CARBON FIBER DISCUSSION

[Piotrsko]

Gotta sleep sometime

 

[telnar1236]

Never to my knowledge had a horizontal stabilizer stall. Might cause a loop uncontrolled, either up or outside, OR, on full wing stall mode, become useless with massive pitch excursions

Shoe airfoil: take right edge of right shoe, trace on piece of paper the outline. Stop at heel. Can use left shoe if left handed. Remove shoe, connect front to back with straight line (ease of building) viola shoe airfoil. Since we are model size, all the NASA airfoil research data doesn't really apply, although I expect @quorneng to disagree (rightly so)

Most designs, it's pretty hard to do - almost all full-scale planes have proportions that prevent this and this carries over into rc planes too. Even if you manage it, it might not do anything - rc planes tend to have the cg farther forwards than their corresponding real counterparts, so if the cg is forwards of the wing aerodynamic center, then you might not even notice. I think I've only ever done it doing cobras on a thrust vectoring plane, and then only because I was stalling all the flying surfaces.

 

[Zoom Master]

Never to my knowledge had a horizontal stabilizer stall. Might cause a loop uncontrolled, either up or outside, OR, on full wing stall mode, become useless with massive pitch excursions

Shoe airfoil: take right edge of right shoe, trace on piece of paper the outline. Stop at heel. Can use left shoe if left handed. Remove shoe, connect front to back with straight line (ease of building) viola shoe airfoil. Since we are model size, all the NASA airfoil research data doesn't really apply, although I expect @quorneng to disagree (rightly so)

Alright.....shoe airfoil....never heard of that before and this is my first time hearing about it. What are its advantages and disadvantages ? And why should one use it over a regular airfoil ?

 

[telnar1236]

Alright.....shoe airfoil....never heard of that before and this is my first time hearing about it. What are its advantages and disadvantages ? And why should one use it over a regular airfoil ?

If I understand correctly, it's basically this profile (first shoe picture I found on Google, so it may vary by the shoe). Overall, if you round the leading edge a bit, it's probably a pretty good airfoil.

It's probably less well optimized than something like a Clark Y airfoil (my go-to), a NACA airfoil or some of the more modern airfoils designed for things like pylon racing, but to be honest, most pilots (including me) probably wouldn't notice a big difference for most things. It probably won't be as good at inverted flight as something symmetrical, or as good at high angles of attack as something with a larger leading edge radius, or as low drag as a laminar flow design, but unless you're really pushing your plane very hard, those things won't matter.

The most important thing to remember about airfoil selection, is that it needs to match the rest of your plane. You're always going to be able to reduce stall speed more through weight reduction and adding flaps than through airfoil selection. And if you have a very draggy fuselage, it doesn't matter if you put a low drag airfoil on your wing - you're still not going fast.

There's a lot of room for aerodynamic optimization relative to most rc planes flying today, but the impacts from that will mostly be felt in the middle of a plane's envelope in flight time increases, and towards the top in speed increases, but they won't really make the plane fly any nicer and they might actually make the design feel weird to fly.

 

[Zoom Master]

If I understand correctly, it's basically this profile (first shoe picture I found on Google, so it may vary by the shoe). Overall, if you round the leading edge a bit, it's probably a pretty good airfoil.
View attachment 253888
It's probably less well optimized than something like a Clark Y airfoil (my go-to), a NACA airfoil or some of the more modern airfoils designed for things like pylon racing, but to be honest, most pilots (including me) probably wouldn't notice a big difference for most things. It probably won't be as good at inverted flight as something symmetrical, or as good at high angles of attack as something with a larger leading edge radius, or as low drag as a laminar flow design, but unless you're really pushing your plane very hard, those things won't matter.

The most important thing to remember about airfoil selection, is that it needs to match the rest of your plane. You're always going to be able to reduce stall speed more through weight reduction and adding flaps than through airfoil selection. And if you have a very draggy fuselage, it doesn't matter if you put a low drag airfoil on your wing - you're still not going fast.

There's a lot of room for aerodynamic optimization relative to most rc planes flying today, but the impacts from that will mostly be felt in the middle of a plane's envelope in flight time increases, and towards the top in speed increases, but they won't really make the plane fly any nicer and they might actually make the design feel weird to fly.

OHHHH so thats what a shoe airfoil is....makes a lot more sense now and yeah NACA airfoils are better for pylons and maybe high speed aircrafts. And I dont think many people who build rc planes care about drag to much unless they are trying to go really fast or do 3D.

 

[Piotrsko]

If you read and understand all the NASA data, they specify the reynolds number range the profile works at. We are SOOOO subsonic, super low reynolds that any wind tunnel data is irrelevant to what we do. Remember a bee isn't aerodynamic but that don't stop them from flying

Telnars picture needs to go to the heel of the shoe otherwise it becomes a nasa laminar series aft cg profile which totally suck for our needs. Btw: leather shoes don't have all them bumps and lumps.

 

[Zoom Master]

If you read and understand all the NASA data, they specify the reynolds number range the profile works at. We are SOOOO subsonic, super low reynolds that any wind tunnel data is irrelevant to what we do. Remember a bee isn't aerodynamic but that don't stop them from flying

True.....so I guess al those people who do wind tunnel testing for aircrafts that will go 200 at max are wasting their time ? For example that one dude who did a bunch of wind tunnel testing for his 50mm edf jet that he was trying to hit 150 with ?

 

[telnar1236]

If you read and understand all the NASA data, they specify the reynolds number range the profile works at. We are SOOOO subsonic, super low reynolds that any wind tunnel data is irrelevant to what we do. Remember a bee isn't aerodynamic but that don't stop them from flying

Telnars picture needs to go to the heel of the shoe otherwise it becomes a nasa laminar series aft cg profile which totally suck for our needs. Btw: leather shoes don't have all them bumps and lumps.

Interesting - not what I'd expect to work well, but if it flies well, you can't argue with that

 

[telnar1236]

If you read and understand all the NASA data, they specify the reynolds number range the profile works at. We are SOOOO subsonic, super low reynolds that any wind tunnel data is irrelevant to what we do. Remember a bee isn't aerodynamic but that don't stop them from flying

Telnars picture needs to go to the heel of the shoe otherwise it becomes a nasa laminar series aft cg profile which totally suck for our needs. Btw: leather shoes don't have all them bumps and lumps.

It's worth considering what the Reynolds number we're flying at is before completely throwing away all the data out there - Airfoil Tools is really geared towards low Reynolds number data and contains a ton of the MH series airfoils as well as some of the Airfoils developed by Mark Drela (well known MIT professor and aerodynamicist) specifically for rc planes and gliders. A pretty typical Reynolds number for an RC plane is about 500,000.

So this data on the NACA 0012 airfoil from airfoil tools contains information that is completely suitable to use with an rc plane.

 

[telnar1236]

The important thing is dynamic similarity when considering wind tunnel testing. If you can match the Reynolds number and Mach number reasonably well, your results should be applicable. With simulations, it's a lot easier. You just simulate it at 100% scale and the correct speed and the results should be right, at least so long as you stay within the capabilities/assumptions of the simulation. In order for standard CFD not to work, you'd have to get really very tiny or really very fast, or of course mess up your setup which is very easy to do

 

[Piotrsko]

See the twitchies about zero? If you're there, it's a neat airfoil for a 3ft span 9" chord. Used to be my favorite profile.

Simulation is cool, we didn't have the compute power. Remember your phone device is better than anything available for us.

 

[telnar1236]

See the twitchies about zero? If you're there, it's a neat airfoil for a 3ft span 9" chord. Used to be my favorite profile.

Simulation is cool, we didn't have the compute power. Remember your phone device is better than anything available for us.

Yeah, CFD and computers have come a long way even since I started - the stuff I run on my laptop now would have lit my desktop on fire if I'd tried it back when I started building rc planes. Xfoil has been around for a while, but it's only really good for airfoils and you can't analyze finite span wings. I think that part of it is also that the tools have gotten a lot more user friendly - Xfoil barely even has graphics while xflr5 integrates xfoil and the ability to analyze realistic wings into a much easier to use package.

 

[Zoom Master]

See the twitchies about zero? If you're there, it's a neat airfoil for a 3ft span 9" chord. Used to be my favorite profile.

Simulation is cool, we didn't have the compute power. Remember your phone device is better than anything available for us.

Just out of curiosity do you work in the military ? Or something related ? ( you dont have to answer if you dont want to )

 

[Zoom Master]

Yeah, CFD and computers have come a long way even since I started - the stuff I run on my laptop now would have lit my desktop on fire if I'd tried it back when I started building rc planes. Xfoil has been around for a while, but it's only really good for airfoils and you can't analyze finite span wings. I think that part of it is also that the tools have gotten a lot more user friendly - Xfoil barely even has graphics while xflr5 integrates xfoil and the ability to analyze realistic wings into a much easier to use package.

True computers in general have come a LONG way over the past 10-20 years. Heard from someone back in the day that your phone has more than 120,000 times the processing power of the computer that sent man on the moon. And probably more then a lot of fighter jets that are still in service although I do not know the exact figures.

 

[Zoom Master]

If you read and understand all the NASA data, they specify the reynolds number range the profile works at. We are SOOOO subsonic, super low reynolds that any wind tunnel data is irrelevant to what we do. Remember a bee isn't aerodynamic but that don't stop them from flying

Telnars picture needs to go to the heel of the shoe otherwise it becomes a nasa laminar series aft cg profile which totally suck for our needs. Btw: leather shoes don't have all them bumps and lumps.

If what you are saying is true how fast does a rc airplane need to go before wind tunnel testing is actually useful ? Im guessing it might start being useful somewhere around the 400 mph mark...although im pretty sure even 400 mph is still quite slow....maybe somewhere in the 500 mph range ? With something like a really fast dynamic soaring glider like this one.

 

[telnar1236]

If what you are saying is true how fast does a rc airplane need to go before wind tunnel testing is actually useful ? Im guessing it might start being useful somewhere around the 400 mph mark...although im pretty sure even 400 mph is still quite slow....maybe somewhere in the 500 mph range ? With something like a really fast dynamic soaring glider like this one.

Wind tunnel testing and simulation can be useful for any speed of rc plane so long as dynamic similarity is met. However, it definitely isn't necessary for the vast majority of designs - you can optimize most rc planes quite a bit, but in the end, what matters is that they fly nice, and squeezing another 10mph out of a design, isn't really all that important

 

[Zoom Master]

and squeezing another 10mph out of a design, isn't really all that important

For most people but not for people who are trying to break records.

 

[telnar1236]

For most people but not for people who are trying to break records.

True, but even in that case you still need to build a bunch of prototypes to validate any analysis. It doesn't matter how good something looks on a computer screen or how well it performs in a wind tunnel if it doesn't work that way once it gets out into the real world

 

[telnar1236]

Pretty much all the aerodynamics we're discussing are pretty basic - stuff that's 50 or 70 years behind the cutting edge at best. All our rc planes are very firmly subsonic and the fundamentals of that were pretty much understood by the end of the second world war

The only reason records for rc planes are a thing is that we are building them as hobbyists - if you look at the aerospace industry, unmanned aircraft, some not much bigger than large rc turbine jets, have been breaking the sound barrier since the 50s

 

[Zoom Master]

Pretty much all the aerodynamics we're discussing are pretty basic - stuff that's 50 or 70 years behind the cutting edge at best. All our rc planes are very firmly subsonic and the fundamentals of that were pretty much understood by the end of the second world war

The only reason records for rc planes are a thing is that we are building them as hobbyists - if you look at the aerospace industry, unmanned aircraft, some not much bigger than large rc turbine jets, have been breaking the sound barrier since the 50s

That is true but a lot of us do not have the time, money or skills to make an rc airplane go supersonic. Although we are now seeing a lot of people working on going 500 plus for example The Mach Initiative, Ivan Markov, And the most qualified and the most experienced out of all of them isss Joe manor who is currently developing a dynamic soaring glider which can break the sound barrier. So yeah the next few years should be very exciting as we will get to see a lot of these projects completed and in the air !