Stern flaps and interceptors

Ok Terhohalme, here we go (I didn't find the PBB though):

I first saw the interceptor in St Petersburg, Russia in 1992 or -3 in the Almaz shipyard. It was installed on a newbuilding; a 20m rivergoing catamaran for pollution control. The originator explained its function and told me they had been experimenting with the device since early 1980-ies, in particular for lift improvement on supercavitating profiles. In this context, it is mentioned in a Russian text on high-speed propellers, dated 1982.

He is a very sympathetic and humble person, and when I asked if the device was patented, he said "no, we think it is better that it is freely available to the industry; I have food on my table, a roof for my dear ones, why should I be greedy?".

I made a few notes on the dimensioning principle (a mix between PBB and Grigorenko's original), that I attach, and have been successfully using the interceptor, both on a number of boats, and for supercavitating and ventilating propellers since then.

In the conference papers from the "Fast-95" gathering, you find it mentioned in an article by A.V. Ponomarov, D.Y. Sadovnikov and Y.M. Sadovnikov; pp 1189.

Hope that might get you started!

In addition: As long as the interceptor height is "inside" the boundary layer of the main body, its Lift/Drag ratio is higher than the lift-equivalent normal flap

 

Very intersting. Thanks for references.

Anybody know how the name "interceptor" originated for these devices?

Similar, though generally fixed, devices have been used for over forty years on high speed automobiles to increase downforce. The are usually attached at the upper rear edge of the rear deck/trunk lid, and are commonly called "spoilers".

 

In the original Russian texts, the term "interceptor" is used in the sense of "cutting off, separating".

The racing counterparts were first (as far as I know) applied by Dan Gurney on the rear foil of his car. In these circles hence a "Gurney flap".

An interesting parallell to the Russian work can be found in:

Linearized theory for flows about lifting foils at zero cavitation number, by Tulin and Burkart, published (and classified 1955, later allowed for open publication). In this document, the conditions for a flapped foil is studied. If we manipulate the original math a bit beyond where T and M stopped, we find that the maximum L/D of a flapped foil will occur when all lift is produced along a line at the trailing edge of the main foil and with the main body close to zero angle of attack.

This result was long regarded imaginary and non-realistic; "engineering imagination" making halt at what came to be known as "propeller cupping". But the Russians took it to its logical finale.

I might add that in the case of single-phase flow (Gurney flap) the pressure distribution over the suction side of the foil is changed due to the presense of twin vortices along the exit side of the foil. In supercavitating flow there is no such interaction.

 

Thanks baeckmo, you made my day!

 

Thanks for the information about the origin of the term. I suspected it might be more obvious in a language other than English.

Gurney is credited with being the first to apply a vertical flap to the trailing edge of an airfoil, though a patent was granted for the same concept in 1931.

However the use of a vertical flap at the rear of the body (not a wing above the body) started earlier, before wings were used on race cars. Some give credit to Jim Hall for originating the idea on the Chaparals though I would not be surprised that there was an earlier use. The pressure on the body ahead of the flap increases for more downforce on the body, very similar to how the interceptor increases pressure on the bottom of a boat to change the trim.

http://chaparralcars.com/images/index3.jpg

 

Well, in "my book", what you see on the Chaparall is a flap, although at a steep angle. It is in fact an extension of the "ducks tail" starting on the rear lid. You should distinguish between a flap, which is mainly redirecting a substantial part of the main flow (ie outside the boundary layer), and the interceptor with its abrupt "non-streamlined" cut-off, having its main impact upon the boundary layer flow and the static pressure distribution. There is a fundamental difference. The interceptor can even have a negative "rake" in some applications, which would not make much sense for a flap.

 

As I said in my previous message, in automotive terminology a vertical or near vertical surface at the rear of the decklid/trunklid is usually called a spoiler. The spoiler may be low enough to be "inside the boundary layer" or it may be taller. In either case the static pressure is increased ahead of the spoiler. They may or may not be exactly vertical but the static pressure ahead of the spoiler is increased.

If need be I'll find another photo of a more vertical spoiler. However the configuration shown would have higher pressure on the rear deck with the device present than if it was removed.

 

Baeckmo, where can I find papers or books from which you have derived the formulae in your attachment to the post #16 ?

 

I'm afraid you can't. The expression for flap lift is directly from Savitsky (Procedures for hydrodynamic evaluation of planing hulls in smooth and rough water, in Marine Technology vol 13, no 4, oct 1976). The algorithm is from personal communication with the original designer. Empirical validation in field conditions show reasonable results within practical engineering standards. It should not be too complicated to reach a theoretical solution, based on first principles; someone who needs a subject for a thesis out there.......?

 

out of my sediments:

PPB February/March 2002 Trim Control Dawson & Blount
PPB April/May 2001 Stern Flaps Karafith & Cusanelli

js

 

Yepp, there you go, thanks Jürgen, your sediments are more structured than mine!

 

Here is an illustration of the pressure distribution over a NACA 4412 profile
js

 

Is this discussion about interceptors on a hull at the transom, or interceptors on an airfoil. There are some fundamental differences. On an airfoil the interceptor will increase the pressure ahead of where it's mounted, but also change the flow and pressure on the opposite side of the profile. On a hull the interceptor will increase the pressure ahead of where it's mounted, but there isn't flow on an opposite side to affect, assuming the speed is high enough the flow is separating at the base of the transom.

 

Yes, that is the essense of my concluding remark in post #18. There is an interesting similarity between two-phase flow with a gas phase on the low-pressure side (read supercavitating or ventilating foil), and the flow we have past a planing bottom with a sharp transom.

 

hello Baeckmo,

The formula in the attached file from post #16 is derived from savitksy and tested in field conditions, can you tell me, based om the test result, if the formula overestimate the pressure or not?