William Robison

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Ernie:

Quote:

ORIGINAL: Ernie Misner
Why the 5 sec. or so lag time on a 4-stroke when changing the HS needle setting?

The best explanation I can offer for the delay is the length of the intake pipe. The engine will draw a quantity of air/fuel mixture through the carb and intake pipe, then push an amount back into the pipe before the valve closes. So if you are running rich at a 6:1 ratio and want to go to 7:1 mixture, it takes a while for all the trapped fuel/air volume to change. If you turn the needle too rapidly you'll find you have gone suddenly from 7:1 and reached 9:1 (lean) with no in between time to let the engine tell you it was at an 8:1 f/a ratio.

I've noticed the engines with stacks installed are a little slower yet in their response, further confirming the greater volume of the intake tract.

Bill.

Hobbsy

My Feedback: (102)

If you don't allow for that delay you'll find your prop in your neighbors yard faster than you can get it back rich.

William Robison

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BruceL:

Five seconds is a number I pulled from my "Nether reaches." It may well not be that long, but sometimes it seems longer yet.

Bill.

gjeffers

besides 5 seconds from fiddling with the needle, i also notice that with my saitos when im idling on the tarmac if i goose it a little it also takes about 5 seconds to return to the previous rpm idle, is this normal, all three of my saitos do this, but does not bother me

loughbd

Yo Torque, I don't wait 5 seconds and I've never had to get any of my props from the neighbor's yard or anywhere else for that matter. I must be doing something wrong because I don't seem to have all the problems you other guys all talk about. Oh well, such is life.

Honestly, that pipe is no more than an inch and a half long. At the speed that air is moving through the system it would take a fraction of a second to go from the opening of the venturi into the cylinder. Figure the engine is turning 8000 rpm. That means the engine is inducting at 4000 times a minute or or 68 times a second. That means it would take 1/68th of a second for the charge to get from the outside to the inside. In 5 seconds it would have done it 333 times. Not too much of a wait seems to me. In my experiance it takes very little time from turning the needle to seeing(hearing) a result. Certainly not 5 seconds. I have been playing with four cycles since 1976 when they first came out.

Actually I have had the props fly off but it was only with an Enya 120R and they were known to do that and with an old OS FS90 but the intake pipe was loose and the engine leaned out.

William Robison

My Feedback: (3)

Gary:

If the delay is just the last 500 rpm or so in dropping to idle you might be able to tune it out, but I wouldn't think it worth the effort.

On the other hand, if your engine slows to around 4K rpm and then slowly falls to the idle speed, that's another matter. Often you'll find the air throttle is open a good bit, to keep the engine running with a rich LS adjustment.

With the throttle drop the idle as low as you can, then do the "Pinch" test. The engine will probably rise a good bit in rpm before it falls. o ahead and peak the LS, but remember the delay in response will be even longer than it is at full throttle because of the lower flow. Go back and forth between the LS adjustment and the throttle, work it to get the +/- 2K idle speed, always trying for the lower throttle setting. Check the HS now and then while doing it. When the idle is as low and smooth as you can get it check the transition, richening the LS as little as you can.

You may well notice improved fuel economy too.

Bill.

gjeffers

thanks bill, maybe i made it sound worse than it really is, im talking about approx a 2-300 rpm increase for a bit after revving it a little, i can idle down to a smooth 1800 now and even lower before she dies now and it has good trans and economy,(10 min on about 12 oz on both the 100 and 125)

my 180 should arrive friday for my new pitts monster 120 that should arrive tomorrow

loughbd

Now you have me curious. I'm going to go down and fire up a 45 and a 65 and see how long I have to wait for a reaction to turning the needle valve

William Robison

My Feedback: (3)

Gary:

For 300 rpm I wouldn't bother with it. As I said, even 500 rpm could well not be worth the trouble.

Bill.

PS: I still say that S-12 should have a radial. wr.

gjeffers

i know Bill but i couldnt find a 400 horse 9 cylinder that would fit

William Robison

My Feedback: (3)

Gary:

I guess you overlooked the Vendenyev M-14. Can't get any more scale than using the engine from the real plane, can you?

Haw.

By the way, another point about the S-12 - it has a three blade prop.

Bill.

gjeffers

im going with a mas 16x8 3 blade with a tru turn 3.25 should pull the 11.5lbs real well(i think)

William Robison

My Feedback: (3)

BruceL:

Quote:

...Figure the engine is turning 8000 rpm. That means the engine is inducting at 4000 times a minute or or 68 times a second. That means it would take 1/68th of a second for the charge to get from the outside to the inside. In 5 seconds it would have done it 333 times...

I'm aware of this, and that's why I said "The best explanation I can offer..." in my earlier post.

Whatever the reason for the delay, it's there, we have to allow for it.

Bill.

Hobbsy

My Feedback: (102)

Bruce, I never said it was a problem, I just said it can happen, if it never happened to you, you've been very lucky.

NM2K

My Feedback: (14)

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Our engines do not have very tight ignition timing when compared to gas/spark engines. Temperature has a lot to do with our engine's timing. What you are experiencing is called "Thermal Dwell". It takes a while for the engine to cool down, retarding the ignition point, for the engine to slow down. The same thing, but opposite, occurs when the engine hesitates a bit during acceleration.

gjeffers

thanks Ed, that makes sense.

Timthetoolman1

My Feedback: (6)

Vibration Question on the 1.25 Several of us on the kit building are building Ultimates from CG, I've reinforced the fire wall and used larger bass stringers than the balsa stringers. I put a bead of silicone under the servo tray. Do you think I need a soft mount for the engine. Everything is packed in foam except the servos and the switch (which is an MPI heavy duty). Have any of you guys actually experienced problems from vibration.

6187gary

im looking for some tunning tips for my saito 182 twin cylinder engine

gjeffers

Rangerman, mine never vibrate bad enough to hurt anything, if i did have a problem i would check prop balance first and then i would tune my saito, my 100 and 125 both run smooth

no soft mount required imo.

NM2K

My Feedback: (14)

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Here is the answer that everyone hates - it depends. <G>

Fortunately, the Goldberg Ultimate is a largish model with enough mass to dissipate the vibration harmlessly, for all practical purposes. I would go for a solid mounting of the engine to the airframe, or use a Dubro isolated metal engine mounting system.

Of course, which particular soft mounting system you choose can make all of the difference in the world. The Dubro mounting system is pretty good, for something that is not real expensive. You do have to change out the rubber/elastomeric snubbers every once in a while. But that isn't a terrific hardship.

More expensive engine mounts with vibration damping systems can easily cost more than your kit and covering. I wouldn't waste the money unless I had to comply with very stringent noise rules.

Be aware that engines tend to shed fasteners and needle valves easier when soft mounted.

I used a J'Tec aluminum mount with through the firewall snubbers in my first CGM Ultimate Bipe. The Enya R120 went invisible when it started. I couldn't tighten the system up enough to set it satisfactorily. Others' experience with this mount may have been better than mine, but I don't think I want a repeat of that experience. I usually use and enjoy J'Tec products, so I'm not slamming the company or their product. Just my experience.

I am far from being expert in the field of vibration damping engine mounting systems.

gjeffers

well i just recieved my first big block(180) and i guess i need a 10mm prop reamer for a 8mm shaft, learn somthing new everyday

HDB

I am just attempting to put my first Saito in the air and need some words of wisdom! I have an FA-65 in a cub which was tested and supposeably broke in and never has been in the air - decided today to tune it up and something felt funny when turning the prop over - very little to no compression. I pulled the valve cover off and the push rods are only moving a very little bit! Checked a new FA-82 and those rods move up and down about 3/16 of an inch, much more than the ones on the 65. What should I look at or do? I have never had a 4 stroke apart. Also - when adjusting the tappet gap, I understand that it is measured at the top of the stroke - correct or not? I have been flying for a few years, but have been using 2 stroke which are definitely different!.

loughbd

The old 65 didn't have a hight lift cam like the new engines have. Push rods didn't move as far.

loughbd

Good evening Ladies and Germs. I just got back from my little spearmint. Here's what I found with MY engines. First one; Saito FA45, Master Airscrew 11x6 wood, White Lightning fuel (10% nitro, 20% castor oil, no synthetic). Got engine warmed up and tweecked in a hair rich. I quickly opened the needle 1/4 turn. The engine slowed down almost immediately. Quickly set needle back 1/4 turn lean to original position. I had a stop watch ready to time the lag but the time lag in both directions was so short the watch wasn't needed. Second engine: Saito FA65. Master airscrew wood 13x6. White lightning fuel. Same routine. The lag between turning the needle valve a 1/4 turn in either direction was negligable.

I don't know why others take so long to speed up or slow down but at least two of mine don't. From what I remember of the others I have, they are the same.

At least that's what mine do. Wish I could record it so I could show all those who ain't gonna believe me. Wish I could upload pictures too but my puter must suck.

blw

My Feedback: (3)

Ed,

The engines may hesitate also because of the larger than normal slug of air that is being rushed into the cylinder before the fuel can catch up.

Loughbd- let us know if you sling any good props. You will need a good tach to be accurate with what is happening if the props stay put....