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Nov 04, 2018, 12:20 AM
http://bit.ly/2zm6gK7
Quote:
Originally Posted by springer
Corsair, i have to admit i haven't watched your video yet, but allowing for the editing of other posters, think that if you are/can run studies of KFM wings the way we actually build them at size and speed we actually fly them that would be valuable. (And present your results, of course). While we "all" have our anecdotal, empirical data that we rely on for our opinions of the value of KFM it would be nice to see data from the "official" math side of it all. That has been the missing piece of the puzzle to me. Even the work by maguro et al used an implementation that no one i know actually uses (step in mh32 airfoil). If you have the time and access to the cfd tools, it would provide some interesting info and you could format it in a way to guide noobs into best imementations. As others have said most folk use kfm because it is dead easy to build, and flys surprisingly well, perhaps even better than conventional, in the characteristics that help beginner flyers stay aloft (not neccessarily in efficiency).
Thanks for your input Springer. In the other thread in Modelling Science I am discussing the possibility of running some wind tunnel tests. I plan to test various types of KF airfoils at a real world use Reynolds number and compare to a more "traditional" airfoil, as well as moving steps forward and aft and serrating the steps trailing edge etc.


Obviously I could also build some drag polars which would be nice

If I were to run some wind tunnel tests, what are some things yall would want me to test?
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Nov 04, 2018, 11:12 AM
Registered User
I keep hearing this term "low rsynolds numbers" everyone is throwing it around in relation to models. Oh models behave differently because of the low Reynolds numbers.

So I just looked it up and hasn't see how this allies to anything more people mention them in context with.

From what I can find it's a mathsamaticsl method to predict a fluids flow. Low numbers is more laminar and higher more turbulasnt.

So models exhibit need turbulent sir than larger? Whstbdoesd that mean. So models fly better?
Nov 04, 2018, 04:41 PM
Build straight - Fly twisty
Whiskers's Avatar
In our application RN is important because as we scale down wing sizes the air we fly in is not scaled down to keep everything constant.
Put very simply, a large wing performs very differently than a a small wing, and data valid for one will not be valid for the other unless factors such as RN are factored into the calculations.
Also we fly at very different air speeds. Typically a full size plane's slowest speed is higher than our fastest speed, and this also makes for variations in behaviour.
In practice a scale model of a high-performance sailplane may get nasty to fly because its wing cord near the tips becomes too small to work well.
The (say) 18 inch cord near the tip of the 'big one' becomes, (say), 1 inch. This will make the model a nasty 'tip staller' that will not be a joy to fly.
Last edited by Whiskers; Nov 04, 2018 at 06:42 PM.
Nov 04, 2018, 05:59 PM
treefinder
springer's Avatar
To add to what my older bro has so eloquently said, You are right that it is 'thrown around' a lot, In most instances it is essentially short hand for recognition that our models fly differently in not scaled down air than full size. While the Reynolds number was developed to help describe fluid flow in pipes, it also works for flow of objects in air (another fluid). In that sense it has no particular value in the vernacular (other than what we said) although the folks who get into the mathematical side of aerodynamics will use it in their calculations. Since most of us are more TLAR 'engineers' understanding that there are some things that work well at one scale and not at another is more useful than an arcane number. I tend to think of KFM's that way, I have found them to be nice and forgiving wings, and in the few times I've made both conventional and KFM for the same plane, the KFM always is more forgiving, while the conventional is typically more exciting and challenging to fly.

For Corsair, what I'd like to see is some evaluation of the differences among the several KFM variants as to life/drag, stall speed, etc. While I think i have some understanding of which one to use when, It would be interesting to see the numbers.
Nov 04, 2018, 06:29 PM
http://bit.ly/2zm6gK7
Quote:
Originally Posted by bazsound
I keep hearing this term "low rsynolds numbers" everyone is throwing it around in relation to models. Oh models behave differently because of the low Reynolds numbers.

So I just looked it up and hasn't see how this allies to anything more people mention them in context with.

From what I can find it's a mathsamaticsl method to predict a fluids flow. Low numbers is more laminar and higher more turbulasnt.

So models exhibit need turbulent sir than larger? Whstbdoesd that mean. So models fly better?
To add to what the others have already described very well, in this context we are discussing Reynold's number because at certain Re (let's say >50,000) KF airfoils begin to shed their trapped vortex as the intertial forces overcome the viscous forces (Reynold's number is the ratio of Intertial forces / Viscuous forces in the fluid).

So in theory, a KF airfoil does not perform well at 'higher' Re #'s because it's big design point (the step and recirculation region) begins to fall apart as the vortex sheds and reforms
Nov 04, 2018, 06:30 PM
http://bit.ly/2zm6gK7
Quote:
Originally Posted by springer
.

For Corsair, what I'd like to see is some evaluation of the differences among the several KFM variants as to life/drag, stall speed, etc. While I think i have some understanding of which one to use when, It would be interesting to see the numbers.
Okay I hope to get to do this sometime this winter! I will keep this thread updated as I make progress and continue to look for feedback and suggestions as I begin to develop the models and plan the tests
Nov 05, 2018, 12:10 AM
flyin' fool
goldguy's Avatar
C2014:

Might find some useful info here to work from …………….. https://www.rcgroups.com/forums/sear...olds%20numbers
Nov 05, 2018, 08:56 AM
http://bit.ly/2zm6gK7
Quote:
Originally Posted by goldguy
C2014:

Might find some useful info here to work from …………….. https://www.rcgroups.com/forums/sear...olds%20numbers
Sorry Gold, this is what I get when i click the link
Nov 05, 2018, 02:42 PM
An itch?. Scratch build.
eflightray's Avatar
I think the few people who use the Modelling Science forum are starting to get frustrated with arguing with each other. They could be attempting to break out of their padded cells

Though it can be interesting forum in an amusing way, (certainly a much safer place than LTUP ),

Perhaps a case of, "Resistance is futile. You will be assimilated, and your knowledge upgraded."


.
Nov 05, 2018, 03:08 PM
Retired CAD guy
birdofplay's Avatar
Hey !
We ARE ALL flyin' Foamys here,
So let's NOT GET SERIOUS, PA-Leaze !

Airobatic Foamys work JUST fine without the benefit of a Curved airfoil - FLAT SHEETS Rock !

Slow flier Foamys work SLOWER with a seriously Curved UC "foil" .
What particular curve ?
Who knows, who cares. They all work pretty good.

Heavy models put their wings at 2-3 deg up because they need a little more Flat Plate lift.
Flat Plate as in KITE type lift.

To achieve that tiny bit more lift one can add a "true" airfoil shape of some sort.

But Seriously folks, at the Power to weight ratios that we have at our disposal,
Who cares ?

Who can actually measure A Difference that actually MAKES a Difference.

Now wouldn't you rather just be out Flyin' :>}
Nov 05, 2018, 06:55 PM
http://bit.ly/2zm6gK7
Quote:
Originally Posted by birdofplay
Hey !
We ARE ALL flyin' Foamys here,
So let's NOT GET SERIOUS, PA-Leaze !

Airobatic Foamys work JUST fine without the benefit of a Curved airfoil - FLAT SHEETS Rock !

Slow flier Foamys work SLOWER with a seriously Curved UC "foil" .
What particular curve ?
Who knows, who cares. They all work pretty good.

Heavy models put their wings at 2-3 deg up because they need a little more Flat Plate lift.
Flat Plate as in KITE type lift.

To achieve that tiny bit more lift one can add a "true" airfoil shape of some sort.

But Seriously folks, at the Power to weight ratios that we have at our disposal,
Who cares ?

Who can actually measure A Difference that actually MAKES a Difference.

Now wouldn't you rather just be out Flyin' :>}
Idk, I actually really enjoy this side of the game too (;
Nov 05, 2018, 09:26 PM
flyin' fool
goldguy's Avatar
It's just a search on the KF Theory/Science form, three pages of links. Lots of reading there, including some good input.

https://www.rcgroups.com/forums/show...ory-Science-**
Last edited by goldguy; Nov 06, 2018 at 02:53 PM.
Nov 06, 2018, 07:24 AM
gpw
gpw
“There’s no place like Foam”
gpw's Avatar
Once we found out it worked , we gladly accepted it and used it happily on many models … I don’t need to understand why the sky is Blue to appreciate it …
Latest blog entry: Lost plans
Nov 06, 2018, 09:46 AM
low tech high tech
vtdiy's Avatar
There are a lot of different reasons to build models. And lots of ways to do it.

The "best" way to build a model, of the "best" type with the "best" foil, with the "best" gear of the "best" material according to the "best" science is the subject of endless debate. As is even the "best" forum and the "best" thread to do that in.

One hopes to even write the "best" post in that thread. But, so many doubters!

Where's the fun all gone? I say let the guy speak. He'll be wrong to some people, right to others, and not interesting to others. Just like the rest of us.

Meanwhile, there is foam in the workshop, still in sheet form, doing nothing. Time to alter that.....
Nov 06, 2018, 10:04 AM
http://bit.ly/2zm6gK7
Quote:
Originally Posted by vtdiy
There are a lot of different reasons to build models. And lots of ways to do it.

The "best" way to build a model, of the "best" type with the "best" foil, with the "best" gear of the "best" material according to the "best" science is the subject of endless debate. As is even the "best" forum and the "best" thread to do that in.

One hopes to even write the "best" post in that thread. But, so many doubters!

Where's the fun all gone? I say let the guy speak. He'll be wrong to some people, right to others, and not interesting to others. Just like the rest of us.

Meanwhile, there is foam in the workshop, still in sheet form, doing nothing. Time to alter that.....
Thank you I appreciate that. And I'm not trying to say KF airfoils are good or bad. Like I said in the video, EVERY airfoil, and EVERY aircraft is designed for a specific mission and will ALWAYS have trade-offs to optimize for ONE flight regime


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