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Jun 05, 2019, 07:22 AM
Red Merle ALES VII SJ
Curtis Suter's Avatar
Since you've decided on a very nice wing airfoil it would really help to determine tail moment arm length and sizes of your tails, if you haven't done so. An easy way to calculate this is with Sailplane Calc. Some easy measurements of your model design is placed in an Excel Spreadsheet and then it'll determine Vh and Vv values as well as Dihedral Angles for your main wing. It gives suggested sizes and it's not perfect but it's certainly better than guessing.

It's free at www.TailwindGliders.com
The file is on the Files page and an tutorial on the Articles page.

All the best with your project.

Curtis
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Jun 05, 2019, 07:34 AM
Sagitta Fanboy
Quote:
Originally Posted by jroshier
Okay. But why does it matter? What does it measure? Is a higher or lower number bettor or worse?

Thanks for the feedback and help.

J
Higher is better for us (not always, but in our small models that's pretty much universal). You really want at least a 6 digit Reynolds number if you can get it.

It's not really a direct measure of anything, but it is a general predictor of efficiency. and since airfoil performance plots (aka polars) are generall plotted in terms of Reynolds number, it's something you must know to predict performance.

It's as important for tail surfaces as for wings.
Jun 05, 2019, 07:59 AM
Registered User
Numbers for Curtis

"Dihedral Sizing Criteria - Spiral Stability (B=)" 5.07
"Horizontal Tail Sizing Criteria (Volume) - Pitch Stability (Vh=)" 0.46
"Vertical Tail Sizing Criteria (Volume) - Yaw Damping and Rudder Power (Vv=)" 0.026
"Dihedral Sizing - Roll Control (VvB=)" 0.13
"Equivalent Dihedral Angle (EDA=)" 7.63

Things may vary a little from this point.
Last edited by RaceMag; Jun 05, 2019 at 08:17 AM.
Jun 05, 2019, 07:57 PM
Registered User
jroshier's Avatar
Thread OP
Quote:
Originally Posted by mawz
Higher is better for us (not always, but in our small models that's pretty much universal). You really want at least a 6 digit Reynolds number if you can get it.

It's not really a direct measure of anything, but it is a general predictor of efficiency. and since airfoil performance plots (aka polars) are generall plotted in terms of Reynolds number, it's something you must know to predict performance.

It's as important for tail surfaces as for wings.
Thank you so much for replying!

I tried researching it for quite a while, but there was no real simple answer like the one you posted.
Is it okay if I use that definition in my folio?

Cheers,
J
Jun 05, 2019, 08:37 PM
Registered User
jroshier's Avatar
Thread OP
Quote:
Originally Posted by Curtis Suter
Since you've decided on a very nice wing airfoil it would really help to determine tail moment arm length and sizes of your tails, if you haven't done so. An easy way to calculate this is with Sailplane Calc. Some easy measurements of your model design is placed in an Excel Spreadsheet and then it'll determine Vh and Vv values as well as Dihedral Angles for your main wing. It gives suggested sizes and it's not perfect but it's certainly better than guessing.

It's free at www.TailwindGliders.com
The file is on the Files page and an tutorial on the Articles page.

All the best with your project.

Curtis
Thanks for the help.
My Dad had downloaded the program and has put the measurements somewhere on this thread. His username is RaceMag.

I should be able to upload more photos this weekend, after I have finished all my exams. I am currently in a dry spot where I haven't done any work for about a week, so I will get back into it soon.

Cheers,
J
Jun 06, 2019, 11:55 AM
Sagitta Fanboy
Quote:
Originally Posted by jroshier
Thank you so much for replying!

I tried researching it for quite a while, but there was no real simple answer like the one you posted.
Is it okay if I use that definition in my folio?

Cheers,
J
Feel free to use it.
Jun 06, 2019, 09:26 PM
Registered User
Specifically, Reynolds number is chord (or characteristic length if it's not a wing) times velocity times a viscosity term, and maybe a constant. You end up with a "dimensionless" number, as I recall. This means that the Reynolds number comes out the same regardless of the units you're using. As I recall, for sea level and 60 degrees Fahrenheit, chord in inches times velocity in feet per second times 532 is about right.

An incorrect description was given before. Low Reynolds number flows are more likely to stay laminar, unless there is separation. Unfortunately, they're more prone to separation. For large, fast aircraft, a great deal of effort is required to maintain laminar flow. On the other hand, it's probably inescapable on an indoor rubber model with a chord of 3 inches and a speed of 2mph. Turbulent flow stays attached better than laminar flow does, so sometimes it pays to have a discontinuity or "trip" to make the flow go turbulent. This could be a piece of tape, thread, etc. at the right place on the wing. Generally, you don're really need this with the best airfoils used correctly.

On an airfoil, flow can separate completely, which gives you a stall, or it can separate and then reattach, creating a separation bubble. As I understand it, the flow usually transitions to turbulent in the vicinity of the bubble. The dimples on a golf ball promote turbulent flow, which stays attached to the ball a bit longer, this lowering the drag. You can probably find lots of fun pictures of objects in smoke tunnels illustrating separation. If you fly an RC glider, at constant speed throughout the flight, just when the dew is starting to form, you can get interesting patterns in the condensation showing the separation bubble and the effect of a rough spot on the flow. Much less messy than using oil or paint as is sometimes done.

You've probably noticed that large RC sailplanes have higher aspect ratios than small ones. This is because the small ones need a high enough Reynolds number for the airfoil to work well. Some airfoils, particularly thin ones, work better at low Reynolds numbers than others do, so the best aspect ratio depends on the airfoil and how well it's made. Of course, if the model gets large enough and you run out of carbon fiber, thicker airfoils and/or lower aspect ratios will be necessary.

For the tail, I suggest one of Mark Drela's HT airfoils. A couple of them are almost an ellipse with two tangent lines back to the trailing edge. I think the ellipse is something like 40 percent of chord. Just over half the ellipse is actually used, the rest is covered by those tangent lines.
http://www.charlesriverrc.org/articl...t-airfoils.htm

You may want to stay with mostly fiberglass and/or kevlar for the fuselage. Carbon fiber may interfere with the radio's reception.
Jun 06, 2019, 09:51 PM
Registered User
BTW, do you really want to take on a huge oil company? They might want to defend their trademark. Come to think of it, will your glider crash into something and spill amazing amounts of oil all over the place? That's what I think of when I think of Exxon. ;-p
Jun 08, 2019, 03:47 AM
Registered User
jroshier's Avatar
Thread OP
Quote:
Originally Posted by lincoln
BTW, do you really want to take on a huge oil company? They might want to defend their trademark. Come to think of it, will your glider crash into something and spill amazing amounts of oil all over the place? That's what I think of when I think of Exxon. ;-p
I actually didn't know there was a company called Exxon.
It was originally named Exon, after the RNA coding sequence in genes, but I thought it looked cooler as Exxon.
Jun 08, 2019, 02:15 PM
Brett
bjaffee's Avatar
Exxon is kinda infamous for causing one of the biggest ecological disasters in US history, when one of their tankers hit a rock in Alaska and spilled it's oil back in the 1980's.

The parent company, ExxonMobile, owns Esso, which you may know in Australia.
Jun 12, 2019, 07:28 PM
Registered User
jroshier's Avatar
Thread OP

Massive update:


Alright,
I haven't posted in a couple of days.
I've been working on my blue foam buck for a while, but the night before last I decided it wasn't good enough, so I proceeded to make another one, this time out of yellow foam. (slightly lower melting point, no other real noticeable difference)
Unfortunately this stuffed up for me when I was doing the post cure last night: I had the heater used for our heat box set waaay too high, so the foam melted and stuffed up. The buck was in an almost perfect shape as well.
Photos of that buck are attached, including the one of the melted buck.

Other news:
I GOT 100% ON MY FOLIO!!!!!!!!!!!!!!!!!!!!!!!!
Perfect score. Thanks to my Dad for really helping me.
I decided not to go into the complexities of reynolds numbers because that's a bit too complex for a high school project. Now I have a bunch of time to work on actually building my plane. My Dad has been cutting out practice wings cores on out hot wire CNC cutter, so I will take photos of those at a later date and upload them.
I will keep all of you posted on what I am doing today: I have the whole day to make a replacement buck and put a layer of resin on it tonight. Then I have all day tomorrow as well. I might be able to start building the tailplane sections as well in the times where I'm waiting for resin to dry.

Please feel free to make any comments below, they're always welcom.

Cheers,
J
Jun 13, 2019, 02:00 AM
Registered User
If I remember correctly, Dow Styrofoam starts to get soft at something like 160 or 170 Fahrenheit. Is that compatible with the recommended post-cure temperature of your epoxy?

BTW, another way to think about Reynolds numbers is that they reflect the relative magnitude of viscous and inertial forces. If I remember correctly, someone wanted to do flow visualization for the unsteady flows of insect wings. So they made a scaled up model of an insect wing and ran it in heavy (i.e. relatively viscous) oil. Reynolds number was the same.

It's probably irrelevant, but for examples of flow completely dominated by viscosity, check out pitch drop experiments:
https://en.wikipedia.org/wiki/Pitch_drop_experiment.

A lot of fluid mechanics calculations, especially when you're being introduced to the subject, are done for zero viscosity. (i.e. inviscid flow). That's another one of those simplifying assumptions that can sometimes give approximately correct results. It makes the math a lot easier. However, if there was such a thing as inviscid flow, there'd be no drag! The closest we can get to this with aircraft is probably something really large going as fast as possible without compressibility (Mach) effects messing things up. Or maybe just a really large dirigible, even though it can't go that fast. The Hindenberg's hull Reynolds number was in the hundreds of millions.

BTW, to get an idea just how much Reynolds numbers can affect design decisions, check out the different shapes of "optimized" airship hulls in this paper:
http://lars.mec.ua.pt/public/LAR%20P..._6_97_lutz.pdf
I'm kind of glad that the designers of the 1930's dirigibles and Zeppelins never saw a paper like this. Or, at least, I assume they didn't. Maybe they just suspected that marketing an optimal shape might have been a challenge.
Jun 13, 2019, 06:54 PM
Registered User
jroshier's Avatar
Thread OP

Update:


That's a photo of the new buck that I have made. The second layer of q-cell resin will be applied this afternoon and I should be able to get a third layer on tomorrow, then sand it down on Sunday and attach a wing mount. Then I will create a mould in dental plaster to create the fibreglass fuselage.

While the resin is setting, I should be able to start working on my wings. I think that a reason for me being behind on my estimated timeline table is that I have only been working on a single part of my project at one time, which means that I have fallen behind. This has to be rectified because I want Exxon finished ahead of my timeline.

I have also attached the photo of my marks, in case anyone wanted to have a look.
Jun 14, 2019, 10:06 AM
Registered User
Gratter's Avatar
Questioning the use of dental plaster. Look on line on how to make a mold and do it out of fiberglass. think it might work out better.
Jun 14, 2019, 03:40 PM
Registered User
Hydrocal or Hydrostone are good materials for making low production volume molds. Not sure how they compare to dental plaster.


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