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handprop

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What a nice airplane! Not many airplanes offer the visual advantages the wings offer. Mike
 

bmcj

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Personally, I have mixed emotions about the benefit of forward sweep in regard to visibility. Yes, it may give you a little better view upward (for a high wing) or downward (for a low/mid wing), but the tip is now forward of its original location and it obscures your view laterally. I'm just saying...

Bruce :)
 

Jim Williams

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Atlanta ga.
I want to thank all how critique my design. But gentlemen Iam not trying to design a jet aircraft only a light sport aircraft weighting no more then 850 lb. GW. If I am reading it right, I should try to have the wing airfoil AC close to the aircraft CG. but not ahead of it. I would appreciate any help on this design.You all know more about aircraft design then I do. Thanks. Jim W
 

Canuck Bob

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Jim you want the CG and the wing's center of lift, the MAC (I believe the Mean Aerodynamic Chord) to corespond to a range of a few inches of each other. It is stated as a percentage of the chord. The CG falls within the chord line NOT ahead of the wing. The flying plane balances within a small range of the wings center of lift say about the 25% chord point.

A straight rectangular wing will have a MAC that is the chord. A common range for the CG might be 10% to 30% of that chord from the leading edge. With a 60" chord the CG would fall within a range of 6" and 18" behind the leading edge.

With your design the CG must fall within a range behind the leading edge or the MAC if you have a swept or tapered wing. With the pilot and engine forward of the leading edge it makes balance a bit tricky as the plane is quite nose heavy. The actual range is a function of the design, sizing and location of the tail feathers.

That is why some people went to forward sweep when facing this balance problem. It is simply a way to sit forward of the midwing spar and have the center of lift moved forward to help with the balance issue.

I never wanted to suggest needless complexity to your design process. I can think of only one small aircraft in the proposed size range that has the pilot and engine as far forward of the spar as in your drawing that does not have a forward swept wing or moved the engine behind the wing. That is the Aurore Souris-Bulle MB 04 from France.

Air Souris Set - le site officiel de l'association Air Souris Set

The design has a VW based engine and the pilot ahead of the wing and a very long fuselage to place the tail weight far to the rear to offset the weight forward of the wing for balance. So you can find ways to solve the balance issue with a simple straight wing.

All the rest of the succesful designers I can think of, faced with this problem, resorted to forward swept wings, using configurations that kept the engine weight as close to the wing as possible, and light weight engines. Hope this helps.
 

orion

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Just make sure to calculate the allowable CG range and don't use rules of thumb. The above examples may be OK to use with something like an RC airplane or as a first cut guess, but certainly nothing you want to sit in. The values that represent the allowable CG range will vary greatly as a function of section, flap size and deflection, tail geometry and position, etc.

You will need to calculate all this about the ac of the MAC, since the ac is that point along the chord where the pitching moment is constant regardless of the angle of attack. And for accurate calculation, you should make sure to know the position of said ac - it's generally not considered "close enough" to assume it's at the quarter chord point. True, it's possibly not too far off but it's better to know for sure than not.
 

Jim Williams

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Iam sorry, I didn't make myself clear. The design airfoil is a 4412. The MAC is 42" and the AC [ Aerodynamic Center] some times called the neutral point is about 25% of the a MAC. Sweping the wing forward 3-5 degrees would move the AC forward. I know the aircraft CG should be forward of the AC. But what should the range be in inchs?
The parameters for the Aircraft are.
EW=480 lbs.
GW=800lbs.
1 seat
60-65hp VW engine
Wing span=24ft.
Wing area=84 sq ft.
Stall speed=45-50mph [hope]
Cruise speed=115-120 mph
Range=350 mi.
Fuel=15 gals.
All metal-Strut braced wing.

Thanks again. JimW
 

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orion

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The MAC is 42" and the AC [ Aerodynamic Center] some times called the neutral point is about 25% of the a MAC. Sweping the wing forward 3-5 degrees would move the AC forward. I know the aircraft CG should be forward of the AC. But what should the range be in inches?
1) The Aerodynamic Center is never referred to as the "Neutral Point". The Neutral Point is vastly different and must not be confused with anything else except what it represents - it is that position on the MAC where the airplane is neutrally stable (static stability criteria). If the CG is located forward but near the NP it may be twitchy and difficult to fly. If it's at the NP it will have no significant stabilizing behavior and will most likely be close to unflyable. If the CG is past the NP then the airplane is unstable and uncontrollable.

2) The aerodynamic center is as I described previously. It can be located near the 25% chord point but for most airfoil sections it rarely is. It's generally not a good idea to calculate stability criteria based on a guess. You should get sufficient data on the section you plan on using to accurately define where that point is. Fortunately, Theory of Wing Sections does provide that point.

3) The CG does not have to be forward of the AC but it must be forward of the Neutral Point, usually at least 5% MAC or more.

4) The range of allowable CG envelopes varies significantly depending on quite a few variables associated with the airplane configuration and the mathematical derivation. I've seen forward CG envelope limits vary from close to the leading edge (about 5% aft of the MAC leading edge) to nearly the quarter chord point. And I've seen the Neutral Point locations vary from about 28% MAC to about 45% MAC. This is why you don't want to guess here.

Even though your airplane might be relatively conventional, this is one area that might bite you. do your homework and take your time in doing this bit of derivation. About a month ago i posted an excerpt from one of my programs that shows some of the calculation steps used in deriving this part of the airplane's characteristics (I forget now which thread that was in - anyone?). Doing it right does involve a bit of work.
 

lr27

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Just to make sure I've got the terminology down (I'm pretty sure I've got the principles down):
-the AC is a property of the wing?
-the neutral point is a property of the whole aircraft?

-or is it that AC is for the aircraft geometrically, and NP accounts for pitcing moment and stuff?

I've flown models of conventional tractor configuration where the c.g. was at greater than 100 percent MAC. It's true they had very large tails.
 

orion

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Just to make sure I've got the terminology down (I'm pretty sure I've got the principles down):
-the AC is a property of the wing?
-the neutral point is a property of the whole aircraft?

-or is it that AC is for the aircraft geometrically, and NP accounts for pitcing moment and stuff?

I've flown models of conventional tractor configuration where the c.g. was at greater than 100 percent MAC. It's true they had very large tails.
The ac is the property of the airfoil and also of the whole wing - part of the stability calculation involves finding the ac position of the three dimensional wing. For a rectangular planform it's pretty much the same as for the airfoil section but I have seen slight shifts also, so just in case, it's a good idea to go through the calculation exercise anyway.

Yes, the Neutral Point is a property of the stability analysis for the whole airplane and it represents the basis for the aft-most limit of the allowable CG range (the aft limit is generally set some distance in front of the forward-most Neutral Point number - usually it's the stick-free case). It combines effects of pitching moments (wing, tail, fuselage, power effects, ground effect, etc.), controls and control effectiveness factors. There are several Neutral Point variations including power-off, power-on, stick-free, and stick fixed.

The other end of the allowable CG envelope (the forward limit) then includes flaps up, flaps down, ground-effect, and of course, free-air.

And assuming I'm reading it right, your last statement is highly improbable. Which airplane are you referring to?
 

lr27

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You're reading it right, but it's QUITE probable, since I've done it. Note that I said models, and in models, we have a bit more leeway to mess around. Many rules for events say you can have a stabilizer up to 50 percent of the area of the wing. If you put that on a long boom, the c.g. ends up WAY back. In events like EZB, you can have the c.g. at 150 percent MAC or so. Imagine a wingspan of 18 inches, a length of perhaps 20 inches, with the horizontal stab all the way back and the wing only an inch or so behind the propellor!

Here's a fairly typical example:
http://www.indoornews.com/uploads/lbarr_millerlite_ezb.pdf
Note the c.g.!
snip
And assuming I'm reading it right, your last statement is highly improbable. Which airplane are you referring to?
 

bmcj

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That plane (ezb) is a microlite with dual lifting surfaces. CG's between dual lifting surfaces is normal (or at least common). It is, in essence, a canard with the canard sized larger than the wing.

Bruce :)
 

orion

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Sorry, my mind (and I was assuming our discussion) was rooted in conventionally configured aircraft. Yes, it is true the cg can be located in a wide variety of locations (including off the main wing), depending on wing sizing and loading, and lifting configuration, but the actual "allowable" range of motion tends to be very small for those layouts in order to still arrive at a safe and controllable airplane. Case in point, the allowable ranges of most canards are about three quarters to half of what an equivalent conventional airplane would have.
 

etterre

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St. Louis, MO, USA
1) About a month ago i posted an excerpt from one of my programs that shows some of the calculation steps used in deriving this part of the airplane's characteristics (I forget now which thread that was in - anyone?). Doing it right does involve a bit of work.
Ironically enough, it was a month ago on this thread - just a couple of pages back.
http://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/5175-critique-my-design-4.html#post41309
 

lr27

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Hope this only comes through once. THe computer is giving me funny messages


Well, I did write that the tail was very large. Not sure where the borderline of conventional is. Mark Drela has a chart that shows everything from a canard to a conventional configuration to a flying wing as a continuum. I seem to recall that the ends are actually equivalent and you could cut and paste it* into a continuous loop.

*Cut and paste in the old sense, i.e. with scissors and paper.

Sorry, my mind (and I was assuming our discussion) was rooted in conventionally configured aircraft. Yes, it is true the cg can be located in a wide variety of locations (including off the main wing), depending on wing sizing and loading, and lifting configuration, but the actual "allowable" range of motion tends to be very small for those layouts in order to still arrive at a safe and controllable airplane. Case in point, the allowable ranges of most canards are about three quarters to half of what an equivalent conventional airplane would have.
 
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