I feel it could do with a little more ext air on the cowl, But you do have a lot in the can tunnel, But an extra inch in depth would be all it should need.

But this wont help the current issue, But will help perhaps keep cooler in the air

Thanks for the extra info and posting those pictures. I agree with the Hodge on a couple of points. If the throttle transitions smoothly without stuttering I would feel confident moving forward with testing in flight and then dialing in the engine from there. The chances of you having to slam the throttle from idle to WOT will be extremely rare. You will also have the advantage of clean moving air through the inlets when the plane is flying, something you don't get on the ground. However, I also feel that you should open up more of the cowl for exit air. The canister tunnel is kept open to help keep the mufflers cool but it doesn't allow for much of the engine air to exit due to the restrictions of the canister mounts. I've attached a pic of the cowl holes on both my 105" and 123" Lasers.

On a completely separate note, I was really surprised to see so much extra weight on your firewall! My setup being identical to yours up front didn't need any extra weight. Only thing I can assume is that you set your plane up with push/pull rudder servo in the tail?

FWIW I have the same exact airframe and set up as you. At first I tried a low pressure lip at the cowl a lot like yours and a second exactly like the white triangle stock and in the same location. On the ground was good in the air it sucked. Tried everything like to drive me nuts...…. With no cowling everything was good...…..So after some thought I removed the front ( cowling ) lip made a flight and the problem was gone some how the two were fighting each other in air flow.....Well that's me story I hope it helps in some way believe me I know how frustrating it is...!

Thanks for the suggestions guys. And I should also thank DA who's been super generous with their time and expertise.

The engine transitions fine when I’m less aggressive with the throttle so The Hodge’s suggestion of slowing the servo should at least solve the symptom. I’ve seen enough “punchy” engines with near instantaneous acceleration to figure that’s what I should be aiming for. And I tune by Terry’s Custom’s method where he recommends flicking the throttle stick like a light switch. Maybe I just need to dial back my expectations a bit. Warwix is correct that I don’t -need- that kind of response. I just naturally try to aim for top performance and I’ve had enough deadsticks (I mentioned there's more history) to not want to “settle” for less than optimal. I’ve slowed the servo at the Tx and will test it tonight. I’m not sure if it’ll solve the root cause or just the symptom in which case the problem might show up somewhere else later. Only way to know is try it and fly it.

I’ll also try removing the cowl lip since that worked for AJT. I hadn’t even considered that the lip could be hurting things. It’s installed with Goop just in case I ever needed to remove it so this will be easy.

Warwix: I’m using comfort mounts that keep airflow through the tunnel pretty much unrestricted. Photos attached. The stock ply mounts are sitting on my spare parts shelf. Separate/related: I don’t see low pressure lips in your cowl photos. Are you sure those are functioning as air exits rather than inlets? I’m no aero engineer (that's obvious from the problem I’m having) but I would have thought the area below the cowl to be high pressure, thus vents without a lip acting more as an inlet. Not criticizing, just curious. If it works, it's good.

DA offered a bunch of other suggestions that have worked for people throughout the years so there’s more to try if these don’t solve it. It seems the key is to get the airflow to where the diaphragm vent and induction airflow work together reliably. The knowledge has to be out there. If it were only packaged and documented for us…

Originally Posted by Warwix

On a completely separate note, I was really surprised to see so much extra weight on your firewall! My setup being identical to yours up front didn't need any extra weight. Only thing I can assume is that you set your plane up with push/pull rudder servo in the tail?

Experiment in progress. I had time to tinker while the engine was away at DA. I installed a lightweight pilot & panel for IMAC this year, painted the rear of the canopy (my one nod to aesthetics on this airplane), and the big one, switched from the stock fuel tank to dual tanks in series, sitting directly over the CG (aside: as I mentioned above, the 2nd tank has been disconnected for the engine testing to eliminate that as a variable). At full fuel, that shifts 26oz rearward by 2". I had 3oz of weight even before these changes. The extra that you see is an estimate to counter the mods. I haven't flown it yet and yes it might be too much. The fuel-over-theCG approach is a tradeoff between light weight vs. predictability. I haven't flown it this way yet so don't know if I'll like it or not. I'm sure others have tried it at some point. Curious about their experience.

My batteries are in the motor box, rudder is pull-pull, and engine is 1/4" forward from stock, so I was also surprised to need some nose weight originally. Chalked it up to the canisters and lightweight Falcon spinner.

The only other variable I can think of is elevator servos which are Savox 1270 which I specifically installed because they're lighter than the MKS 380s I originally had installed. If there are lighter without skimping on strength/centering/reliability, I'd consider it.

Good stuff Random. Those comfort mounts are great and really open the tunnel up for cooling, so strike my comment about that, haha.

I cut my cowls with a larger hole and no lip mainly to act as both an exhaust at low speeds and also as a ram air inlet to the pipe tunnel at higher speeds (as long as there is another place for the air to exit behind the cowl) I've setup all my models like this and have never had heat issues. I'm very curious to see what changes you may see removing your lip on the cowl.

Very cool to see you tinkering with the planes layout, I'd really like to see the entire plane together with the canopy painted! I'm still not sure why you have to add so much nose weight. Where are you happy with the balance point? My setup is, again, almost identical (DA120, 3086 cans and headers, 26x12x3 prop, Falcon spinner, 32oz Fortitude single tank back against the wing tube, dual pull/pull servos, and two 2500 mAh A123 packs between the wing tube and the rudder tray). With that setup, I am about as perfectly dialed in for IMAC as I could ask for.

Either way, I'm sure it's still performing very well for you. Glad to see another IMAC fan out there!!

Matt

Originally Posted by randomguy

Thanks for the suggestions guys. And I should also thank DA who's been super generous with their time and expertise.

The engine transitions fine when I’m less aggressive with the throttle so The Hodge’s suggestion of slowing the servo should at least solve the symptom. I’ve seen enough “punchy” engines with near instantaneous acceleration to figure that’s what I should be aiming for. And I tune by Terry’s Custom’s method where he recommends flicking the throttle stick like a light switch. Maybe I just need to dial back my expectations a bit. Warwix is correct that I don’t -need- that kind of response. I just naturally try to aim for top performance and I’ve had enough deadsticks (I mentioned there's more history) to not want to “settle” for less than optimal. I’ve slowed the servo at the Tx and will test it tonight. I’m not sure if it’ll solve the root cause or just the symptom in which case the problem might show up somewhere else later. Only way to know is try it and fly it.

I’ll also try removing the cowl lip since that worked for AJT. I hadn’t even considered that the lip could be hurting things. It’s installed with Goop just in case I ever needed to remove it so this will be easy.

Warwix: I’m using comfort mounts that keep airflow through the tunnel pretty much unrestricted. Photos attached. The stock ply mounts are sitting on my spare parts shelf. Separate/related: I don’t see low pressure lips in your cowl photos. Are you sure those are functioning as air exits rather than inlets? I’m no aero engineer (that's obvious from the problem I’m having) but I would have thought the area below the cowl to be high pressure, thus vents without a lip acting more as an inlet. Not criticizing, just curious. If it works, it's good.

DA offered a bunch of other suggestions that have worked for people throughout the years so there’s more to try if these don’t solve it. It seems the key is to get the airflow to where the diaphragm vent and induction airflow work together reliably. The knowledge has to be out there. If it were only packaged and documented for us…

Awesome stuff Glade it seams you have a resolution, re the servo speed you current .007 ish speed only probly needs if change to around the 0.14 or so should be perfect and will still be punchy, Just experiment a bit get to a point where you can go idle to Wot (Fast) and it transitions well.

When the Video and write up Terry did was done most servos were not as fast as your one

Now go fly her

With my 105 I don't have anything cut out in the cowl. Just the stock locations for vents. I also opened up the last hole in the tunnel to allow full unrestricted air flow out of it.

I flew 3 flights tonight. Slowing the servo helped keep the engine running during most idle > full transitions on the ground. I probably need to slow it a tad more.

With the original carb and last year’s more extensive baffling, the engine ran very smoothly no matter what I asked of it, both on the ground and in the air (aside from a number of very sudden dead sticks…). Now the midrange in flight is very rough – seems quite rich, very different from on the ground. Tonight I’m going to reinstall a portion of that baffling and see if it helps. If not, I’ll try removing the low pressure lip. I’m also going to call Tim to see if he’s had others with similar problems. After all that, I’ll tune it based on in-flight rather than on the ground. Need it to be “good enough” for a contest this weekend.


Matt – My CG before the mods was about .15” ahead of AJ’s recommendation (so, 1.65” ahead of the wing tube) without fuel. I was very happy with it. Very stable inverted with just a touch of down, and inverted 45 test gave a gentle arc down. A couple very good higher level IMAC pilots who flew it thought it was fine - one of them suggested it flew better than his Carden. I’m curious where you have yours set? It would seem that with a second rudder servo and batteries more rearward (not sure if I mentioned but my 2x Pulse 3600’s are in the engine box), your CG should be a good bit aft of where mine is. What elevator servos are you running?

Excellent glade its going well. Sounds like you need to lean the Low end a touch,

Servo Speed wise you may find that in flight you may not be going that violently from Idle to Wot so may be fine.

She looks great.

It's just opinion of course, but I think it looks much better with a single place canopy.

Solid progress today.

Last night I mounted a metal plate to the rear of the carb, extending out about 1/2" to each side and below (DA's idea)- kind of like a backstop. I velcro'd a smaller plate to the diaphragm cover on the front of the carb to baffle the area around the vent hole. And I reinstalled a foam splitter between the bottom surfaces of the cooling baffles.

I also slowed the servo a bit more. It's now at 7 (Futaba tx). I'd bet this one was the game changer. Someday I'll confirm that by subtracting one airflow device at a time until something bad happens.

I was able to lean much more aggressively. The midrange is a lot better but still has noticeable roughness at certain RPMs which it never had before - and it's precisely at the speed I want to fly at. I may try slowing the servo further in hopes of being able to lean the low end needle more. This begs the question of why is servo speed an issue now when it wasn't last year. But I'm happy to be flying again.

The dual tanks are new and working flawlessly. They're plumbed in series. The tank plumbed furthest from the engine gets emptied bone dry before the other tank starts to empty which is perfect. Gotta love felt clunks.


Matt was dead on about the CG (no surprise there). I ended up removing all but 70g of the nose weight. There might be room to go even further when I have a calmer day for trimming. I go pretty far to eliminate weight on any airplane and more so on a Laser - I feel like I'm offending Leo's legacy - so I'm feeling much better about that.

Just a note regarding servo speed. The speeds are of course at no load, which a carburetor does not present any level of loading to a servo (unless there is a spring, in which case the load is still minimal). Depending on how you have your linkage geometry, there is a potential that the carburetor itself is seeing 0.02-0.03 faster transition times than the servo itself is rated at. So, if using a 0.05 servo, there's a possibility that your carb is seeing 0.03 transition times, or greater. If this is the case, you could potentially be opening your carburetor at such a speed that fuel is unable to flow and your engine leans out to the point that it either sags severely at first, or dies completely.

The solution to avoid this is to use throttle servos with 0.1 transitional speeds or slower, and pay close attention to your linkage geometry. A shorter servo arm than your carb arm is far more beneficial than it may seem and helps with utilizing a wider range of servo resolution to provide finer control over your throttle.

In your instance, you can help alleviate the servos lightning fast speed by shortening your throttle linkage movement at the servo arm end. I would move your linkage in to about 0.7" out from the servo and widen the servos EPA's in your transmitter to achieve the same movement at the carburetor. You can progressively move the linkage in at the servo arm if you need to slow it further, and continue to widen your EPA's.

On my 105 Laser with DA120, I'm using a Hitec HS-5565MH with about a 0.625" long servo arm, and I have flipped the carb arm on the 120. It's been rock solid for 800+ flights and revs as quickly as anyone elses DA120

Just some food for thought

ok fellas... I have seen some threads about this issue before but would like to know what the current fix is or IF it can be fixed.
105 Laser DA120 on KS3086 canister set up. Here is the deal...After running about a minute in the air engine starts surging like mad....I mean bad. Have tried both sides off of 1/1/2 out on carb t o no avail. Printed baffles, all exits open. Going to put standard mufflers on if I can't get this taken care of...5 flights so far. Love the plane but can not do anything with it due to this terrible surging. Thanks for any info out there.
Mack Daddy.....

Originally Posted by Big Mack Daddy

ok fellas... I have seen some threads about this issue before but would like to know what the current fix is or IF it can be fixed.
105 Laser DA120 on KS3086 canister set up. Here is the deal...After running about a minute in the air engine starts surging
Mack Daddy.....

What happens if you lean your low needle in small increments?

What throttle servo and how long is the arm?

What carb do you have? See photo. It'll be stamped with D, D2, or nothing. Also do you have the natural metal diaphragm cover or the bright red anodized one?

I'm curious what prop you're running.

Could you post pictures of your air inlets, outlets, and baffles?

I'm having what sounds like the same issue. I would describe mine more as roughness and lack of power in the mid range.

1) I've been able to narrow the RPM range where roughness occurs by leaning the low needle which required slowing my very fast throttle servo. This was only a few days ago, I don't yet know if slowing it a bit more will allow further leaning and eliminate the problem. I made a few other changes at the same time, all described above, so I can't be 100% certain the improvement was due solely to slowing the servo. I expect it was the primary reason. EDIT: I hadn't seen Xpress' post when I wrote this. He's spot on. Slow it down and use a short arm to maximize usable servo travel.

2) Look at the successful installations above by Warwix (AJ team pilot Matt Komar) and AJT. Both have cut cooling vents in the bottom of their cowls. Normal recommendation is to add a low pressure lip to improve air extraction. Warwix has no low pressure lip and no problems. AJT had roughness which he says was resolved when he removed his low pressure lip. I think it's worth trying. I'm going to remove the lip from my cowl before the next flight.

3) I fully believe it can be fixed, for two reasons. One is Warix and AJT both have similar setups and seem to be running fine. If they can do it, we should be able to also. The other is that I never had any roughness at all when I had my original extensive custom baffling and just the stock cooling vents, though I did have several deadsticks. The roughness only started when I made a bunch of cooling-related changes to try to solve the deadsticks, including adding the lower cowl vent and large-ish low pressure lip. Ironically, the changes were designed to make my setup as close as possible to setups that other people seem to be using. I guess that didn't work out so well . It also runs fine on the ground without the cowl. So this very much appears to be an airflow issue which of course can be resolved. We just need to find the right solution.

DA has been even more specific, suggesting the root cause is that the airflow around the diaphragm vent hole is out of sync with the induction air entering the carb throat. There's no shortage of potential fixes. Any of them might work but none of them seem to have been proven for this setup yet. Again my next step will be removing the low pressure lip.

It would be really interesting to cut cooling holes to mimic one of Warwix' pictures above because we know it works. Meanwhile I'm still speaking with DA about some things and will contact Tim tomorrow to ask about others' experience. He's out of the office today.

I want to say for the record that I'm not pointing fingers at AJ or DA or anyone else for the problem. It's a wonderful airplane and a great engine, and there isn't any I'd rather own. Most installations seem to work just fine. The problems seem to be the minority and are likely due to some uniqueness of our setups. My goal for raising this here was simply to solve the problem. Now that there seem to be others with similar problems, hopefully we can find a solution that will help everyone.