefit than any airfoil selection for a Sagitta. The Allegro wing design has the airfoil design integrated with the aspect ratio, planform, twist, etc. for a far more efficient wing, both aerodynamically and structurally.

Because the Allegro has substantially less parasitic and profile drag than a Sagitta, It will penetrate far better than its light wing loading would lead you to believe. The low wing loading will allow you to work much smaller, weaker thermals than the Sagitta.

The main advantages of the Sagitta are that the material cost and labor to build it will be several times less than an Allegro.

[scififlyer]

Originally posted by Ollie
Any of the airfoils that you have mentioned will not be reproduced accurately with the Sagitta wing construction because of covering sag. The E205 is very well suited to the Sagita planform aspect ratio and twist (none).

So the covering sag isn't as big a deal with the Eppler 205, compared to the others? Also how can a E205 section be built any more accurately than the others, or is it more tolerant of errata in construction?

My goal is to expand the possibilities of the Sagitta without going to a foam-and-glass wing, while keeping it RES. When I want something more capable, I have rib and shear web sets for the Allegro Lite. Right now the Sagitta is going to be my next glider. Might as well get as much out of it's planform that's there.

[ICTHRMLS]

With the "D" tube construction of the wing sheeting I don't recall sag being a big issue. To expand the possibilities I recommend good ol' carbon fiber and lots of it. Sandwich it between the longerons and the fuse sides. Cap the top & bottom of the spar before sheeting the wings. The idea is to get a plane that can darn near launch full out oon the winch and stand up to the occasional dork landing. I still can't believe I sold my 900..... Who knew there'd be this thing called RES????????

[Ollie]

ICThermals gives good advise. A stronger wing that will take a hard zoom launch will give you more benefit in duration from the extra altitude than any airfoil selection.

I should have started a new paragraph when I said the E205 is well suited to the Sagitta. The opinion I was trying to get across is that with the Sagitta structural design and planform, trying other arfoils won't get you much and may even cause some slight degradation. If you want to try to prove my opinion wrong, please go right ahead. I've been wrong more than a few times in my life.

See:
http://my.athenet.net/~atkron95/pcsoar.htm
Download a copy of the PC soar program. Put the Saggita 600 in and try it with various airfoils. Then use the one you think best.

[ICTHRMLS]

Ollie - did you ever see an aeirleon (sp?) flattened wing version of the Sagitta fly worth a darn?? I never did. They always looked very strange too.

[Ollie]

As long as you are not doing aerobatics where yaw to roll coupling is a disadvantage, generous dihedral or, the equivalent polyhedral, has much to recommend it. It greatly reduces the pilot work load. It makes the plane much more stable and predictable in thermal turns. This makes the plane safer to fly near the limits of vision. Maybe that's why almost every successful cross country ship has generous dihedral, ailerons or not.

We can learn something from watching turkey vultures work a small low level, turbulent thermal. They greatly increase their dihedral under these flying conditions. This observation lead Don Stackhouse to investigate the question. His conclusion was that dihedral is actually slightly more efficient in steeply banked turning than a flat winged aileron arrangement.

[BMatthews]

Back when Soartech 8 came out the Saggita was one of the top models and the Eppler 205 was the airfoil of the day for many designs other than the Saggita. Selig had a fair amount to say about the use of D tube construction with this airfoil. I don't have it all at hand but the gist was that there was much to be gained by extending the upper sheeting of the D back to about the 60% chord point to better support the covering around the critical high point and sharper curved area immedialtley behind the high point.

This same philosophy is apparent in Drela's Allegro Lite airfoil with it's extended upper sheeting.

I had a Top Flite Metric with the Saggita type D tubed 205 and it had a number of low speed issues that I remedied with turbulators. But it would be nice to know just how much better it could have been if the upper sheet had extended back further.

As for updating it I'd have no qualms about using Drela's AG 34(?) off the Allegro Lite. It's very close to a 205 in any event and takes advantage of a few years extra evolution. With any sort of care at all you should be able to build that 2M Saggita down to the mid 30 oz range which should put your model well with the wing loading range between the very light Allegro Lite and the electric version of the same model.

[markdrela]

Originally posted by BMatthews
I had a Top Flite Metric with the Saggita type D tubed 205 and it had a number of low speed issues that I remedied with turbulators. But it would be nice to know just how much better it could have been if the upper sheet had extended back further.


The need for turbulators is not surprising. The E205 is more at home on a ballasted XC ship, or even a human-carrying microlight. On a Sagitta-size glider, it has a rather severe separation bubble thoughout its speed range. The bubble is worst at the low speed end, and will likely be further aggravated by covering sag.

Xfoil predicts the AG34 or S3014 have about 10% improvement at the high speed end, and over 20% improvement at the low speed end. The AG35 is a few percent better still, but will have to be heavier because of the smaller thickness.

If you really want to make a new wing, then I'd go for the AG34 in the center, transitioning to the AG35,36 at the tip. Like Oliie sez, the structure will have to be reconsidered to have adequate stiffness and strength with the smaller thickness. You also want the top sheeting to 40-45% chord, and the rear support strip at 65% chord.

[Bernie Wolfard]

My limited memory seems to remind me that Dr Selig’s 7037 is a smoothed E205. It is a better all around airfoil but more critical in shaping the LE. In fact, I don’t think it can be hand sanded accurately enough. Dr Selig also said the E205 would benefit significantly from turbulation. I have an E205 winged 2.5 meter E-Glider with a bagged wing that was significantly improved by taping a line of 20 lb monofilament fishing line on the top of the wing at 20% cord back from the LE.

My 2 cents worth is it would be easier to built your Sagitta 900 with the stock wing, add a trubulator strip or two and go fly.

[BMatthews]

Mark, that bubble bit is just what I found and what led me to try the turbulators with great results. The model used to fly fast with no problem and had a glide slope that "almost matched the curve of the earth" (or so it seemed in those pre composite model days :D) at higher speeds but slow it down and you started wondering where the droque chute came from ! ! !

If I recall the Saggita wing used cap strips for the rear area behind the leading edge sheeting. If that IS the case then it would be super easy to just go for a wider upper sheeting and shorter cap strips. That way you also have a better surround for the spoilers as well. The extra weight of that 1/16 by 1/1/2 inches or so over the span would be negligable and the possible gains considerable.

[markdrela]

Originally posted by BMatthews
Mark, that bubble bit is just what I found and what led me to try the turbulators with great results.

If you will be just adding extra sheeting to an existing Sagitta wing, the turbulator makes sense. But if you're making a whole new wing from scratch, you will get better performance with an airfoil matched to the operating Re, so that it doesn't need band aids.

[scififlyer]

Thanks for all the input! Since I'm building the whole thing from plans, the AG34-35-36 with generous spoiler will be my first choice. Got Carbon? Yep.

[steelhead]

Anyone built the wing with foam?

Dean

[scififlyer]

Several examples of foam winged 900's (used to) exist, I've even seen an unbuilt foam wing set at a friend's house (along with a NIB 900 and 600, he has my number if he needs quick cash!) Strictly aftermarket, or do it yourself stuff. I haven't seen any fly since the early 80's, and I don't recall their flights since at that age I didn't know what to look for.

[Lance Prior]

The AG34 is 9.28% thick and the E205 is 10.48% thick.

You will need a stronger spar system.

Look at the spar in the thread "build Allegro-Lite" that should give you some ideas.

[fly1milehi]

Hey Dr. Sprocket

I'm not sure if I totally understand the actual differences between 9.28% and 10.48% thickness in airfoils. I thought that meant that the % is the ratio of thickest point vertically, versus the chord of the airfoil. So in the case of my current Sagitta 600 project the max vertical thickness is 25mm(measured on the plans) and 223.5mm chord (measured on the plans) so 223.5 x 10.48% should equal 25mm? (well that doesnt add up I only get 23.4mm for the thickness)
Anyhow.. doing the math between 10.48 and 9.28 I only get a difference in physical dimension of 23.2mm.... (25mm x .928 = 23.2mm) so would this be a way to compare the thicknesses of the two airfoils? or should I do the 223.5 x 9.28% = 20.7mm?
Okay so if the difference between the 10.48% and the 9.28% airfoil is only 1.8mm than I dont see a significantly smaller spar and shear webs to build. If the difference is the 25mm versus the 20.7 then I see a tougher build but not impossible with standard construction techniques and some carbon caps and such..

Somebody help me with the math? and then we will all understand this better
thanks
Greg

[Lance Prior]

I am pretty sure the depths are correct.

Here are some pictures with numbers from a program called Profili.

You can check my E205 thickness number by going to:

http://www.nasg.com/afdb/show-airfoil-e.phtml?id=113

[fly1milehi]

Wow! Very cool, thanks!

Two things jump out at me. One, my next sagitta model will not have an E205 airfoil on it. Two, I am really amazed that the AG34 is not significantly different (other than the thickness and overall camber) Don't know exactly what I was thinking that it would look like??? Visually it seems nearly like a thinned 205 or the E193 on my Larry Jolly Meteor.
I think with all the info I have been gathering on this thread that the next Sagitta will be with a Drela airfoil.

[Lance Prior]

All things being equal a 10.48% deep spar is roughly 45% stronger then a 9.28% deep spar.

Unless I am tired and fouled up the numbers.

Some one check me on this.

The AG 34, 35, 36 is a good choice but you need a strong spar system.

Do you live in Denver?

[Lance Prior]

If you want here are some free airfoil toys you can look at (and analyze) all kinds of airfoils with:

http://www.profili2.com

http://www.geocities.com/xflr5/xflr5.htm

http://www.dreesecode.com

http://www.mh-aerotools.de/airfoils/javafoil.htm

To get a whole bunch of airfoils (1839) go to:

http://www.jean-claude.etiemble.com/tracfoil/telechargementE.htm#pack

and download "Free Pack Airfoils"

He also has a neat program for plotting airfoils you can demo for free.

[lp3]

Unless I am tired and fouled up the numbers.

Dr. have you been drinking again?

[Lance Prior]

Indubitably

[ChuckA]

The original Satitta used a modified E205 airfoil. A friend of mine built one of the original kits in 1983. At the time I was writing my original Airfoil Plotting Progarm and I did a comparison with the true E205. The only modification was to increase the leading edge radius. I never saw an Airtronics kit that didn' at least increase the leading edge radius of the airfoil.

[tomcat5109]

Mark, you stated previously that using an AG34 in the center might be an idea in designing a new wing for the Sagitta. Question: On the Charles River website, there is a table describing the AG35-38 airfoils and the flat facets and percent of cord they cover. What are the flat facet percentages for the AG34? Thanks.

TL

[markdrela]

What are the flat facet percentages for the AG34? Thanks.
Same as for the AG35

[fly1milehi]

Just for fun I had one of my engineering buddies figure the strength difference between a spar of 25mm depth (approx. the depth of a Sagitta 600 with E205 airfoil) and a 22mm depth (approx. the depth of a Sagitta 600 with the AG34 airfoil) Assuming a chord at the root of 224.mm. Here is what his calculations found.......

"I calculated the Elastic Section Modulus for the spar shapes, a geometric description of the spar's resistance to bending independant of the applied load. I got 1113.47mm^3 for the 22mm spar and 1346.89mm^3 for the 25mm spar. The bigger the Modulus the greater the load. And yes, as the web distance grows so does the strength of the spar. The taller spar is about 21% stiffer/stronger than the 22mm spar, not bad for an increase in height of about 14% (25/22=1.139----).

[Lance Prior]

Opps!

Sorry about that.

Rule of thumb is twice as deep is four times stronger.

But you should probably check that considering my track record.

[gouda]

Ollie - did you ever see an aeirleon (sp?) flattened wing version of the Sagitta fly worth a darn?? I never did. They always looked very strange too.

Yup, that's a understatement. Mine flew like, well, a rock. But that's not the e205's fault in my opinion. Seems like a very nice flying plane from that vintage was a Windsong with a flat E205.

Wish I could still get ahold of one. Wth today's radios.....

[fly1milehi]

Thats like the second or third negative post about Sagittas with ailerons..... I am still planning to build my 600 that way but maybe I should reconsider. Is it a question of the flattened wings not having enough dihedral? Or maybe the cord witdh of the aileron itself is critical or wrong no the plans? I had a cumic with ailerons and visually they looked like barn doors instead of ailerons on a glider and to tell you the truth I hated the way that glider flew.
Im planning to do polyhedral on the 600, 2 degrees center and 3 or 4 degrees on the tips..
Any thoughts or comments?
greg