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Laser Engines - Technical questions

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Jon - Laser Engines19/04/2018 09:05:09
3920 forum posts
151 photos

Bert, that dosent look bad. Performance is in line with what i would expect. How is the throttle response?

Denis Watkins19/04/2018 09:14:07
2906 forum posts
141 photos
Posted by Chris Walby on 18/04/2018 22:17:15:

Noise test today with the Dual Ace (twin 70's) and it failed at 85dB (needs to be 82)

The airframe is not helping as it resonates being balsa open structure with covering.

Advice a

  1. Do something to the airframe to stop it "drumming" so much?

 

Bath sponges Chris

In the open spaces, especially the tail

Cheap sponges weigh nothing from the £1 shop, you get 2 or 3 in a packet

Also, I know people Hate this one, and I will be berated

But Soft Mount the motors, it halves the noise

Edited By Denis Watkins on 19/04/2018 09:16:14

bert baker19/04/2018 09:31:20
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1176 forum posts
249 photos

Throttle response is poor,,,ok if altered slowly,,and low idle unobtainable, I have both needles out 2-1/2 turns,, suspect signal pressure is not god enough,,i may try the carb of my NGH 30 four stroke,, I would like a second nipple in back plate and a pipe to go to inlet manifold and a vent between front bearings

Jon - Laser Engines19/04/2018 09:52:47
3920 forum posts
151 photos

Bert just drill another hole in the backplate and stick a 2nd nipple in there. Thats what i did on my early testing.

Denis, soft mounting may help but in our experience it usually causes excessive engine vibration which ultimately damages them. Damping is normally only an advantage on metal mounts as nylon mounts do flex a little. Personally i wouldnt recommend it, its more trouble than it worth. Filling the model with sponges, or other damping material is likely to be worth while but only if the drumming is from the airframe itself and not a rattling cowl or similar.

Bob Cotsford19/04/2018 10:15:00
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7327 forum posts
419 photos
Posted by Jon - Laser Engines on 19/04/2018 09:04:20:....Again i assume they were the 12x7 apc props? If so you cant really do much more. 13x7 would slow them down for sure but ground clearance will be an issue. 3 blade is not an option as they make more noise anyway.

DO you have a video of the model running?

 

You would think so wouldn't you? I had noise test issues with the 120 Surpass III in my Skybolt and a 120 Surpass II in a Ta152, Even using APC props and 180 size exhausts I couldn't get below the club limit with 2 blades but swapping to Master 15*7 (iirc) 3 blades dropped both the revs and prop tip speed enough to get the models through.

If I remember rightly my ASP 70 is turning a 13" prop of either 7 or 8" pitch. I'd have thought a Laser should be ok on 13*8 - if they will clear the ground that is!

Edited By Bob Cotsford on 19/04/2018 10:15:39

Bob Cotsford19/04/2018 10:22:24
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7327 forum posts
419 photos
Posted by Denis Watkins on 19/04/2018 09:14:07:

Bath sponges Chris

In the open spaces, especially the tail

Cheap sponges weigh nothing from the £1 shop, you get 2 or 3 in a packet

Also, I know people Hate this one, and I will be berated

But Soft Mount the motors, it halves the noise

Edited By Denis Watkins on 19/04/2018 09:16:14

I like the sponge idea, I was thinking of thin self adhesive foam or vinyl sheets but the weight of the glue can add up. When I first built my Acrowot in the '80s I put 2 strokes in it and found that a layer of sheets rubber between the mount and firewall cut the resonance down. On my Jungmeister I used rubber well-nuts (sold as Rawlnuts in hardware shops or as motorcycle fairing fasteners) to mount the engine. That worked too.  Jon, these both used short and very rigid grp mounts, very little flex in these old Irvine units!

Edited By Bob Cotsford on 19/04/2018 10:25:52

Martin McIntosh19/04/2018 11:05:17
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2485 forum posts
947 photos

If this is of any help, back in the 70`s I used highly tuned OPS 60`s on pipes. Changing from 11x7 Rev Up wood to 9 1/2x7 GRP dropped the noise by over 3dBa, big thrust loss but still plenty. Strangely, rounding the tips slightly increased the noise by 3 dBa. Squaring them off again brought it back down.

I would suggest that you try 11x8 to reduce the tip speed.

Most reasonably priced sound meters are rarely within 3 dBa of each other. You need a DOE calibrated one to be sure. Also the test conditions such as wind strength, length of grass etc have a huge bearing on the results.

The SMAE eventually reduced the test distance to 1m at the time so that wind noise became negligible. Cannot remember what the new limit was.

Jon - Laser Engines19/04/2018 11:50:29
3920 forum posts
151 photos

Bob, i have a 3 blade 14x7 on my Laser 80 and it howls like mad. Even at idle you can hear a whine from it. In the air is fine, but on the ground its much louder than a 2 blade

The rubber behind the mount form of damping would be fine if you dont mind tightening the bolts periodically as they will come loose after a short time. Im still not a fan, mind you

Martin Harris19/04/2018 13:32:56
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7696 forum posts
191 photos
Posted by Jon - Laser Engines on 19/04/2018 09:04:20:

Martin, i understand that the test is being done 'by the book' but that does not mean it is being done correctly. I have seen it at clubs before where meters were set to record peak sound and if a butterfly broke wind nearby you failed the test. Some clubs average all 4 results, others do not. There is no real standardisation. Given that the noise test records of a club offer no protection of any kind against a noise complaint the whole thing is, sadly, pointless. Im not saying that noise is not a consideration as it is very important, but running flat out on the ground is hardly a representative test. I understand why we use it though as sadly a perceived noise test with the model in flight is not really quantifiable so cannot be tested very easily, even if it is perceived noise and nuisance that actually cause complaints.

And i know my phone app isnt offical but its all i have. If i get a reading of 79 for example there is no way the app is that inaccurate. Thats all i was looking for.

In the meanwhile Chris angle the exhaust deflectors back very slightly and see if you can work out which part of the model is resonating and sort it out. As before, double check you dont have a prop out of balance and then as a last resort set some sort of throttle limit. Make sure its easy to turn on and off as you need to be able to open both throttles fully to tune the engines. Were you able to check the rpm of the engines? i assume they were pretty close to eachother? if not this will create more noise. Again i assume they were the 12x7 apc props? If so you cant really do much more. 13x7 would slow them down for sure but ground clearance will be an issue. 3 blade is not an option as they make more noise anyway.

DO you have a video of the model running?

 

I did say it was done to the test specifications - slow response, dBA weighting and common sense when transient noises boost the readings, so no peaks. Noise level not to exceed 82dB in any of the 4 orientations.

Factors which can affect readings include proximity of sound reflective material - even including bystanders...we sometimes resort to standing out in the middle of the field for marginal models!

Very often, increasing pitch rather than diameter will drop the noise level significantly unless you hit resonance of course! RPMs drop and with the lower diameter, there's a double whammy on the speed of the tips, which is where most of the prop noise is generated.

I'm glad you gave such confidence in your phone app - it sounds impressive that all microphones come out of the factory equally calibrated!  Comparisons between our meters and phone apps have been less than impressive when we've tried them.

Edited By Martin Harris on 19/04/2018 13:34:09

Bob Cotsford19/04/2018 13:55:13
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7327 forum posts
419 photos
Posted by Martin Harris on 19/04/2018 13:32:56:even including bystanders...we sometimes resort to standing out in the middle of the field for marginal models!

We have one member, Ken, who has been recorded at 84dBa in normal conversation disgust

Anything that can reduce tip speed and resonance will help but I suspect that not much short of converting to electric will get a twin 70 model through a to-the-book noise test. My club specifically lists limiting the throttle movement as an automatic fail. The model must be full throttle, ie max rpm, no richening the mix to drop revs either!

cymaz19/04/2018 17:53:18
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7701 forum posts
988 photos

Any good trying a 3 blade prop?

Jon - Laser Engines19/04/2018 19:36:57
3920 forum posts
151 photos

As I have not tested my twin in any form I cant say if 85 is right or wrong, but it sounds really high to me. Loads of customers have used two 70's in models and we have not had any issues before with noise tests and I cant believe they are louder than a pair of two strokes running higher revs.

12x8 props would certainly bring the revs down but my goodness it would be a hot ship!

Martin, I know they are supposed to use slow response, my question is did they? I think very few actually read the full details of how to perform the test in full compliance with the guidelines.

bert baker19/04/2018 19:50:47
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1176 forum posts
249 photos

I had positive comments on the sound of the petrol conversion I have been doing,, I shall noise test it soon,,,

Jon - Laser Engines19/04/2018 20:48:16
3920 forum posts
151 photos

I have been able to fit my 155 petrol prototype into my pulse 125 and intend to get it in the air soon. Testing it on the ground it was peaking at 8800 on a 16x8 using aspen fuel. I was surprised by that but I will take it

TartanMac19/04/2018 21:44:34
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228 forum posts
142 photos

Is there any difference in power between Aspen and petrol ?

Jon - Laser Engines19/04/2018 22:06:57
3920 forum posts
151 photos
Posted by TartanMac on 19/04/2018 21:44:34:

Is there any difference in power between Aspen and petrol ?

I think normal petrol is about 100 revs faster but have not really tested it that much

Chris Walby19/04/2018 22:36:42
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541 forum posts
96 photos

Seagull Dual Ace update, it passed its noise test today and the changes were:

  • Exhaust deflectors pointed reward
  • 12 x 8 props fitted
  • Upper servo arm travel limited (this is menu driven and can't be adjusted in flight or accidently) currently shows about 2 mm of throttle barrel when in wide open position
  • Standing in the middle of the flying field with just the model and the tester

Additional measures had the above not been sufficient frown

  • Fill the airframe with water proofed cheap sponges
  • Gone to 12 x 9 or 13 x 8 props
  • Limited the throttle even more

I have a couple of other challenges to sort out, one being the wing bolts keep working loos/falling out! and then I'll be ready to go.

Thanks to everyone who contributed suggestions, some I have used and others I'll keep for the next time smiley

Jon - Laser Engines19/04/2018 23:00:11
3920 forum posts
151 photos

loose wing bolt is more than a challenge!

Manish Chandrayan20/04/2018 02:35:36
452 forum posts
56 photos
Jon
What happened to the 180 testing? Is it on? I mean engines out with individuals and being given good exercise?
Manish Chandrayan20/04/2018 02:39:12
452 forum posts
56 photos
Chris, is the wing bolt metal? If it is, replace that with a nylon one and that is likely to solve your issue. I would also suspect that there is some considerable vibration for the wing bolt to keep coming loose. This may also have been a factor in the noise problem you were encountering

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