Prop Length VS Cowl/Fuse Width
05-19-2003, 02:32 PM
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Damnathius,
The CL values that I used in this equation have been taken at absolutely Zero AOA, and it is completely independent of a certain aircraft's wing incidence. If the Sig Cub has to fly with it's thrust angle at a slightly downward angle to achieve Zero AOA on the wing, then so be it: those Zero AOA CL values will still hold true! I have no tricks up my sleeve here. Your incidence argument is unfounded, because my figures for Zero AOA are absolute and are completely independent of the thrust line. If the Sig Cub in my previous problem has an incidence of 1.5 degrees, then at the 54 MPH I quoted, the thrust line would have to sit at negative 1.5 degrees in order to maintain level flight and bring the wing AOA to Zero. Incidence is irrelevant when you focus strictly on wing AOA.
I have proof that bigfoot exists, but I don't remember where I found the information. Sources and explanations please; anyone can play that game. I would really like to hear more about the Damnathius 400 MPH Cessna wing theory.
I do not blindly trust all sources, and therefore I put all of my information out for all to critique if I have come to my conclusion in error. Feel free to critique my methods, but please try not to attack me as a person.
This is correct for almost any airplane, until you reach a certain speed, which I calculated to be 54 MPH for this Sig Cub. When you reach this speed, your AOA will have to be zero to maintain level flight, and the faster you go, you will have to maintain an increasingly slightly negative AOA to maintain level flight. If you maintain positive AOA at this speed and above, your lift will be greater than your weight, and the aircraft will climb. I have noticed this nose-down attitude at high speeds on lightly loaded high-wingers such as Cubs. Giving the plane more incidence will only increase the nose-down appearance as the thrust line will have to become more negative to maintain level flight at this speed.
I have observations to back up my claims, but those won't hold much water in an argument. You may think you've seen bigfoot, and you may truly believe it, but just stating what you believe won't get you very far in a skeptical world. Your thesis needs backing other than your opinion. Two people watching the exact same event may be seeing two completely different things. I'm throwing out what I think I saw and solving the problem in a logical way.
The only way to QUANTIFY real-world physics is through math! Otherwise it's all just an unfounded guess. The math I used figures in logical parameters such as altitude and Reynolds number and conservation of energy. I'm not sure how physics can get more "real-world."
I for one am glad that Aerospace Engineers use mathematical equations to calculate flight characteristics and structures required. It is in this way that airframes can be optimized for maximum performance, and safety factors can be numerically built in. I will take mathematically designed aircraft over "it looks right" any day. The ideal situation being real-world experience used as a logic check against what the math says. My real world experience confirms in my mind that what I have figured is correct.
If you would like to further attack my credentials, and accuse me of living in textbooks, I have been flying R/C for seven years and am a student pilot through solo, with time in a wide variety of light aircraft (including Cessnas, and aircraft equipped with Clark-Y airfoil sections). My Father and I have built and flown our own experimental aircraft, and I have experience in the restoration of classic aircraft. I have also designed and built my own R/C aircraft utilizing AutoCAD. I just completed my first year of an Aerospace Engineering major at Mississippi State University, and was a crew chief on a design-build-fly team that took 8th in an international competition of universities.
But none of this matters in this discussion. If that were the case, it would turn into a "my Dad can beat up your Dad" style argument. The information that I put forth is what matters here.
Mark VanZwoll
The CL values that I used in this equation have been taken at absolutely Zero AOA, and it is completely independent of a certain aircraft's wing incidence. If the Sig Cub has to fly with it's thrust angle at a slightly downward angle to achieve Zero AOA on the wing, then so be it: those Zero AOA CL values will still hold true! I have no tricks up my sleeve here. Your incidence argument is unfounded, because my figures for Zero AOA are absolute and are completely independent of the thrust line. If the Sig Cub in my previous problem has an incidence of 1.5 degrees, then at the 54 MPH I quoted, the thrust line would have to sit at negative 1.5 degrees in order to maintain level flight and bring the wing AOA to Zero. Incidence is irrelevant when you focus strictly on wing AOA.
If you decrease to 0 degrees AOA in level flight, the plane will descend. I am talking practical application here, not unattainable theory like the 400 MPH Cessna wing. I don't remember where I found that information, but it was the same place I found the information that stated that a wing would have to be very fat (almost 1/2 of it's chord length) to produce lift via low pressure on the top of the airfoil at 0 degrees AOA.
I do not blindly trust all sources, and therefore I put all of my information out for all to critique if I have come to my conclusion in error. Feel free to critique my methods, but please try not to attack me as a person.
Try building a Cub model with an actual 0 degree incidence, (chord line parallel to the thrust line) and I guarantee you it will have to fly with a nose up attitude in order to maintain altitude.
You say I am offering nothing but heresay, and I say you are offering nothing but mathematical equations. My "heresay" is based upon observation, (many, many years of it) which is "real world" stuff, not numbers on paper. Numbers on paper OFTEN have nothing to do with the way a particular thing works in real world conditions.
If the rules allow logic, practical application of ideas and real-world physics to factor in, then I still await your argument.
I for one am glad that Aerospace Engineers use mathematical equations to calculate flight characteristics and structures required. It is in this way that airframes can be optimized for maximum performance, and safety factors can be numerically built in. I will take mathematically designed aircraft over "it looks right" any day. The ideal situation being real-world experience used as a logic check against what the math says. My real world experience confirms in my mind that what I have figured is correct.
If you would like to further attack my credentials, and accuse me of living in textbooks, I have been flying R/C for seven years and am a student pilot through solo, with time in a wide variety of light aircraft (including Cessnas, and aircraft equipped with Clark-Y airfoil sections). My Father and I have built and flown our own experimental aircraft, and I have experience in the restoration of classic aircraft. I have also designed and built my own R/C aircraft utilizing AutoCAD. I just completed my first year of an Aerospace Engineering major at Mississippi State University, and was a crew chief on a design-build-fly team that took 8th in an international competition of universities.
But none of this matters in this discussion. If that were the case, it would turn into a "my Dad can beat up your Dad" style argument. The information that I put forth is what matters here.
Mark VanZwoll
05-19-2003, 02:43 PM
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Prop Length VS Cowl/Fuse Width
Originally posted by Damnathius
If the rules of argument require that one must use only mathematical equations, then you win. If the rules allow logic, practical application of ideas and real-world physics to factor in, then I still await your argument.
If the rules of argument require that one must use only mathematical equations, then you win. If the rules allow logic, practical application of ideas and real-world physics to factor in, then I still await your argument.
I'm afraid, the days are gone when it was sufficient to stick a wet finger in the air to test for wind direction, as lamentable as that may be.
-David C.
05-19-2003, 02:47 PM
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Originally posted by MarkVZ
If you browse through the polars, you will see that there isn't a *huge* difference in Zero AOA CL between the highest Re and lowest Re offered, the lowest being 0.25 at the Re corresponding to a Cessna-sized aircraft.
If you browse through the polars, you will see that there isn't a *huge* difference in Zero AOA CL between the highest Re and lowest Re offered, the lowest being 0.25 at the Re corresponding to a Cessna-sized aircraft.
I didn't realize before, that the scale effect, resulting in a different Re number wasn't, actually, very large. It may be in other aspects of scaling (like wing loading, for instance) but not in the Reynolds number.
Interesting!
-David C.
05-19-2003, 03:08 PM
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David,
I believe that you're correct in saying that wing loading has a lot to do with scaling factors. I mean just look at the wing loading of a lightly-loaded full scale aircraft such as the Piper J-3 Cub:
With a Wing Area of 178 square feet and a gross weight of 1100 lbs (taken from http://www.moxon.org/aircraft/cub.htm ) the J-3 has a wing loading of 6.18 lbs per square foot. That's 99 ounces per square foot! That's very heavy by R/C model standards, yet the full scale Cub has been known to be described by it's pilots as a "kite" referring to it's very light wing loading and tendancy to "float."
For reference, our Sig 1/6 scale Cub has a wing loading of 16.7 - 18.3 oz/sq ft listed by Tower.
This leads me to believe that wing loading does have a very distinct effect on flight characteristics of scaled airframes.
Mark VanZwoll
I believe that you're correct in saying that wing loading has a lot to do with scaling factors. I mean just look at the wing loading of a lightly-loaded full scale aircraft such as the Piper J-3 Cub:
With a Wing Area of 178 square feet and a gross weight of 1100 lbs (taken from http://www.moxon.org/aircraft/cub.htm ) the J-3 has a wing loading of 6.18 lbs per square foot. That's 99 ounces per square foot! That's very heavy by R/C model standards, yet the full scale Cub has been known to be described by it's pilots as a "kite" referring to it's very light wing loading and tendancy to "float."
For reference, our Sig 1/6 scale Cub has a wing loading of 16.7 - 18.3 oz/sq ft listed by Tower.
This leads me to believe that wing loading does have a very distinct effect on flight characteristics of scaled airframes.
Mark VanZwoll
05-19-2003, 06:14 PM
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Okay, I found "Damnathius' 400 MPH Cessna wing theory".
http://www.paraborne.com/how_airplanes_fly.html
We seem to be going back and forth, much like a tennis match. The arguments do not change but are simply rephrased. Perhaps if each of us draw enough pictures we can get our points across.
Mark, I did not mean to "attack" you or your education and experience, and if it came out that way I apologize. It is often that people rely only on what they read, and think no further about what they are reading while taking it as the gospel truth. (Honestly, from your inital post this is what I took you for... Sorry.) At other times, people will expand upon a theory (or mathematical necessity) and apply it to real-world physics. Now I know we are on a different page regarding "real world physics", and my definition of the term includes the old "hand out the window" experiment. This is what I meant... I do not need mathematics to tell me that the angle of my hand at a given speed may or may not produce a "lift", but I understand the physics which are acting upon my hand.
David, I don't believe I ever said there was no need for mathematics regarding flight, but I might have said that mathematics alone will not make a plane fly. Mathematics is a great tool for design, but flight testing is the ultimate tool for adopting what really works, which can easily vary from what looked good on paper. Holding one's wet finger to the wind might be archaic and relatively innaccurate, but it still works.
http://www.paraborne.com/how_airplanes_fly.html
We seem to be going back and forth, much like a tennis match. The arguments do not change but are simply rephrased. Perhaps if each of us draw enough pictures we can get our points across.
Mark, I did not mean to "attack" you or your education and experience, and if it came out that way I apologize. It is often that people rely only on what they read, and think no further about what they are reading while taking it as the gospel truth. (Honestly, from your inital post this is what I took you for... Sorry.) At other times, people will expand upon a theory (or mathematical necessity) and apply it to real-world physics. Now I know we are on a different page regarding "real world physics", and my definition of the term includes the old "hand out the window" experiment. This is what I meant... I do not need mathematics to tell me that the angle of my hand at a given speed may or may not produce a "lift", but I understand the physics which are acting upon my hand.
David, I don't believe I ever said there was no need for mathematics regarding flight, but I might have said that mathematics alone will not make a plane fly. Mathematics is a great tool for design, but flight testing is the ultimate tool for adopting what really works, which can easily vary from what looked good on paper. Holding one's wet finger to the wind might be archaic and relatively innaccurate, but it still works.
