tandem vs. side-by-side considerations

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yankeeclipper

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Pepsi or Coke?

...no, not that kind of "vs" thread. But I do want to hear what considerations there are regarding the choice between them. For example, assuming an LSA friendly design (under 650Kg ;) ), will a .7 meter fuselage vs a 1.2 meter fuselage make an appreciable difference in aerodynamic efficiency? It could make it less towable...

The obvious difference is the social aspect. What else?
 

addaon

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My views:

Side-by-side:
Pros: Better for flying with the girlfriend, both pax near CG, no replication of instruments, huge amounts of space when flying alone
Cons: Higher drag, off-centerline flying

Tandem:
Pros: On the centerline (I love it, personally), easier to put second pax directly on CG for less CG change, lower drag, uses fuselage volume you're "paying for" (weight) anyway, generally wider per-person
Cons: Potentially larger CG range to deal with, less popular, may be narrower than the engine (in extreme cases) which looks weird, harder to get in and out, harder to duplicate all controls (brakes and rudder pedals can be awkward) and instruments
 

wsimpso1

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addaon covered most of it.

Another issue is what baggage you want to carry. My criteria included a bundle with four pair of cross country skis. In a side-by, that means a somewhat long tail cone in, but in a tandem, I would have had to put a ski pod on the belly...

While the x-c skis are pretty neglected lately, that is because snowboards and alpine skis have taken their place, and they fit the smae space pretty well.

My wife is a pilot, and we fly together, with flying, navigation, and radio duties shared, so side-by-side works well for that too.

Billski
 

Canuck Bob

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I'm a tandem fan, learned on a tandem plane and a decent intercom solves the social issue for me.

The tandem configuration is superb for the pilots visibility if it designed for solo operation from the front seat. Rear visibility is great for the rear passenger if its a high wing. I like travelling in a Supercub rear seat but I also like to sightsee more than chat.

I would add though that the modern LSA side by sides are easier to get in, offer large panel real estate, and I believe much better resale demand and price.

I don't think the performance difference is enough to make that the deciding factor. Kitplanes compared two similar RV's with each config and the differences between say a Champ and Chiel are very marginal.
 

Inverted Vantage

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Hard to get a nice looking side by side though, if you're looking for a sporty sexy look...
 

Inverted Vantage

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I guess, yea. Generally you're gonna get what essentially looks like a sexy airplane ran into a wall, though :D

Here's a question by the way; does anyone have tips for balancing a puller tandem aircraft so that the pilot is actually in the front seat when no passenger is present? Just wondering if it's even possible on light aircraft (I know the Shorts Tucano can do it, but that's not really "light").
 
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radfordc

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My views:

Side-by-side:
Cons: Higher drag, off-centerline flying

Tandem:
Pros: On the centerline (I love it, personally)
There is a side by side design that does allow for center-line solo piloting. The Sonex when configured with a single stick allows for either side by side seating for two; or when solo the pilot can fly from the center. You use the two outboard rudder pedals for yaw.

Charlie
 

yankeeclipper

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Some good input, thanks.

Any thoughts on whether the aerodynamic losses are substantial in a side-by? I see many high performing motor-gliders now in this config, such as this noe (not bad on sex-appeal either):

Pipistrel - Gallery

Though this is undoubtedly a very high performing glider (motor-retracted), would it have gained much in performance by "pulling it" thin? Yes, I know, I crossed the River Vague a while back, but I'm really driving at the difference between the two configs, rather than the limits of one or the other. If a design doesn't stand to gain much efficiency (or...) by going thin, why do it, balancing aside?
 

addaon

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If you're going for a high-performance sailplane, you can probably tell the difference in glide if you measure it accurately. If you're powered, you'll be hard pressed to notice the difference in fuel consumption.
 

Topaz

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Hear is a great looking (if not a baby Lancair) relatively fast EAB or LSA (just this year) 2-place.
Pretty quick to build, seemingly good support and 40 or 50 flying (completed).
Lightning Sport Plane by Arion Aircraft
Larz
Wow. Looks like the legacy Lancair 360 with fixed gear and a slight change to the lower vertical fin. Lance Neibauer would be pissed if he were still associated with Lancair...
 

bmcj

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It's amazing how a successful plane spawns copycat companies, like the Varieze (Cozy, E-Racer, Berkut, etc), the Lancair (Lightning, etc), and the Avid Flyer (Kitfox, Highlander, etc).

Bruce :)
 

Denis

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Some good input, thanks.

Any thoughts on whether the aerodynamic losses are substantial in a side-by? I see many high performing motor-gliders now in this config, such as this noe (not bad on sex-appeal either):

Pipistrel - Gallery

Though this is undoubtedly a very high performing glider (motor-retracted), would it have gained much in performance by "pulling it" thin? Yes, I know, I crossed the River Vague a while back, but I'm really driving at the difference between the two configs, rather than the limits of one or the other. If a design doesn't stand to gain much efficiency (or...) by going thin, why do it, balancing aside?
The tandem configuration has higher drag than a best optimized side by side. Indeed one needs to obey certain design rules to utilize the full potential of side by side layout. The wing should cross the fuselage at its widest section, the fuselage width must be constant along the root chord and the wing to fuselage interface must be ideal for reducing the interfrence drag. The Pipistrel motorgliders are the examples when these requirements are completely fulfilled thanks to the advantages of high-wing configuration as a major factor.

If we imagine such plane reduced to a single seater, by narrowing the fuselage, we'll get almost zero reduction in equivalent drag area. This result is explained by the decrease of wing area covered by fuselage, which is excluded from the total wetted surface. the profile drag of this area now placed into airflow will almost or completery compensate the decrease of the fuselage drag due to its reduced wetted area. Such reduction is small here due to this particular fuselage shape, which has maximal width and perimeter only around the cabin, but similar conditions apply to nearly all light planes designed with great attention to parasite drag reduction.
If now we'll make a tandem two place cabin in this narowed basic fuselage, we'll inevitably increase its wetted area without any compensation from covered part of wing area. This will result in an increase of the airframe drag area. The smaller frontal area will not help.

Denis.
 

Inverted Vantage

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Well to be fair a lot of GA planes look very similar; look at a lot of the LSP's coming out, they all look like the same plane practically :)

As for tandems, I really like the idea of them; only flown in them twice (a 1930s Tigermoth, and a Waco biplane of a type I don't remember), I personally think it's great. Yea it's less social but it's much more about flying with those aircraft. Plus I think a single cockpit where the back seater is up against and above the front seater really negates the social issue quite a bit. It's only when you get those cabins that actually have the people separated by fuselage does it get solitary.
 

gschuld

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Back in the states UnknownTarget?

I do find it interesting that a plane designed 25 years ago still appears to be the basis for so many planes. The looks and performance of the original Lancer/Lancair 200 is hard to beat, even today.

Denis's post suggesting that a properly designed side by side would offer less drag than a tandem really got my attention. I wonder if this is the prevaling opinion of aircraft designers? Though the total whetted area of a comparable scale side by side and a tandem may be fairly close to equal, the total frontal area of a side by side would certainly be much greater. Not easy to overcome I'd imagine. I suppose the extent of the width of the side by side would skew the comparison somewhat.
 

orion

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This is really where attention to detail does pan out. While it is true that the larger frontal area of the side-by-side loft and the higher projected area of the windscreen might present a drag penalty, with proper shaping of the of the body components, this could be minimized to be on the same level as that of a tandem, since there you'll most likely have more wetted area simply due to the larger (longer) cockpit. The tandem will of course have less frontal area but the typical longer length may counter the smaller cross section's benefit.

I don't think that this is something that can be analyzed off the cuff since I think that both configurations can be optimized to deliver nearly equal numbers. However, if you fix some variables like overall length (equal tail volume coefficient), the tandem will most likely come out ahead. So here I'm assuming that the two configurations would be optimized as required.
 

Inverted Vantage

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Yep gschuld, just got in yesterday :)

As for the tandem vs. side by side, I'm pretty sure it boils down to what the designer wants. Seems to be that no matter how you try to spin it, it always comes to them being about the same after proper analysis.
 

Denis

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Tandem never can be optimized for drag reduction. to same extent than side by side. There is no direct relationship between drag and frontal area for the complete airframe. Such dependence can be established only for individual parts, which are simply saclable and are particularly dirty, like open LG wheels. Their drag is mostly determined by flow separation at the most part if their surface.
The optimal length of the tail section is also quite short. A well optimized side by side plane will never benefit form longer tail. The total drag fo aft fuselage and tailplanes varies just slightly, anf the more important design criteria are minimal weight and moments of inertia while maintaining necessary stability margins and control authority.
 
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