F2D Combat Rules

Published: 10/2 2009
Last update: 10/2 2009
From the Yahoo Group: Combat-l - July 19, 2008 - Message #16160

Re: F2D Technical Rule Clarifications Part 1 Venturi and muffler check

Venturi and muffler check:

Two weeks ago The Chairman of the C/L Subcommittee and the F2D Judges Group sent out a document with Technical Rule Clarifications for F2D at the World Champs in France this year.

In this document you will find the sentence: "In case of doubts whether the diameter exceeds 4.00 mm or not it will be checked with precision gauges".

This "Clarifications document" makes it clear that a check with "precisions gauges" will be a possibility at the World Champs. Actually it makes no difference for the pilot if the test is made at random or as a general test for all pilots at the processing. The only concern for all pilots is if his equipment will pass this test or not.

Most pilots don't have a clue on the exact measurements on their venturi or mufflers. It was probably OK when the engine was new but after some hard landings no one can be sure if all parts can pass a closer inspection. As most pilots I do not have the necessary "precisions gauges" and will not be able to verify my own equipment before I am going to France. I know that all my engines will pass the 4.05/8.05 gauge test but with the new "check with precisions gauges" that is not enough to be sure that everything is OK.

Let's start with a look back to the introduction of the 4 mm venturi rule.
As far as I remember the discussion back then, the reason for the 4 mm venturi inlet and the 4.05 mm gauge was to make it easy for all pilots to produce new venturi inserts for their engines. 4 mm holes with good surface quality, good roundness and a small variation could easily be made with a 4 mm reamer. A standard 4 mm reamer will make a hole with a H7 tolerance. This hole will be between 4.000 mm and 4.012 mm.

I don't think it was the intention to define the maximum inlet opening as 4.00 mm but for some unknown reason the rule was written with 4.00 mm venturi and 4.05 mm gauge.
I think the intention was to let it be easy to manufacture venturi inserts with a 4 mm H7 reamer and check for oversized venturi openings with the 4.05 mm gauge but the rulebook is not clear on this point. The two tailing zeros on the 4.00 mm dimension is the reason for the controversy we have now. The 4.00 mm can be interpretated as a sharp limit for the size of the venturi inlet opening.

With the "Clarification document" we now have two different views on the venturi/muffler verification. A low-tech approach where anything that passes the 4.05 mm gauge test is OK and a high-tech approach where anything bigger than 4.00 mm is prohibited.
From my point of view it is not possible to tell which approach is right and which is wrong. It is a matter of personal taste or style!

The low-tech test:

Today we have a tradition with a well functioning low-tech method. The test with the 4.05 mm gauge is easy to do and can be done with very simple tools and under simple conditions. It is accepted by all pilots and has been used for decades without any problems.

There is a small chance that some pilots will be able to use venturi inlet openings that is a few hundreds of a millimeter oversized. These pilots run the risk of being disqualified during a competition. The benefit they get from this breaking of the rules is so small that almost no pilots see it as a problem.

The venturi on a typical combat engine has a very short zone with the restricting diameter. This zone is typically 1-2 mm high. Above this critical zone the venturi opens up to a much wider diameter.
When this type of venturi is being checked with a 4.05 mm gauge the fins of the gauge will make contact to the venturi above the critical zone. Any wear or marks from the inspections will not affect the critical zone. Only the area just above the minimum diameter will be affected. With the low-tech inspection a venturi can withstand a lot of inspections without affecting the critical zone.

The high-tech test:

The high-tech test can only be done in a proper manner with specialized measuring tools and special trained officials. With the right tools and skilled technicians the venturi check can be done without damaging the engines and there is a very low chance that an oversized venturi will pass the test.

There will probably be a lot of discussions on tolerances and methods in the start. One of the first issues will be to decide if the tolerance (from tools and methods) shall be regarded in favor off the pilot or the official. Will the pilot have the "benefit of the doubt" if his venturi is measured too big but inside the measuring tolerance or is it the pilot's duty to present equipment that never will give an oversize measuring?

For a precise measurement of the venturi opening a three-point internal micrometer gauge must be inserted into the venturi. Three measuring points will contact the internal wall surfaces and measure the inner diameter with an accuracy of +/- 2 micrometer (typical).

This contact might wear or mark the surface in the critical zone and this will tend to make the venturi opening bigger for each successive measurement. One rough measurement can spoil the venturi.

For engines with the crankcase and venturi cast as one piece damage to the critical zone will result in an almost useless engine. This is why many pilots are very skeptical of this measuring method. Some pilots will even prohibit the officials to performing any measuring inside the venturi with measuring tools that involve contact with the surface in the critical zone.

If a venturi inlet starts to have some wear in the critical zone it will come closer and closer to the 4.00 mm limit. This will encourage the officials to do even more inspections to make sure that the venturi is not oversize. This will accelerate the wear process and result in a spoiled venturi.


The "F2D Technical Rule Clarifications" document has de facto replaced the inspection with an 4.05 mm gauge with a new inspection with a "precision gauges".
At this moment we don't know how the inspection will be done in France so all pilots must be prepared for a worst case scenario. The consequence of this confusion is that no pilot will dare to go to France with a venturi that is even close to 4.00 mm. Most pilots will probably have to change venturi on all engines.

The high-tech test is a very big change in verifications method and it does have some influence on the equipment. I find that it is a very drastic decision to introduce this change just a few weeks before the world champs. All pilots should be given more time to understand the new procedure and have time to make sure that their equipment is OK.

I think that the best solution to this problem will be to state that the inspection will be done only with the 4.05 mm gauge (the classic low-tech inspection).
If the venturi does not accept the 4.05 mm gauge it is OK. There is no need for "checking with precision gauges".

See you in France!

Henning Forbech


The Clarification document (pdf):

Mitutoyo three-point internal micrometer:

Benefit of the doubt: