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After seeing the below image of the "Landseaire" flying yacht here I was curious to know why the small boat mounted to the underside of the plane was mounted the way it was. At least to me (and @Dave who mentioned it in his answer containing this picture), I found it interesting that the boat would be mounted "backwards" and also so far from the body of the aircraft. Can someone explain why this position and orientation was necessary or why it was chosen?

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    $\begingroup$ If you want to read more about the story of this specific aircraft read that. Unfortunately it doesn't give any reasons about why the boat is mounted like that, although one photo seems to suggest that the contour of the boat matched the wing closer in that configuration. $\endgroup$ – Ron Beyer Dec 19 '17 at 19:55
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    $\begingroup$ I doubt it was due to the mounting points on the wing and the boat, and it was simply easier to mount it that way. It may not be a performance / aerodynamic decision. $\endgroup$ – kevin Dec 19 '17 at 21:13
  • $\begingroup$ Maybe they planned to build a boat leading edge out of foam, but never had time to do it. Like done on space shuttle trailing fuselage on 747 cargo plane. $\endgroup$ – qq jkztd Dec 19 '17 at 21:41
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    $\begingroup$ My first thought was that it keeps the rate of change in the cross section smooth, as it appears right behind the engine. Then again, I highly doubt this aircraft was worried about supersonic performance =) $\endgroup$ – Cort Ammon Dec 19 '17 at 22:19
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The reason is the "flat tail" of the boat. The drag this "flat tail" produces when mounted "reverse" is much less then the dynamic drag it would induce the other way around.

https://en.wikipedia.org/wiki/Drag_(physics)


I don't see how the link helps explain why the flat side produces less drag than the pointed side.– Ron Beyer

The published cw coefficients (see answer of @jwzumwalt ) show the best value for the drop shape facing towards the air stream.

There is no entry for a reversed drop. But it is evident that is would have a higher cw coefficient.

A closer look at the boat reveals it also has a drop shape. But this is facing to its tail. Therefore the boat creates less drag when mounted "reverse".


If this was the case, wouldn't the boat cause less drag moving backwards through the water also? – andy-m

It might but the usually draft of the boat is no so deep that a relevant part of the "flat tail" is actually in the water. enter image description here source: https://upload.wikimedia.org/wikipedia/commons/2/2d/Bundesarchiv_B_145_Bild-F056332-0004%2C_Bonn%2C_Bundesgartenschau%2C_Seen.jpg

On the other hand a little turbulences at the boats tail could help to keep direction in the absence of a steering paddel...


This explanation is wrong - an anemometer spins with the forward facing blunt half of the hemisphere causing the most drag - en.wikipedia.org/wiki/Anemometer – jwzumwalt

The most important differences between the anemometer an the drag experiment is:
The half bowls in the anemometer are hollow which "catches" the wind way better than the flat surface of the filled half bowls in the experiment.

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    $\begingroup$ I don't see how the link helps explain why the flat side produces less drag than the pointed side. It could be that this is correct, however it seems more like a statement than any kind of explanation as to why... $\endgroup$ – Ron Beyer Dec 19 '17 at 15:41
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    $\begingroup$ If this was the case, wouldn't the boat cause less drag moving backwards through the water also? $\endgroup$ – andy-m Dec 19 '17 at 16:43
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    $\begingroup$ If the way the boat is fixed is more aerodynamic than the other way, then you should explain this to Nasa. They believe a prism with the flat face ahead generates 4x the drag of a bullet (though the Re number is not mentioned) :-) $\endgroup$ – mins Dec 19 '17 at 19:26
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    $\begingroup$ This is interesting. You cite a demonstration which shows something rather peculiar. jwzumwalt shows a chart of Cd for various object which shows the more intuitive answer. How exciting! Two answers that disagree, AND cite their sources =) $\endgroup$ – Cort Ammon Dec 19 '17 at 22:18
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    $\begingroup$ @CortAmmon: Fluid dynamics (including aerodynamics) are always dependent on the flow regime. In particular, neither case mentions the Reynolds number (aka Re number, as mins calls it). $\endgroup$ – MSalters Dec 20 '17 at 8:13
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With regards to the lesser question as to why it is mounted so far from the body, in this case it's actually the only available place; the Catalina is amphibious so mounting it on the fuselage/hull would cause issues with off-water operations. The internal structure of the wing also needs to be considered: the boat can only be attached were there is a structural hard point, and some method of releasing the boat. In this case, the boat is mounted where the underwing stores would normally be fitted, so there's already a release mechanism and internal supporting structure: underwing.jpg

As for the primary question, my phd is in hypersonic design so I'm a little fuzzy on subsonics, but although mounting the boat backwards increases frontal drag, having the sharp prow facing aft reduces flow separation. flow separation reduces pressure drag This would help to reduce wake turbulence and reduce buffeting over the tail. Possibly it also minimises disturbances in the prop wash, since the boat is mounted so close to the engine. In this case, although the flat front isn't optimal for drag, the whole boat is shaped more like a tear drop fairing, with a sharper trailing edge. Without wind tunnel data or cfd analysis to back up this theory, it is only my surmising: as others have stated, it could simply be the contour of the boat matched the wing better that way.

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    $\begingroup$ Any reason why they couldn't have made the entire boat to be a little more aerodynamic or put in an aerodynamic shell? $\endgroup$ – dalearn Dec 20 '17 at 1:22
  • $\begingroup$ @dalearn yes, possibly. But with the bureaucracy that goes into military decisions when it comes to design changes, getting a completely new boat design certified would have been a hassle. They could have just as easily attached a disposable fairing to the boat to make it more aerodynamic, if it was really an issue. Custom lifeboats for asr work definitely did exist. The B-17 was used for asr with a custom boat. link $\endgroup$ – Nathan Dec 20 '17 at 1:29
  • $\begingroup$ @Nathan Do you think the boat was part of the original military airplane? Or was it added later? $\endgroup$ – user7241 Dec 20 '17 at 10:39
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The attachment of the boat as seen in the picture to reduce drag might seem to be common sense. However truth is sometimes stranger than fiction. We are given the drag of various 2d and 3d objects on page 3-17 of "Fluid-Dynamic-Drag" by Hoerner.

Both the 2d and 3d objects with the blunt end towards the rear - just like the boat moves through water - has the least drag. The drag in the water is going to closely approximate the drag in the air, and the boat should have been pointing into the wind to have the least drag.

In the 3d drag profile the #2 & 3 illustrations have CD of ~.40 while the opposite direction (#8 & 9, blunt first) has a Cd ~1.20, or about three times as much.

If the intent was to reduce drag, the designers or mechanics who installed the boat without wind tunnel data got it wrong! But, it is also possible the boat is situated in that direction due to the prow being higher and fitting the wing camber better. The boat does appear to match the camber quite well. Note the curve of the boat rail in the other picture.

We can look at a real world example and see that the wind tunnel data is correct. Hoerner in the same book provides Cd for various "boat tail" bullet shapes and they also have the least drag with the blunt end trailing. Bullets could use a different shape than we are accustom to, but tests show a bullet with a sharp point and blunt rear have the least Cd.

In fact, the Bell X-1 was "a 'bullet with wings', its shape closely resembling a Browning .50-caliber (12.7 mm) machine gun bullet" - https://en.wikipedia.org/wiki/Bell_X-1

An anemometer spins because the forward facing blunt half of the hemisphere has the most drag, meaning the boat is in the wrong direction for the least drag - https://en.wikipedia.org/wiki/Anemometer

Note the triangular shape with its sharp corners is also counter-intuitive to what we would normally expect.

I think the drag chart accurately depicts the boats drag. What appears to be causing a misunderstanding is we normally find a forward facing blunt blended body to have the least drag, however in this instance the boat has sharp corners that can "trip" the otherwise smooth flow. The sharp corners make this an exception to the rule as depicted in the drag chart.

enter image description here

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    $\begingroup$ Aerodynamics is very different at 108 knots of Catalina's cruise speed, 832 knots of Bell X-1's max speed and 1730 knots of bullet leaving the muzzle of M2 machine gun. The first case is subsonic, where the trailing edge matters a lot, while the later two are supersonic, where separation always occurs after the thickest point anyway and most important is sharp point to minimize contact with the shock wave. Also your image does not really show anything really resembling the boat. $\endgroup$ – Jan Hudec Dec 19 '17 at 23:02
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    $\begingroup$ You are grossly misinterpreting the chart. Streamlined fittings, wheel pants, etc are installed rounded end forward and tapered end back. The boat approximates that shape and is installed in the orientation matching everything else of similar shape. $\endgroup$ – Chris Stratton Dec 20 '17 at 16:51
  • $\begingroup$ @JanHudec I'd say that the boat kinda resembles the 'streamlined' image $\endgroup$ – Aequitas Dec 21 '17 at 1:42
  • $\begingroup$ I agree that this does not convincingly imply the boat would better be oriented forwards, still +1 for some actual wind-tunnel numbers. $\endgroup$ – leftaroundabout Dec 21 '17 at 9:38
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As for why is at that position, the answer seems simple- there is where the hardpoints for bombs and torpedoes are- so mounting the dinghies that makes sense, both from structural and maintenance point. From the linked article:

Slung under each wing, where bombs and torpedoes used to hang, are two 14-ft dinghies.

As for the orientation, it looks like it was selected so that the boat fits snugly against the underside of the wing. It is repeatedly alluded to in multiple articles:

Here

Each boat fits snugly against the wing and is raised or lowered by a built-in electric hoist.

and here

The Landseaire had 14-foot dinghies under each wing, hoisted to fit flush by cables that had once lifted torpedoes and bombs ...

Note that the photo in question shows a product demonstration and looks like boats were attached to the wing in both orientations- bow first

Boat stern fist

Landseaire with the boat fitted stern first; image from flightglobal archives

and also stern first:

Boat stern first

N68740 at Ontario, California in April 1957 as a "Landseaire"executive air yacht conversion;image from Ed Coates Collection

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    $\begingroup$ Interesting indeed that the boat in your pictures is closer to what i would expect (e.g. snug fit to wing, more aerodynamic shape, etc) while the boat in my picture looks like a rowboat or similar. $\endgroup$ – dalearn Dec 20 '17 at 13:44
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    $\begingroup$ the one in your top picture looks more like grandmas bathtub than a boat to me... $\endgroup$ – dalearn Dec 20 '17 at 13:46
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    $\begingroup$ The boat you are claiming as bow first is actually stern first, as evidenced both by the hull shape and the nameplate position. $\endgroup$ – Chris Stratton Dec 20 '17 at 15:03
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Fundamentally this question is related to Reynolds number and dynamic similarity.

In order for the flow about two similar geometrical shapes to be similar and thus for their drag coefficient $C_D$ to be similar both the respective flows' Reynolds numbers $Re$ and Mach numbers $M$ must be the same. Mach number is related to compressibility and for the case at hand (subsonic aircraft and boat) compressibility effects are negligible, so lets focus on $Re$.

Reynolds number is essentially the ratio of inertial to viscous or friction forces present in a flow: $$Re=\frac{\text{Inertial forces}}{\text{Viscous/friction forces}} \: .$$

Reynolds number is a function of the body (in this case boat) length, the speed of the flow and the viscosity (thickness/stickiness) of the fluid.

Lets estimate $Re$ for the boat in the water and mounted to the aircraft wing:

Assuming the boat is about $3m$ long and moves in the water at $2m/s$, and assuming the aircraft cruises at $200$ km/h at $10\:000$ ft, we can estimate $Re$ using this handy calculator and the fluid properties of water and air found here (you could also just do it by hand, its an easy formula).

It is found that:

$$ \begin{align} Re_{\rm water} &\approx 6 \:000\: 000\\ Re_{\rm air} &\approx 10\: 000\: 000 \end{align} $$ These values for $Re$ are of the same order of magnitude and the flow is in fact nearly dynamically similar. Note that $Re$ values typically span many orders of magnitude and these values are actually quite close.

Based on this, the same streamlining and drag reduction principles should hold and they would probably have been better off placing the boat the other way, i.e. the same orientation as it is made to be streamline in water.

It might however be that they simply placed it that way because it was easier to mount, as opposed to any aerodynamic reasons.

PS: I was half expecting the $Re$ of the boat mounted to the aircraft to be a lot higher than in the water, in which case I would have made the point that if the $Re$ of a body immersed in a flow is vastly different, the same streamlining principles would not hold, although in this case it would seem that they do.

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    $\begingroup$ Boats made to travel rapidly or efficiently in displacement mode don't have squared off transoms - traditional human powered boats have tapered sterns, as do naval vessels and (at the waterline) most keeled sailboats. A squared off transom on a rowboat is more a manufacturing simplification to give a broad passenger area and convenience for mounting an outboard motor, in a hull that would be planing anyway if it were powered to move at any real speed. For that matter on rowboats sometimes the bow is squared off, too. $\endgroup$ – Chris Stratton Dec 20 '17 at 16:55
  • $\begingroup$ Interesting, thank you! $\endgroup$ – Mike Dec 20 '17 at 17:09
  • $\begingroup$ Note that even if the Reynolds numbers are similar, the situations are quite different because the boat sits on the surface of the water, rather than immersed in it. In particular, this means wave drag is important even at low speed (basically, you have high mach numbers WRT surface waves, because these travel so slowly). Thus optimising the stern is little use – the bow will first be the bottleneck, adressed by en.wikipedia.org/wiki/Bulbous_bow. $\endgroup$ – leftaroundabout Dec 21 '17 at 9:43
  • $\begingroup$ Interesting @leftaroundabout, now that you mention that it seems obvious, thank you. For interests stake, is wave drag for aircraft and for boats caused by the same physical mechanism? $\endgroup$ – Mike Dec 21 '17 at 18:01
  • $\begingroup$ Broadly speaking, yes, but there are a couple of important differences. Water waves propagate in the 2D plane and have a nonlinear dispersion relation, which is why there isn't such a hard subsonic/supersonic distinction as there is for aircraft. $\endgroup$ – leftaroundabout Dec 21 '17 at 18:56
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If the boat was the other way round, it may produce some lift of its own, causing the wing to be pulled downwards, or may have an effect of the lift produced by the upper surface of the wing, refusing lift in that way. Making the airflow turbulent under the wing reduces the tendency for the pressure to drop.

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    $\begingroup$ +1 That's an interesting idea (really) however while it would prevent a rolling tendency, it now creates a yawing tendency (which could maybe be balanced by an asymmetric thrust setting). $\endgroup$ – mins Dec 20 '17 at 13:18
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This boat is subject to sub-sonic aerodynamics in the region where compressibility effects are negligible. In this region, streamlined bodies are teardrop shaped with a round front providing suction forces and a tapered rear for preventing the air stream from stalling.

The boat does not have a rounded end on either side. It does have a tapered side on one end - position it to the rear of the flow and at least one of the two properties of subsonic streamlined bodies is utilised.

And the stern has partially a rounded shape, which helps with streamlining when in front, not in rear. The flat surface does not help either in front or in rear.

So without knowing the exact Cd from wind tunnel measurements or CFD and based on an Order Of Magnitude estimation, my money would be on that the boat has least resistance with the bow pointing backwards.

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