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Author Topic: Design Parameters II  (Read 12225 times)

Online Howard Rush

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Design Parameters II
« on: January 10, 2013, 04:30:14 PM »
I have been locked out of Mike's topic, presumably because I posted something saying that MAC is an important design parameter.  Nevertheless, here are answers to his questions:

Quote
What if you have a tapered wing with flaps?  Is the width of the flap at the root end taken into consideration for the total chord width?

Yes.  

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In figuring these parameters for the wing, what is the difference in figuring a constant chord wing and a tapered wing?Aspect ratio?  

Use span squared over area.  That is the official definition, and it works for both.

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Pertaining the the fuselage:  I have been told that the fuselage overall length should be 75% of the wingspan.  Is this right and does it make any difference if it is a profile or built up fuselage?

1) Not necessarily. 2) The profile will tend to be shorter, because the amount of stiffness you can get for a given weight will be less, so there will be a bigger penalty for length.


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I have read where the nose moment should be 20% of the fuselage length.  Is this true?

No, mostly because "nose moment", whatever that is, has no physical significance.  It appears nowhere in aeronautics other than stuff written about stunt.

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I also have read that the the stab and vertical fin should be located behind the wings trailing edge at a distance of 40% of the fuselage length.  

That also has no physical significance.  Stab and fin locations can be meaningfully discussed in terms of mean aerodynamic chord.

Quote
Laying out a profile fuselage was taken from an article by Ted Fancher relating to his IMITATION.

Determining the Thrust Line for a Profile - Measure down 1 5/16 inch from the top of the 1/2" board and draw a straight line.
 Measure down 1 5/16 inch from the top of the 1/2" board and draw a straight line.

Determining the centerline of the wing - drop down 1" from the thrust line and draw another straight line the length of the board.

Determining the leading edge of the wing - measure back from the front edge of the board 5 7/8" and draw a vertical line the width of the board and then trace the wing root rib.

Determining the center line of the stabilizer - this is the same as the thrust line.

Determining the leading edge of the stab - Measure back from the nose 29 5/8" on the thrust line and draw a vertical line the length of the board.

Although this was given for the IMITATION, would this be a guideline for similar sized profile stunters as well?

Sounds just right for an Imitation.  It would probably work for an airplane of a similar size if the quarter-MAC location is the same as the Imitation.  If you scale it up or down, use the ratio of MACs as the scaling factor for the tail length and vertical dimensions, but locate the engine so the CG comes out right.  
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Offline Serge_Krauss

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Re: Design Parameters II
« Reply #1 on: January 10, 2013, 04:37:30 PM »
I'm late, but left message on SSWF. Locked out??? Over there? 'confused. Agree here.

SK

Offline Igor Burger

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Re: Design Parameters II
« Reply #2 on: January 11, 2013, 05:40:40 AM »
and where is part I ?

Offline Tim Wescott

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Re: Design Parameters II
« Reply #3 on: January 11, 2013, 09:20:53 AM »
A side discussion developed about the true meaning of the term mean aerodynamic chord, which lead the originator of the thread to feel that it had been hijacked, so he deleted the thread.

Given the way he worded his questions, I think that what he needs is a basic guide to how to take a plane that works and copy its aerodynamics to another plane.  Basically, the guide would let someone know what's important (aspect ratio, areas, moment arm distances, etc.), tell how to measure things (MAC to MAC, which is what upset the OP), what's not so important aerodynamically (nose length, although it matters to balance and possibly in a minor sort of way given whatever aerodynamic effects a prop has). 

I'm walking a minefield here with the usual denizens of the engineering board, because even before the OP got upset with the MAC discussion it was obvious that what he needs is a simplified guide, while the usual crew here tends to get into all the esoteric little details.  Worse, to some extent the minefield is justified, because this is aerodynamics, which means that some of those seemingly little, seemingly esoteric details actually make a difference.

For all that, I think one could come up with some guidelines that would let an aerodynamic beginner start with a set of plans for an Impact and a goal of (for example) designing a semi-scale stunt Dewoitine D.510, and do a good enough job of transferring the aerodynamically important features of the Impact to his frenchy Warbird that at the very least he's encouraged not to stomp the thing into the ground after the first flight.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Jim Thomerson

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Re: Design Parameters II
« Reply #4 on: January 11, 2013, 10:23:05 AM »
I think I started the MAC discussion, and apologize for any hurt feelings.  I would still like to know the definition of MAC. 

Online Howard Rush

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Re: Design Parameters II
« Reply #5 on: January 11, 2013, 10:36:44 AM »
For all that, I think one could come up with some guidelines that would let an aerodynamic beginner start with a set of plans for an Impact and a goal of (for example) designing a semi-scale stunt Dewoitine D.510, and do a good enough job of transferring the aerodynamically important features of the Impact to his frenchy Warbird that at the very least he's encouraged not to stomp the thing into the ground after the first flight.

Interesting idea.  One could do the same thing for a radio, I guess.  Do you suppose anybody would ever use it?  It would be a lot of work just to @#$% people off that you thought you were helping. 

I don't think I'm up to it.  I struggle just to get an Impact that flies like an Impact.
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Offline Tim Wescott

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Re: Design Parameters II
« Reply #6 on: January 11, 2013, 10:57:55 AM »
Interesting idea.  One could do the same thing for a radio, I guess.  Do you suppose anybody would ever use it?  It would be a lot of work just to @#$% people off that you thought you were helping. 

I don't think I'm up to it.  I struggle just to get an Impact that flies like an Impact.

The ARRL has done it for radios, which is why I can't just shrug and say "it can't be done".  Check out The ARRL Handbook.  People who know nothing about radios can pick up that book, and a year later be building working radios.  Granted, they have to be pretty bright people to start with, and the radios either don't work as well as commercial ones or are vastly simplified, but they're still doing it.

You struggle to get an Impact that slides into home with the same style and panache as Paul's.  My goal would just be to get someone into the same infield as the Impact.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Serge_Krauss

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Re: Design Parameters II
« Reply #7 on: January 11, 2013, 12:57:39 PM »
OK, guys. As moderator, I probably should have been closer to these various threads, but none of us can always be everywhere, and I'm not sure what one could have done. At least I now know something about what happened. Arriving after the fact was pretty confusing.

I agree that MAC is very important here and do feel that a reasonable design philosophy can be mapped out. I'm going to try that, either on a forum or via personal e-mail, but I'm still stuck doing other "stuff". It's easy to write long pieces, but much more time-consuming to be concise and cover the important things at the appropriate level. So my contribution will have to wait. I believe that Howard has addressed some of the specifics well, but I have little idea what was already said about MAC. I'm about ready to look at the adjoining thread, but understand that it can be discouraging to try to glean a consistant understanding by sorting through a lot of diverse material. That's how I learned what I think I know, but it took a long time, and I still get caught up once in a while. Perspective is much of that battle, and that's what I'd aim to create.

BUT...this moderator is bowing out again to finish a very late club newsletter.

SK

Online Howard Rush

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Re: Design Parameters II
« Reply #8 on: January 11, 2013, 01:07:45 PM »
I think I started the MAC discussion, and apologize for any hurt feelings.  I would still like to know the definition of MAC.  

It's probably the most significant aerodynamic parameter for a stunter.

A problem with formulas is they're hard to type here, and finding good references on the Web is tedious.  Here is a general set:

Where b is wingspan, S is wing area, y is distance to the right, x is aft distance from the leading edge at the root, xMAC is aft distance from the leading edge at the root to the leading edge of the MAC,

MAC = (1/S) * integral from left tip to right tip of (c2(y) dy)

xMAC = (1/S)  * integral from left tip to right tip of (xLE(y) * c(y) dy)

yMAC = (2/S)  * integral from centerline to right tip of (y * c(y) dy)

This comes from Roskam's book.  Integration usually assumes wing symmetry and is specified from -b/2 to +b/2 or centerline to +b/2.  I took liberty with the places where the integral stops and starts, because we often have asymmetrical wings.  I may have the wrong sign on x, but I think it cancels out.

To account for flying in a circle, you probably ought to multiply c(y) by (r(y)/R)^2, where r(y) is the distance from the control handle to y, and R is the distance from the control handle to the center of the airplane.  r(y)/R is squared because aerodynamic forces are proportional to the square of airspeed.  This is getting a little picky.  One could get even pickier, but I don't think it's warranted.  

For straight-tapered wings (assuming the usual symmetry), you can find and locate MAC easily with the drawing on this page: http://airfieldmodels.com/information_source/math_and_science_of_model_aircraft/formulas/mean_aerodynamic_chord.htm

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Online Howard Rush

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Re: Design Parameters II
« Reply #9 on: January 11, 2013, 01:17:27 PM »
I agree that MAC is very important here and do feel that a reasonable design philosophy can be mapped out. I'm going to try that, either on a forum or via personal e-mail...

You may be underestimating the task. 
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Mike Griffin

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Re: Design Parameters II
« Reply #10 on: January 11, 2013, 03:58:38 PM »
Thanks for picking it back up Howard.  I am listening and learning.  Thanks

Mike

Offline Tim Wescott

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Re: Design Parameters II
« Reply #11 on: January 11, 2013, 04:11:31 PM »
This comes from Roskam's book.

Title?  Maybe even a full cite?  Do you recommend it?
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Mike Griffin

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Re: Design Parameters II
« Reply #12 on: January 11, 2013, 04:13:14 PM »
Maybe this will help.  Found this today while doing some searches.
Maybe this will benefit everyone.  Click on the link.

Mike


http://webcache.googleusercontent.com/search?q=cache:VP9UDuUCY5YJ:www.control-line.org/admin/Portal/LinkClick.aspx%3Ftabid%3D22%26table%3Ddnn_CsDocument%26field%3DItemID%26id%3D131%26link%3DNetzeband_Stunt_Design_Summary.pdf+&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a

or Google these words:
A SUMMARY OF AERODYNAMIC AND GEOMETRIC DATA
FOR 101 C/L PRECISION AEROBATIC OR STUNT DESIGNS,
ACTIVE BETWEEN 1946 AND 1996.
Prepared by WildBill Netzeband

Online Howard Rush

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Re: Design Parameters II
« Reply #13 on: January 11, 2013, 04:23:20 PM »
That's the one I said I'd look for on another post.   Thanks for finding it. 
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Mike Griffin

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Re: Design Parameters II
« Reply #14 on: January 11, 2013, 05:58:22 PM »

The excerpt below was taken out of the article written by Bill Netzenband.  It sound like he prefers a straight constant chord wing to one with a swept back leading edge.  Am I wrong?

The C/4 SWEEP ANGLE is the accurate way to determine the geometric properties of the wing. By itself it defines the dihedral effect of the wing during yawed flight. When yawed, the swept wing tends to roll in the direction of the yawing. This is not necessarily a positive feature for CL STUNT! My observations of many wing configurations have led me to design wings with a ZERO sweep angle. These seem (to me) to be the most resistant to rattling around in roll during level flight under non-turbulent, steady wind conditions. Turbulence from trees, buildings etc. are best handled by the highly-loaded wings, at maximum comfortable speeds. Wings with REAL dihedral are a game in their own right, and have been successful for several folks. The ability of a stunt plane to be visibly rock-steady under windy conditions is a whole chapter in the book of knowledge.

Offline Tim Wescott

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Re: Design Parameters II
« Reply #15 on: January 11, 2013, 06:05:14 PM »
The excerpt below was taken out of the article written by Bill Netzenband.  It sound like he prefers a straight constant chord wing to one with a swept back leading edge.  Am I wrong?

The C/4 SWEEP ANGLE is the accurate way to determine the geometric properties of the wing. <snip>

Not necessarily.  That's the sweep angle of the quarter-chord (the quarter-chord is the line that goes through the wing 1/4 the way back from the leading edge at each point).  If you sweep the trailing edge forward three times as much as you sweep the leading edge back, then the quarter-chord will neither be swept forward nor back.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Howard Rush

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Re: Design Parameters II
« Reply #16 on: January 11, 2013, 07:54:58 PM »
Furthermore, each of these three sentences is wrong:

The C/4 SWEEP ANGLE is the accurate way to determine the geometric properties of the wing. By itself it defines the dihedral effect of the wing during yawed flight. When yawed, the swept wing tends to roll in the direction of the yawing.

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Mike Griffin

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Re: Design Parameters II
« Reply #17 on: January 11, 2013, 09:37:41 PM »
OK I am confused.  Howard are you saying that Wild Bill was wrong?

Mike

Online Howard Rush

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Re: Design Parameters II
« Reply #18 on: January 12, 2013, 01:24:17 AM »
OK I am confused.  Howard are you saying that Wild Bill was wrong?

Yes. For those three sentences,

1. The dihedral effect of sweep (rolling moment due to sideslip that I keep going on about) is a function of more than the sweep of C/4, the line a quarter of the chord back from the leading edge: it also depends on aspect ratio and taper ratio.  Here's the NACA report I cite every month or so: naca.central.cranfield.ac.uk/reports/1952/naca-report-1098.pdf .  For the dihedral effect of sweep, c/2 (the sweep angle of halfway back on the chord) might be a better rule of thumb, because it combines the effects of wing sweep and taper, but I can't find where I read that.  I have some references, but they wouldn't hold up in court.
 
2. Actual dihedral is an important part of the dihedral effect, too.  The sweep effect is also proportional to lift coefficient.  Here's a reference: http://books.google.com/books?id=AUWCXVepMKEC&pg=PA32&lpg=PA32&dq=rolling+moment+due+to+sideslip&source=bl&ots=sM7l7amIuP&sig=ikwkFTmTxDropmMR3fE1mDkDfvg&hl=en&ei=t3FTS6meNJPSsQOv8qH1Bw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CAsQ6AEwAjgU#v=onepage&q=rolling%20moment%20due%20to%20sideslip&f=false

3. This comes up a lot.  I've read it in the local newspaper.  Here it is again: http://stunthanger.com/smf/index.php?topic=25214.msg243558#msg243558 , post 11 .  Yes, if an airplane is yawing to the right, the left wing is going a little faster and makes more lift.  A much bigger effect is lift due to the increased angle of attack of the upwind wing.  Bill should have left off the "ing" at the end of the sentence-- that's probably what he meant.  Better would be to say that in a sideslip, a swept wing tends to roll in the direction opposite the sideslip.  "Yaw" really doesn't tell you which way the wind is blowing on the airplane.  The reference in #2 is a good description of various "dihedral" effects without a heap of math. 

Bill was given to making authoritative-sounding simplifications that could lead folks astray. 

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Online Howard Rush

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Re: Design Parameters II
« Reply #19 on: January 12, 2013, 01:34:24 PM »
Title?  Maybe even a full cite?  Do you recommend it?

Sorry.  I intended to answer this, but forgot to push Post.  I have two old versions.  This looks like the latest: http://www.amazon.com/Airplane-Flight-Dynamics-Automatic-Controls/dp/1884885039/ref=sr_1_4?s=books&ie=UTF8&qid=1358022001&sr=1-4&keywords=airplane+flight+dynamics+jan+roskam

Roskam is a famous aero teacher at the Kansas U.  He is Dutch and ran a sort of underground railroad taking undergraduates from Delft, training them up, and sending them to Boeing.  One of them-- relating this back to stunt-- designed the vortex generators on the last 737 family.  Roskam was the favorite teacher of the guy who's now CEO of Ford.
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Offline Serge_Krauss

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Re: Design Parameters II
« Reply #20 on: January 12, 2013, 04:37:11 PM »
You may be underestimating the task. 

I responded last night, but apparently never clicked on "post". The clever, eloquent, and lengthy response then was, I think, "'could be."

SK

Online Brett Buck

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Re: Design Parameters II
« Reply #21 on: January 12, 2013, 09:51:21 PM »

3. This comes up a lot.  I've read it in the local newspaper.  Here it is again: http://stunthanger.com/smf/index.php?topic=25214.msg243558#msg243558 , post 11 .  Yes, if an airplane is yawing to the right, the left wing is going a little faster and makes more lift.  A much bigger effect is lift due to the increased angle of attack of the upwind wing. 



   While we are at it, I would also point out that the purported "blanking out" effect of the fuselage is also unnecessary to explain the coupling of yaw into roll. That's an even more common notion than the other three.

    Brett

Offline Ted Fancher

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Re: Design Parameters II
« Reply #22 on: February 12, 2013, 06:25:55 PM »
Snip: 

 A much bigger effect is lift due to the increased angle of attack of the upwind wing. 


We're talking a wing with "real" dihedral here aren't we?  A  stunt ship with no "geometric" dihedral wouldn't have an increased angle of attack due to yaw on the leading wing...or would it?  Since we're constantly flying yawed with respect to the flight path any extra lift would pretty much negate the need for any tip weight and Bubba Hunt could build planes even lighter!

Online Howard Rush

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Re: Design Parameters II
« Reply #23 on: February 12, 2013, 11:24:58 PM »
A  stunt ship with no "geometric" dihedral wouldn't have an increased angle of attack due to yaw on the leading wing...or would it?

Yes.  The more sweep, the bigger that effect.  Stunt planes usually have some quarter-chord sweep, but even rectangular wings with no geometric dihedral have a little negative rolling moment due to sideslip.  You gotta have some forward sweep or a lot of taper to have it be zero.  See naca.central.cranfield.ac.uk/reports/1952/naca-report-1098.pdf, referenced above.

Since we're constantly flying yawed with respect to the flight path any extra lift would pretty much negate the need for any tip weight and Bubba Hunt could build planes even lighter!

You'd need more tip weight downwind without the rolling moment due to sideslip.  Do some eights upwind in a nice breeze with the same tip weight and see the effect of the opposite sign of sideslip.
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Online Brett Buck

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Re: Design Parameters II
« Reply #24 on: February 13, 2013, 09:05:48 AM »
We're talking a wing with "real" dihedral here aren't we?  A  stunt ship with no "geometric" dihedral wouldn't have an increased angle of attack due to yaw on the leading wing...or would it?  Since we're constantly flying yawed with respect to the flight path any extra lift would pretty much negate the need for any tip weight and Bubba Hunt could build planes even lighter!

      IF you could keep it that way, possibly. But trying to do that causes "Twister Disease", where lots of rudder offset generates a lot of yaw angle, and to get the wings level, you take out a lot of tip weight. Swell in level flight. Do the first maneuver, change the line tension, and it noses closer to straight ahead, now wants to roll violently in.

     Bob Hazle's "Medic" had exactly the opposite issue, with the tiniest bit of nose-in yaw (by accident). As you might recall, we put on a bunch of tip weight to get the wings level, but it then hinged severely in the corners. And it was generally evil. We fixed it by taking the cardboard off a glow-plug "bubble pack", bending it in half, taping it to the rudder with the tiniest bit of outboard offset. So we had this teeny little rudder tab in the middle of this big fin with a teeny bit of offset. We also took off about 3/4 ounce of tipweight. Flew it again, it flew wing-down in level flight, hinged significantly less, and was generally predictable. More tip weight out, and it flew fine.

     Brett

Offline Ted Fancher

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Re: Design Parameters II
« Reply #25 on: July 27, 2013, 06:54:35 PM »


Sounds just right for an Imitation.  It would probably work for an airplane of a similar size if the quarter-MAC location is the same as the Imitation.  If you scale it up or down, use the ratio of MACs as the scaling factor for the tail length and vertical dimensions, but locate the engine so the CG comes out right.  

Very late, I know, but the quote was taken from the Imitation construction article and was only intended to be a step by step guide to layout the balsa plank ultimately destined to be "the" fuselage for "an" Imitation.  It was to help the builder cut the holes out to match the plans.

Ted


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