# How would you define "close-coupled"?

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#### rtfm

##### Well-Known Member
Hi,
While I intuitively understand what close-coupled means, I can't find any reference to how this condition is defined. Visually one can look at an aircraft and say "that plane is close-coupled" - but this has to do with memory and familiarity more than anything else. Would a given plane which appears to be close-coupled, and which had its nose lopped off in a fit of design weirdness suddenly appears no longer to be close-coupled, even though the relationship between the wing and the h-stab has remained constant.

What defines close-coupled-ness?

Confused.

Duncan

#### Dana

##### Super Moderator
Staff member
Y'know, that's a good question. I don't know if there is an exact definition... we just all "know" what it means. I've heard the term apply to cars and boats as well. I would say it relates to the distance between the wing and tail (it applies to rudder and yaw as well, important in a taildragger!), relative to the size / moment of inertia of the whole aircraft.

-Dana

"Naked" means you ain't got no clothes on; "nekkid" means you ain't got no clothes on - and are up to somethin'.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
From a intuitive point of view, I might use these two figures of merit:

(Tail arm)/MAC;

(Mass moment of Inertia)/ (tail area*tail arm).

Each of these requires that you use coherent units. In pitch, use the arm from CG to the 1/4c point of the horizontal tail, mass moment of inertia in pitch, and are of the horizontal. Once we have computed these for a few well know airplanes, we can check it out. Perhaps someone already has...

The first puts how far back the tail is in terms of the wing size. There are some amazingly short airplanes that do not have a reputation for feeling close coupled. The entire Rutan canard family and all of its derivatives, the Questair Venture and its derivatives, and others.

The second puts the amount of tail power in proportion to mass it has to move.

From a more intellectual perspective, the airplanes that feel twitchy might have another set of metrics that would correlate. One of the most common is tail volume coefficient. Pazmany uses:

Pv = (Aht*Rht)/(Aw*MAC)

Another is tail damping coefficient:

Dv = (Aht*Rht^2)/(Aw*MAC)

And yet a third is static margin:

Lsm = Lnp - Lcg

The tail volume coefficient just indicates if you have enough tail. Tail damping coefficient indicates how deadbeat the response to control inputs, gusts, etc will be, and static margin indicates how much tail moment will be required to move the airplane off the equilibrium point a given amount.

Pazmany listed a bunch of the statistics in his book, and then calculated the volume coefficients. You could take his terms and also calculate the damping coefficients, and then get flight impressions on each airplane, and develop some correlation. Or perhaps it has already been done.

Billski

#### orion

##### R.I.P.
The term "close coupled" is commonly used in design circles but outside of that, I don't recall ever having come across a specific reference or text that addresses the issue. In the design process we tend to aim for a specific set of behaviors that result in an airplane that is pleasant to fly and controllable, characteristics that are defined by either historical data, the FARs or results of flight testing from organizations like the CAFE Foundation.

One of the difficult aspects of the term is that it is not consistently applied so airplanes that exhibit some of the characteristic behaviors may get away with it just simply because the pilots operating them have gotten used to the quirks and don't recognize (or refuse to admit to) the issue as a problem.

In other cases the issue was recognized during the design process and was accounted for in the controls and as such, the behavior is not in evidence in the final product. These aircraft are usually shooting for some type of specific behavior or performance and thus are willing to live with the compromise. That however does not mean it's a viable approach for a wider market.

As is often pointed out, history is a great teacher. If we look at the line of certified and experimental products that have come forth over the decades, we can observe a trend in layout and proportions. These have evolved almost in a sort of Darwinian process, the most successful surviving due to the physical aspects they create for the aircraft. Significant deviations from this "norm" can therefore be somewhat risky, but not so much from a flight control or safety standpoint but more so from the standpoint of customer acceptance. But the former may be an issue so it is up to the designer to make sure the resulting product matches some level of established criteria.

The approaches to this criteria can be varied, which is of course why examining historical or regulatory requirements is good for establishing at least the guideline to shoot for. For instance, I like the way the CAFE Foundation presents its data - the characteristics of the flight tested aircraft are compared directly with equivalent data from other airframes. That way you can make a quick and direct evaluation of behavior and flight quality.

#### rtfm

##### Well-Known Member
Pv = (Aht*Rht)/(Aw*MAC)

Another is tail damping coefficient:

Dv = (Aht*Rht^2)/(Aw*MAC)

And yet a third is static margin:

Lsm = Lnp - Lcg
Billski,
Hi. Interesting formulae, and ones I'd like to play with. Since I don't have Pazmany, could you define what the variable names stand for? I can guess at what they stand for, but I'd only be guessing! :grin:

Cheers,
Duncan

#### Captain_John

##### Well-Known Member
Close coupled is a relative term.

I am no En-ga-near, but I can explain it to you in plain english.

I am a pilot who flies many types of aircraft and a builder of what I consider to be a closely coupled airframe.

To me, it refers to the the distance between the rudder and the vertical axis point and how much AUTHORITY the rudder has upon it.

So totally relative is this definition that every airframe is compared to all others. For instance, I fly the Cherokee Six and the Aeronca Chief.

The Cherokee Six is a long plane with a good sized rudder that is placed about 16 feet from the vertical axis. When you stab the rudder hard, it takes a while for the nose to respond.

The Aeronca is much shorter. Maybe about 9 feet from the vertical axis. It is also a taildragger and authority is generous. A good stab on the rudder moves the nose A LOT!

The Kitfox is even MORE sensitive than the Aeronca!

Not a lotta math or data in this definition, just some pilot's eye perspective.

Hope this helps!

CJ

#### orion

##### R.I.P.
. . . . . although the term usually tends to refer to control in pitch. The most common result of a close coupled airplane, especially one that does not have a whole lot of "stick feel", is a pilot induced oscillation.

For the case of the vertical, it's usually a matter of learning how to fly an airplane that has an effective rudder. One of my instructors referred to students who wee transitioning from a typical Cessna or Cherokee to something like a SuperCub as those who were about to find out what the rudder is really for.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Pazmany wrote a really terrific book. Lessee here:

Pv = (Aht*Rht)/(Aw*MAC)

Pv (should have been Ph) for Horizontal Tail Power Coefficent. Thurston and others also write about this. For the Horizontal Tail, they always formulate it the same way. For the vertical tail there are a couple ways of doing it, one with Wing Area, the other with wing span. The scale of the coefficient changes, as well as what is considered acceptable.
Aht for Area of Horizontal Tail
Rht for distance from CG to 1/4c point on Horizontal Tail
Aw for Wing Area
MAC for mean aerodynamic chord (wing)

Dv (should have been Dh) is Horizontal Tail Damping Coefficient - Terms are same as for Tail Power.

Lsm/MAC = (Lnp - Lcg)/MAC where

Lnp is position of Neutral Point
Lcg is position of Center of Gravity
MAC is Mean Aerodynamic Chord

This gives the static margin in % MAC, which normalizes the term. This tells us how much margin we have to maintain stability and positive stick gradients.

Billski

#### rtfm

##### Well-Known Member
Pazmany wrote a really terrific book.

Billski
Hi,
So I guess this means I'm going to have to invest in this one as well? And I thought the actual building was going to be the expensive bit... :gig:

BTW, Raymer has software available for purchase from Amazon:
http://www.aircraftdesign.com/rds.shtml

I was thinking of getting it. Do you have any experience with this? Any stories (horror or otherwise?)

Regards,
Duncan

#### orion

##### R.I.P.
Personally, I have a high regard for Dan Raymer's work in that not only is he experienced in the field but also in the fact that many of his design/analysis methodologies are very similar to mine and work well in the configuration development discipline (most aero approaches are more optimized to analysis, not design).

The RDS design package he developed should work for all aspects of this field, assuming you can afford it. I notice that the web site does not list prices (I think it used to) but if I recall right, it was not cheap. The other concern I might have is that it runs under a DOS shell rather than in Windows and as such, has some of the peculiarities associated with that operation. I don't know to what extent this may be a problem, if at all, but reading the write-up he has at the web site suggests that there are several issues to deal with.

Something that might be a bit more affordable yet still useful is Airplane PDQ from DaVinci Technologies (www.davincitechnologies.com). I've spoken with the developer several times over the years and found his approach reasonable and quite comprehensive considering it's a conceptual tool. for your particular application, I think the data you'll get here should provide you with a fairly good level of verification and comfort.

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#### rtfm

##### Well-Known Member
Hi,
You're a good man, Mr Orion. Thanks for the background. The RDS software comes in a Student Version (about \$100 USD) so that's not too ororous. I also checked out AirplanePDQ and it is downloadable for free 15 day use. Mmmm Interesting. Since most of the hard yards have already been run as far as the design is concerned, having access to AirplanePDQ for two weeks worth of confirmation would be handy indeed. Especially since it will be free.

However, the download link is broken, and I've written to the web site folks to take a look and get it fixed.

Cheers,
Duncan

#### Hugh Lorimer

##### Well-Known Member
Hi Duncan, see the My random ideas thread for an example of a close coupled design of mine. I would define close coupled as an a/c with a short distance from the ac (wing) to the ac (stab).The formula I used for all my designs is the Volume coefficient formula , which can be seen and explained on Drg. L1-a (Iolaire) on www.hughlorimer.co.uk
Hughie.

#### endwood

##### New Member
Hi All,

In automotive design language 'close-coupled' is a technical term also defined as Point H - this being the distance from the hip-joint of the seated driver to the rear axle. A modern example of this would be the BMW Z series sports cars where that distance is quite short.
How this translates to aircraft is not quite so obvious but like others who have contributed to this thread, the distance from the pilot to the rudder appeals as logical.

#### Joe Fisher

##### Well-Known Member
Close coupled is the relative distance to the tail from the wing in relation to the wing span. Close coupled are planes are usually less stable than those with more reasonable coupling.

#### bmcj

##### Well-Known Member
"Close coupled" is sometimes also used regarding the distance between the main landing gear and the tailwheel (as it can affect ground handling), but more often than not, it is used for the reason Joe stated because it can affect the sensitivity of the elevator.

#### ultralajt

##### Well-Known Member
My personal opinion on the saying "close coupled" is such, that a plane is short coupled when a static stability is so low, and pitch reactions of controll inputs so fast, that ordinary level flight became a bit scarry.
Why scarry?
Because pilot cant thrust to aeroplane that will remain on the flying path excatly..without accidently pitching or diving a bit, and as pilot reactions to counteract, can lead to pilot induced oscilations.

And as we all know, that tails on the long arm generally lead to very stable aeroplane and short dont, an eyeballing (some common sence for aroplanes aesthetic also is needed) of the particular design gives a hunch if aeroplane is short coupled. Of course there are some people that claim, short coupled aeroplane could fly well if horizontal surfaces are of sufficient area to get decent moment with short arm, but such aeroplanes are "twichy" for average pilot anyway. And that "twichy" behaviour tells us that even so (short arm but large tail area), that aeroplane is "short coupled".

Mitja

#### Aircar

##### Banned
Semantics aside I would categorize an aircraft as short coupled if the wing has a major secondary effect on the stabilizing surfaces (the primary effect is simply the 'dart like' effect of tail feathers dCl tail/DcL wing ) --where the tail is close to the wing there is a significant mutual interaction and 'close' is best measured in wing chords as this is the denominator in stability equations . Probably anything less than 3 wing chords to the tail is getting short coupled with one or two definitely so --the flying fleas are classic examples but are not twitchy or hard to fly because the control is not by a small flap with low hinge moments amongst other things . Are flying wings 'short coupled' by this definition ? -- the 'couple' involved is force times distance to point of application so I guess this is so (actually a couple is a set of forces that produce no net force but only a turning moment which approximates an aircraft elevator short period reaction and even though there can not be any interaction between two flying surfaces on a tailess aircraft it can be said that the wing root and the tips are reacting each other.

An aircraft like the Cassutt racer or the GeeBees are distinctly short coupled by the 'number of chords' rule and exhibit the over sensitive behaviour Mitja describes whereas the Questair is short but not short coupled due to high aspect ratio for comparison.