Many years ago – possibly in the '80s – Flying had an article about bush flying. Or maybe it was one on how to go camping with your plane. I don’t remember. In any case, one of the photos (on the cover?) showed a Cessna 206 (?) flying along with an eight- or ten-foot boat strapped to the passenger side of the fuselage. The thing that struck me was that the stern was forward. One would expect that less drag would be produced if the bow were forward. After all, that’s how they go through the water. IIRC, the text of the article explained that testing showed that stern-forward was actually less draggy on the airplane/boat combination.
In the 1970s many cars had the ‘Kamm tail’, where the rear of the car was ‘cut off’ vertically. Apparently this was more aerodynamically efficient than previous designs. The Kamm tail is similar to the transom of a small boat, so points to the bow-forward team. Smooth airflow over the hull, and then the Kamm tail.
On the other hand, by putting the stern forward you have a big flat air block; but you also have a long curved surface to smooth out the airflow. That should reduce buffeting on the empennage.
There’s also the issue that it was an open boat (inside against the fuselage, hull outward) so there are airflow issues there as well; but let’s ignore that. Can someone explain which is more efficient and why, when strapping a boat to an airplane; stern-forward, or bow-forward?
First of all, I’m amazed and somewhat appalled that anyone would strap a boat (or other large object) to the outside of a light aircraft. The effect on the lifting surfaces and control surfaces must surely be adverse and unpredictable.
In addition, in this case the effect would have been asymmetrical, affecting the airflow on one side only.
But from what you’re saying, it seems like they had expert advice and even testing to determine the most suitable configuration for the boat.
If you were going to do it, one reason to consider a stern-forward configuration might be to avoid disrupting the airflow over the control surfaces on the empennage, reducing rudder and elevator authority.
I know nothing about flying boats but often the answers which might seem obvious or intuitive when dealing with fluids are wrong.
I have a sailboat with an auxiliary motor. When I am sailing and not using the motor what would put less drag on the boat and therefore make it go faster: (1) let the propeller turn freely or (2) lock it so it cannot turn?
You would think letting the prop freewheel would create less drag, wouldn’t you? But you would be wrong. A locked propeller is stalled (the fluid does not flow evenly close to the surface but separates from the surface) and does not generate lift whereas a turning prop is generating lift which means drag on the boat. Just like the wing of an airplane does not create lift if it is stalled.
So it could well be that the turbulent air flow around the reversed boat creates less drag than the less turbulent airflow around the boat facing forward.
I know four men who routinely take a canoe trip into northern Ontario. Four men, two canoes, lots of gear, for a two week trip. The thing is, they fly in. In a little Cesna (like a 185, I think), with a canoe, (filled with gear, mind you), strapped to each pontoon. Fly in, canoe out. Neither they, nor the teenage pilot seemed to think much of it.
A C-185 is a workhorse - they use them to haul banners and even other aircraft into the sky - when I took a glider flight I was towed aloft with a C-185 and the glider with the instructor and me in it might have weighed as much as the unloaded, stripped down Cessna. Cessnas don’t go particularly fast (well, their jets do, but we’re talking pistons here) but they can get a remarkable amount of stuff off the ground for their size.
With a canoe on each pontoon, assuming the weight is roughly equal properly distributed, will take care of most of the balance issues. It’s strapping the boat to just one side that would seem troublesome.
Not me, but I can WAG with the best of 'em. It may be that having the stern forward causes turbulent flow. Having the bow forward may cause a bump in the laminar flow which causes more problems with yaw.
IANABP (I am not a bush pilot*), but I seem to remember that the Cessnas and Pipers adapted for pontoon work had larger rudders for their size than other small planes. Could it be that the plane is kept straight just by brute force?
*…or any type of pilot for that matter. Clearly, you guys are the experts here.
Well - not larger rudders. They do have larger vertical fins. More specifically, most planes that are fitted with pontoons have a sub fin attached below the original vertical fin. The reason is that more of a pontoon extends forward of the plane’s CG than aft of it. So, there’s more additional vertical surface forward of the CG, the subfin makes up the difference. But, no additional rudder is added.
As for stern fwd/bow fwd, you need smooth airflow over the tail surfaces (what others are referring to empennage). Having the stern facing aft will create a low pressure area behind the stern of the boat - which is very draggy. I suspect even more draggy than having it face forward.
Hm. Sounds like ‘stern-forward reduces buffeting’ is the most likey answer; that the drag penalty is not as great as the potential penalty of not having smooth airflow over the tailfeathers.
It may be that having the stern forward causes turbulent flow. Having the bow forward may cause a bump in the laminar flow which causes more problems with yaw.