qspilotcmh

I have a Valley View RC 20 in a 65" Extra profile. The engine has approximately 1 gallon of fuel through it. I am using VVRC 17x6 and IBEC.


The engine will idle fine on the ground at 1800-1900 and my max is 7900 RPM. The high is peaked and the low needle is set for smooth transition. The plane has very little four stroking upright and a little inverted. The engine has never quit in flight.


The issue I am having is with the approach and landing. I setup a nice glide path coming down final at idle. The engine RPM will stay a little high until the plane is a few feet off the ground and then it drops to 1800-1900. At that point the plane wants to settle down pretty firm. I have good landing gear and have never broke a prop or caused the engine to quit.


I have tried tweaking the high & low together and separate. I have also tried going to idle earlier on downwind but no change. I am getting a little carb spray on the fuselage if that matters???


Any suggestions?

ahicks

My Feedback: (2)

What you are experiencing is called "high idle hang". Pretty common deal for engines tuned right on the edge. The low is just a hair lean. When I say hair, I mean like a screwdriver blade width, maybe 2?

And yes, this change is likely going to lead to some mid range burble - that can be minimized by gaping your plug to .025"-.026". Don't ask why, but it's easy, cheap, and it works. If you think of it, it's worth a try. It's put a grin on a few frustrated faces....

Lifer

My Feedback: (1)

Plus 1 on ahicks advice.

CK1

My Feedback: (60)

+2

speedracerntrixie

My Feedback: (29)

I agree being that it is a smaller engine. The overheating may not be mixture related either. I doubt it is the same condition but every DA 150 I have owned would hold an RPM until just after touch down and then drop. I think in this case it is either the reed cage design or the prop while it has forward motion is helping the engine stay spooled. Reason why I mention this is if you are sure your engine is not getting too hot you may be able to reduce the issue with changing to a prop with more diameter and less pitch. As for other reasons to overheat, even being mounted on a profile, check engine timing and use good quality air cooled 2 stroke oil mixed at the correct ratio.


Al, thanks for reminding me about the plug gap to fix a rich midrange. I had completly forgotten about that fix. I was just about to open up my carb and start adjusting the metering fork. I may have to do that anyways but a fresh set of plugs gapped correctly should be step 1. Thanks!

ahicks

My Feedback: (2)

SRT,
Agreed, it's a heat issue. The combustion chamber is hot (for whatever reason, e.g. cooling, mixture), and it takes awhile to cool it down once the throttle is closed.

I learned about all this quite a while back on 2 strokes WAY bigger than the ones I'm playing with now. These were 800cc twins and triples (150hp+) used in snowmobiling. In cooler weather conditions (thus leaner mixtures), we would be cruising along at maybe 1/2 throttle, and when you happen upon a stop sign, you would coast up to the sign and stop - but not because the engine wanted too! They'd be doing this same stupid "high idle hang" thing (some times a little disconcerting depending on your rate of closing on the distance to the sign)! No clue what was going on at first, but we learned that a quick blip of the choke would bring the engine to it's senses, or a couple quick blips of the throttle? The cure for the sleds was to jet properly for the cooler weather conditons (richer) and make sure the idle was set for absolute minimum, no higher than necessary. The richer mixture allows for more cooling affect from the incoming fuel charge - cooling the combustion chamber noticeably quicker. Closing the carbs to minimum just starved this borderline runaway condition for air.

When I started flying gas, didn't take long to realize these little RC guys acted like and responded to changes just like the bigger guys I had been messing with for the last 40 years or so... FWIW, -Al

qspilotcmh

Thanks I will try it tomorrow and post results.

av8tor1977

My Feedback: (6)

I agree with the probable border line lean idle setting.

However, check to see if your ignition has the number 4 on it. I highly doubt it since your engine came from Valley View and they are very reliable, but it is worth a check. The number 4 RCEXL ignitions were known to have this problem because of a crappy ignition advance curve that they had.

AV8TOR

ahicks

My Feedback: (2)

The problem with the original #4 modules supplied with DLE 20 engines only (to my knowledge), was not an advance "curve", it was a STEP! The timing changed at about 2200 or so, making it uncomfortably close to some idle speeds. Understanding this, it's pretty easy to tune around it by avoiding idle speeds in the 1900-2100 range, instead going for idle speeds at something under 1900 or so (usually not very hard to do). As mentioned, modules supplied since, with an 8.4 volt input limit on them, were changed to a real curve. It's the #4 modules marked with a 4.8 volt limit that have the issue, and again, this step/curve issue was regarding DLE 20 modules only.

High idle hang can occur with ANY 2 stroke....

av8tor1977

My Feedback: (6)

+1

I just thought it was worth checking, as the pain in the butt #4 might have been sent with that engine by mistake.

AV8TOR

Gizmo-RCU

My Feedback: (27)

I have had a little mid-range burble, very little but will try the plug thing for grins. Great litle engine.

qspilotcmh

I was able to do 4 flights yesterday. The field was pretty quiet with the hot weather we are having in Tampa.


I set the plug gap @ .25 and richened the low a blades width. The engine still had an idle hang on landing but it was not as bad. I continued to tweak the low richer over the next 2 flights. The idle hang improved slightly but never went away. My midrange starting to get rough upright and inverted it was very rough. One the next fill up I set the high with my tach at peak rpm (it was a touch rich from changing the low). The engine still ran rough in the midrange and it appeared to be lacking a little power (possibly too rich). I still had the idle hang but it was improved from a few days ago.


In an attempt to clear up the midrange I leaned out the low. I did it a blades width at a time until it ran smooth upright and inverted. Now I am pretty much back where I started with the idle hang.


Should I try a larger plug gap or possibly change the timing? I want to get rid of the idle hang but not at the expense of the engine four cycling and lacking max power. FYI - I can not reproduce the idle hang on the ground. It will go from 7900 RPM down to 1750 in one second and stay there.

It has a newer ignition rated for 8.4 volts. I am running a tech aero IBEC from my single A123 battery.


Thanks

ahicks

My Feedback: (2)

This could easily have something to do with your weather (105 yesterday, right?)? There is no doubt it's temp related, and I've never seen the problem duplicated on the ground. The reason? On the ground you always have the drag of the prop present. The prop drag is always present unless the plane is coasting. In the air, the momentum of the plane while it's slowing can allow the engine/prop to coast/free wheel - which is going to set up the problem you're experiencing. A 2 stroke engine with no load on it can get into what they call a "run away" condition, where they cannot be shut down even by turning off the key. The high idle hang occurs right on the edge of this condition.

Here's a thought for you to mess with if you like. The issue is about cooling a hot combustion chamber. If it's not as hot, it should cool quicker. So, maybe try running the HS a little richer from peak. Lean it out until you get max rpm, then richen until the rpm just starts to drop. Then test fly to see where you are at with the low speed, running it as rich as you can stand it. I can tell you right now, that all this is a compromise. We're working with a carb design that hasn't changed much since the 30's!

Of note also is the fact that at 105 degrees, your engine is running WAY leaner than when it's 70 or even 80 degrees. The air is REALLY thin, affecting oxygen content, which affects how much fuel you can mix with that thin air and have it burn efficiently...

I can also tell you that whatever we're doing here should have no effect on upright vs. inverted performance. If you're getting something going on there, I would suggest you have a look at your metering lever height and spring tension. If it's set right, it should be able to hold fuel pressure the same no matter what the engine is being put through!

What size prop? Plane? Low pitch with light plane? Like a 3D setup? If that's the case, speedracertrixie posted some ideas that may help as well. Adding more load on the engine with a larger diameter prop might kill 2 birds with one stone. Could have an effect on the high idle hang issue, as well as the mid range burble - which is also due to an engine that's just kind of coasting or has a very light load on it. If there's any doubt on your part regarding that, just listen to the engine's exhaust note change while doing it's burbling thing, by adding enough elevator to cause even a slight climb.... What you'll see is even the slightest additional load will cause the burbling to stop.

Bottom line, going for perfection on your carb adjustment, especially in you weather conditions, is likely going to lead to frustration. Set it up fat, it'll be happy and run forever. Your ears will get used to the way it sounds soon enough....

Last thought. If this is a newer engine, that might still be breaking in, that might be affecting this as well, if it's running a little warmer internally than it would be if well broken in. .

Lifer

My Feedback: (1)

Plus one on trying a bigger prop. It is a method that has worked for me in the past.

av8tor1977

My Feedback: (6)

Actually, when the ambient temperature increases, the mixture automatically enriches, not leans out. It the same as what happens with altitude; the air gets thinner and the mixture becomes richer. I've been told by someone I trust though, that the Walbro carbs rather self adjust for differences in air density to a degree.

Yes, a higher prop load can help with that troublesome midrange burble....

AV8TOR

ahicks

My Feedback: (2)

To maintain max performance with a weather change to the warmer, you would lean the mixture, just as you would richen as the weather cools.

It's about the oxygen content in the air, and trying to maintain that 14:1 (or something close) fuel air mixture. Warm air (or air at altitude) is not a dense, and contains less oxygen because of that. Under those conditions, you don't add fuel to maintain the correct mixture, you remove it. An engine that's been peaked at 105 degrees will be running a leaner mixture than one peaked at a lower temp...

>>

I would agree to a certain extent, but thinking a 20cc carb set up for 70 or 80 degrees might be running a little rich at 105? I know for sure when the temps cool this fall, from eighty something to say, fifty something, my engines will need to be adjusted a bit richer. Don't see the difference with this situation....

Also, I think these smaller engines might be a little pickier on their carb settings, than say, a 55cc + engine?

The bigger sled engines running Mikuni carbs had no self correction at all. If they were set up for zero degree temps, then run on a 30 degree day, they would hardly run at idle. Way fat blubbery rich, won't get out of their own way until they get up over about 4000 rpm. We used iridium plugs to help that situation, make the engines more forgiving of a rich mixture....

And getting a little further off topic, to maybe reinforce the point, we can say 2 identical engines, one running in 50 degree air, the other running in 90 degree air, the 50 degree engine will be making noticeably more power. Why? Because it's burning more fuel, while maintaining the same 14:1 fuel air mixture.

av8tor1977

My Feedback: (6)

I agree with most of what you said. Just a couple of comments really.... The stoichiometric mixture, or the "perfect" fuel mixture is 13.7 to one, which you have rounded off to 14 and that's fine. However, best power is generally achieved at around 12 or 12.5 to one. That is also a much, much safer mixture to be running a two stroke engine at to keep it alive. A moot point really, since few of us use a meter to read the actually read the exact mixture; instead tuning by ear or rpms.

As far as the density issue, something most people don't realize is that these ratios we speak of, 13.7:1, etc. are in pounds. It is 13.7 pounds of air to one pound of gasoline. Since each "gulp" of air the engine takes at a lower atmospheric density weighs less, while the fuel weight stays the same, the mixture automatically richens, and vice versa.

You can find lots of density altitude calculators online, and it is interesting (and sobering) to play with them. Density altitude is the true altitude adjusted for barometric pressure and temperature. (Sometimes humidity is calculated too, as it does also have an effect.) Density altitude is what the airplane experiences in relation to the lift of the wings, the efficiency of the prop, and the power of the engine. (And mixture as mentioned above.) With each 1000' gain of altitude, a normally aspirated engine loses approximately 3% of its power.

AV8TOR

ahicks

My Feedback: (2)

Again, kinda wandering from topic, but....

>>

A richened mixture is something we'd like to avoid, right? If we are attempting to maintain optimum fuel mixture/max power? We would lean the mixture to correct?

Or am I not following what you are trying to get across?
-Al

av8tor1977

My Feedback: (6)

Yes, you have it right. In full size airplanes, there is a mixture adjustment lever or handle (called the mixture control), right next to the throttle. As you gain altitude, you lean the mixture to keep it correct. Coming back down, you have to richen it back up with the mixture control.

Glow engines are much more finicky about having the mixture set just right, and often need some adjustment for temp/humidity changes. Gasoline engines and Walbro/Zama/Tillotsen carburetors generally don't need much, if any adjustment for ambient condition change. If any adjustment is needed for large temperature changes, it would be quite small; probably only a screwdriver width if that, of the needles I would say. Changing props, fuel, oil, etc. might engender a larger mixture adjustment change however. Basically, gasoline engines, unless you change something or something changes in the engine through wear or breakage, are "set 'em and forget 'em". They don't need constant fiddling like a glow engine. (Even glow engines need less "fiddling" than they are often subjected to!!)

To answer your question, we don't want to adjust our engines too rich nor too lean. Too rich loses power and causes carbon build up, too lean causes "sagging", overheating, and likely engine damage. I like to adjust for max rpms, then open the needle about 1 screwdriver blade width richer. I consider that a good, safe mixture setting. We shouldn't be setting the mixture to control engine temps, even though they do have an effect. The mixture should be set properly, and then the cooling system set up so that the engine cools itself properly.

AV8TOR

speedracerntrixie

My Feedback: (29)

Good post AV8TOR, I may add that most of my flying happens at sea level. When I would go to Las Vegas for a contest I never had to tweak needles on my engine. I would however need to fine tune my throws and expo settings.

Lifer

My Feedback: (1)

Quick question on mixture: My 55cc with a 22x10 has the needle opened 1.75 turns. I then bolted on a 4-blade 20x10x4, richened before the start then leaned it out to a good setting. After the flight I closed the needle in order to count the turns. It was 1.75 turns. The 2 blade was turning about 1,000 rpm faster than the 4 blade. Does this mean that the engine needle settings are unrelated to rpm and load?

Thanks to all for any replies received.

speedracerntrixie

My Feedback: (29)

Lifer, yes load will affect mixture as will RPM. In your case the additional load should have required a richer setting but remember that you dropped 1,000 rpm so your fuel requirement dropped. That being said it appears to me that you have yet to fly with the 4 blade prop? If that is the case, be prepared to make more adjustments.

captinjohn

Try something simple. Plan your landing slope so it is not steep at all. Start that final decent out further & I bet that "high Idle hang" will go away. It may also look more like a full scale landing !

av8tor1977

My Feedback: (6)

Yeah Captain John. I do often see people diving at the runway instead of gliding in as you should. That can help an engine not want to idle down, as well as screw up your landing by arriving at the strip with too much airspeed.

AV8TOR

pilotpete2

Uh Oh, I think you've been lurking around my field, haven't you?
Pete