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Author Topic: Some questions about Bob Reeves modification  (Read 11871 times)

Offline Mark Mc

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Some questions about Bob Reeves modification
« on: July 09, 2016, 10:36:20 PM »
At a swap meet, I was hanging around at closing time hoping to get some deals from guys not wanting to cart their stuff home.  One of the things I got at closing time for $50.00 was a .40 sized RC Trainer plane.  It came with a brand new radio and had a Saito FA-40 engine mounted.  I’ve always wanted to try 4-stroke engines, so it was a good starter deal.  I’m thinking I’ll fly the engine on the RC plane a few times to get some 4-stroke experience, then move the engine to a CL plane.  I spent three or four hours last night reading up on the older Saito 40 engines.  I understand that they were heavier than the 40A, but an engine in the hand…

I’ve read through Bob Reeves’ Saito 4-stroke modification of making a new barrel and trimming back the spray bar a couple of times. Unless I’m totally confused, the main purpose of the modification is to be able to match the volume of air intake to that required for the size prop being used.  In the modification, it is done by screwing the nylon bolt in or out.  I do not have the lathe to do the mod myself, and it seems excessive to spend more on having the mod done than I paid for the whole plane outfit.  So, I was wondering about effectiveness of a different method.  The FA-40 that I have has the factory installed slide choke.  If I use that to incrementally limit the air intake, will this have an analogous effect to doing the nylon screw mod?

Also, Bob’s article says to only use props with an effective pitch of 6 ½ or more, and then goes on to recommend the Rev-Up 11 X 7, Thunder Tiger 11.3 X 6.5, or Evolution 11 X 7.  Can someone define “effective” pitch versus stated pitch?  Are these props still the best recommendations for the early FA-40 engines?

Bob says to set the choke to a point that lets the prop turn 8000-8500 RPM in flight.  Is that for the .40 as well as the larger engines?  What static RPM gives an in-flight RPM of 8000-8500?

The statement, “If you are running over 7 minutes on 4 ounces (3 1/2 on the 40) something isn't right.” Not sure what problem this points to?

On setting the needle and choke, he starts with setting the needle on the rich side of peak.  I assume the initial setting is done with the choke fully open?  Initial setting done with one of the recommended props above?  

I suppose those are all the questions I have for now,
Mark

Offline Bob Reeves

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Re: Some questions about Bob Reeves modification
« Reply #1 on: July 11, 2016, 03:13:05 PM »
Hi Mark, don't check this board very often any more sorry I haven't responded sooner.

First please understand my personal experience has only been with the newer Saitos, you are kinda in unchartered water with the older versions. Will try to answer your questions as best I can and in no particular order..

My statement of effective pitch was referencing a carbon 3 blade prop that was sold as a 5 pitch prop but flew my airplane at the same RPM and lap times as a measured 6.5 pitch prop. I didn't know how to better explain a 5 pitch prop working like it was actually a higher pitch.

Except for the Evolution my prop recommendations still hold. After I wrote that I discovered the Evolution didn't pan out as well as my initial tests indicated.

Set it for 8200 to 8400 on the ground and with a good prop and reasonable line length you should be doing about 5 second laps. Lets just say, this is the RPM range the Saito's like to run at for CL.

The only real reason not to use the RC carb is consistency. Doesn't matter how well the barrel fits in an RC carb you have a variable air leak. This doesn't really matter if you are just burning holes in the sky but if you want to fly against Joe Gilbert's T-6 with a Saito 40a or my Latency with a 56 you won't even be in the same ball park.

Always reset the needle to just on the rich side of peak anytime you change the choke setting. Do not try to regulate the speed with the needle, use the choke to set the speed WITH the needle set just on the rich side of peak.

Running over 7 minutes on 4 ounces is telling you that you are not burning enough fuel, too lean, choke set too far open and not sucking enough fuel for whatever reason. A 4 stroke gets all it's power from one big bang and the more fuel you can load into the combustion chamber the bigger the bang.

Hope this helps
Bob


Offline Brett Buck

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Re: Some questions about Bob Reeves modification
« Reply #2 on: August 02, 2016, 08:45:02 PM »
My statement of effective pitch was referencing a carbon 3 blade prop that was sold as a 5 pitch prop but flew my airplane at the same RPM and lap times as a measured 6.5 pitch prop. I didn't know how to better explain a 5 pitch prop working like it was actually a higher pitch.


  I think that this is the explanation:

   http://www.supercoolprops.com/articles/pitchcompare.php

    To summarize, measuring the back of the blade angle doesn't measure the zero-lift line of the prop because of the copious camber of the prop. In some cases with lots of camber, the effective/experimental pitch can be much higher than what you measure, particularly at the tips. I did a bunch of measurements and tests, and my 3.75" pitch prop was actually something like 7.5 inches of experimental pitch. Before anyone says " I told you so, high pitch works", I would note that the same issue happens with 6" of pitch, too, which might be 10-12" at the tips computed the same way.

     Brett

Offline Douglas Ames

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Re: Some questions about Bob Reeves modification
« Reply #3 on: November 05, 2016, 08:45:11 PM »

SNIP>
Always reset the needle to just on the rich side of peak anytime you change the choke setting. Do not try to regulate the speed with the needle, use the choke to set the speed WITH the needle set just on the rich side of peak.

Hope this helps
Bob

The above is a very good piece of info for four strokes!
(in this case - "Choke" refers to a variable throttle which sets the "Speed" (manifold press.).)

Also,
When an engine consumes all the fuel in the mixture and has excess oxygen in the exhaust, it is LEAN.
When it consumes all the oxygen and has excess fuel, it is RICH.
When the are ideal it's called Stoichiometric, however that's is NOT the best power setting.
RICH side of peak (Stoichiometric) is best power.
AMA 656546

If you do a little bit every day it will get done, or you can do it tomorrow.


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