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Hello all!

#1
Hello everyone, long time lurker here but first time poster! I enjoy reading everybody's material here and it is fantastic. I've been a fan of FT since they came to be on youtube and it has been wonderful to get back into the hobby with all my friends. I hope that with my background in aviation I can assist with some forum info for people just learning even though there are plenty of others on here with a wealth of info. My background is all aviation! I fly full scale, RC, and I make my living in the airlines. I love the RC hobby due to its experimental and therapeutic nature. I look forward to interacting with all of you on here. To all, I give a hello!
 

buzzbomb

I know nothing!
#4
Welcome Rawdog! You're in aviation. You have aviation based-knowledge. You say you've been lurking since the beginning (I'm really new). What made you post and get back into the hobby with all your friends? Where have you all been? Sorry for the obvious question, but I'm honestly curious. :)
 

Merv

Legendary member
#5
It would be good if you cloud explain Center of Pressure. I’ve understand the concept but have no idea how to control it. I presume the shape of the wing has something to do with where the CP is.
 
#7
Welcome Rawdog! You're in aviation. You have aviation based-knowledge. You say you've been lurking since the beginning (I'm really new). What made you post and get back into the hobby with all your friends? Where have you all been? Sorry for the obvious question, but I'm honestly curious. :)
Honestly, crashing $300 models really sucked and then you had to hope your hobby store had spare parts or wait for what always seemed to be two weeks for shipping. Replacement parts were always stupid expensive and it just became not worth it. When FT came around and opened up my eyes to foamboard and hobbyking, that's when I started up again with the rc stuff and told my buds about it. All we have done over the past 7-8 years is use their techniques instead of blowing money on expensive models. We aren't exactly the most responsible with our planes....

What made me post is, a lot of people on here have all sorts of neat ideas on building techniques and new models. I wanted to be able to ask questions when I had a specific one in mind.
 
#8
It would be good if you cloud explain Center of Pressure. I’ve understand the concept but have no idea how to control it. I presume the shape of the wing has something to do with where the CP is.
In basic terms, Center of Pressure is the point at which your lift is applying its vector (direction with force) to the wing of your aircraft. Shape of the wing and speed effects the center of pressure. Symmetrical wings are a good illustration for center of pressure. The camber, AKA curve of the wing, has the same top and bottom of a wing on a symmetrical airfoil so your center of pressure on a level line would be equal on top and bottom. the wing relies on angle of attack (angle between leading edge of wing and forward movement) for lift and adjustment of that pressure in this example.
Also, with center of pressure, you can assume that it will move back towards the trailing edge of the wing as speed increases, and move forward towards the leading edge as speed decreases. Meanwhile, CG stays the same. They discovered this phenomenon back when they were trying to break the speed of sound and discovered the aircraft would violently pitch towards the earth. Look up mach tuck sometime.

Aerodynamics gets really complex but that's what I like about DIY. If you have a fundamental use of basic aerodynamics you can just slap a wing on, balance it and go fly. If you have ever wondered why FT balances the planes around the 20-30% back from the leading edge, center of pressure is your answer. This is the typical location of the CP on basic airfoils but as models and airfoils change, that point changes as well and you have to R&D where you balance the plane for best results.

Unless you have a wing design for diy that has been run through a wind tunnel with very specific mathematical formulas, there is really no way of actually knowing where that point is on your model or where it will be at different speed envelopes. CG is really the only thing we can count on for aerodynamic stability. Just some food for thought.
 

Merv

Legendary member
#9
In basic terms, Center of Pressure is the point at which your lift is applying its vector (direction with force) to the wing of your aircraft. Shape of the wing and speed effects the center of pressure. Symmetrical wings are a good illustration for center of pressure. The camber, AKA curve of the wing, has the same top and bottom of a wing on a symmetrical airfoil so your center of pressure on a level line would be equal on top and bottom. the wing relies on angle of attack (angle between leading edge of wing and forward movement) for lift and adjustment of that pressure in this example.
Also, with center of pressure, you can assume that it will move back towards the trailing edge of the wing as speed increases, and move forward towards the leading edge as speed decreases. Meanwhile, CG stays the same. They discovered this phenomenon back when they were trying to break the speed of sound and discovered the aircraft would violently pitch towards the earth. Look up mach tuck sometime.

Aerodynamics gets really complex but that's what I like about DIY. If you have a fundamental use of basic aerodynamics you can just slap a wing on, balance it and go fly. If you have ever wondered why FT balances the planes around the 20-30% back from the leading edge, center of pressure is your answer. This is the typical location of the CP on basic airfoils but as models and airfoils change, that point changes as well and you have to R&D where you balance the plane for best results.

Unless you have a wing design for diy that has been run through a wind tunnel with very specific mathematical formulas, there is really no way of actually knowing where that point is on your model or where it will be at different speed envelopes. CG is really the only thing we can count on for aerodynamic stability. Just some food for thought.

Is there a relationship between CP moving forward or aft & the thickness of the wing (say 9% of cord vs 15% of cord) OR between where the peak of the wing is (say peak at 25% cord vs peak at 35% cord)? I'm sure I'm not using the correct terms for what I'm trying to describe.

If I want to move CP what changes to the wing will make it move?
 
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#10
Is there a relationship between CP moving forward or aft & the thickness of the wing (say 9% of cord vs 15% of cord) OR between where the peak of the wing is (say peak at 25% cord vs peak at 35% cord)? I'm sure I'm not using the correct terms for what I'm trying to describe.

If I want to move CP what changes to the wing will make it move?
Absolutely there is a relationship. the farther back your peak as you stated, the farther back your CP. Also, keep in mind that CP changes as angle of attack changes. Most low speed airfoils like what flite test has mimicked, the CP stays fairly consistent in location. High speed airfoil designs will have a CP around the 40% back mark of the chord so you would not balance this specific plane at 25% like you are used to with a cub, for example. I am no expert and by no means can tell you where exactly CP will be on any given airfoil. I leave that to the calculus experts out there.
 

sprzout

Knower of useless information
Mentor
#14
Welcome, sir!!!

You'll find me offering advice as well, but you'll notice I have a title - "Knower of Useless Information". My wife refers to me as Cliff Claven. :)

That said, I've done a lot of what I call "stupid" research - as in, "Let me try this...Well, that was stupid." LOL I figure if I can tell people, "Don't make my stupid mistakes," things will go better for others.

And, because I haven't really spelled it out before, I've had a lot of exposure to aeronautics that I didn't really realize, via my father, who used to work for General Dynamics/Martin Murrieta/Lockheed Martin/ULA - I saw planes, got exposure via the San Diego Air & Space Museum (and had NO idea how much history San Diego had in the way of airplanes; the Spirit of St. Louis was built here, Ryan Aircraft started right across from the Santa Fe train depot, etc), and just kinda grew up with it.
 
#15
In basic terms, Center of Pressure is the point at which your lift is applying its vector (direction with force) to the wing of your aircraft. Shape of the wing and speed effects the center of pressure. Symmetrical wings are a good illustration for center of pressure. The camber, AKA curve of the wing, has the same top and bottom of a wing on a symmetrical airfoil so your center of pressure on a level line would be equal on top and bottom. the wing relies on angle of attack (angle between leading edge of wing and forward movement) for lift and adjustment of that pressure in this example.
Also, with center of pressure, you can assume that it will move back towards the trailing edge of the wing as speed increases, and move forward towards the leading edge as speed decreases. Meanwhile, CG stays the same. They discovered this phenomenon back when they were trying to break the speed of sound and discovered the aircraft would violently pitch towards the earth. Look up mach tuck sometime.

Aerodynamics gets really complex but that's what I like about DIY. If you have a fundamental use of basic aerodynamics you can just slap a wing on, balance it and go fly. If you have ever wondered why FT balances the planes around the 20-30% back from the leading edge, center of pressure is your answer. This is the typical location of the CP on basic airfoils but as models and airfoils change, that poin
Outstanding explanation... If i understood correctly what you said about symetrical airfoil I now understand why the first really efficient airplanes in history were bi-planes ... or did I miss something ? 😁