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Seagull Mosquito Laser build

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Jon - Laser Engines18/01/2019 08:20:05
4765 forum posts
179 photos
Posted by John Stainforth on 18/01/2019 00:45:42:

...so for a 1/8th scale Mossie (with wingspan around 80 or 81" the spinners should be about/at least 4" or 10 cm in diameter.

Edited By John Stainforth on 18/01/2019 00:46:59

Yup, the brian taylor model uses 105mm spinners

Chris Walby25/05/2019 07:59:28
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952 forum posts
228 photos

With the impending rubbish weather forecast for this weekend and a couple on nice flights with my BH Hurricane the pervious day I though it would be a good idea to get the mossie out.

Last weeks engine test seemed to show up a port idle issue and minor idle to 1/2 throttle RPM difference where the starboard would not be consistent.

At the field the Dual Ace assembled and flew a treat although it was getting buffeted in the breeze (lowish pass roll and loops ok).

Back to the continuing aggravation with the mossie and with the idle and WOT set (good tweak on the port main needle had it all sorted). However it still had the idle to 1/2 throttle inconsistent pickup on the starboard engine of about 1000 RPM. I know what it is....between the mixes and different end point settings the TX delays the initial output on the aux (starboard) output.

The motto don't push a poor decision was well and truly ignored in part as this idle/pick up issue was just winding me up, the cloud had come over and the wind picked up , but the wind was straight down the runway so what the..lets just get on with it.

I can only say the practice take off runs installed little confidence with a couple of wild swings either way so another poor decision was made and I lined up and took off. Lively climb out and gear away with the speed picking up she circled around all be it with a couple lively roll/yaw moments (I put down to wind & slower airspeed).

Lowish pass and nice rolls complete I though I would try a nice big loop and was very slightly surprised with it twisting out of the top of the loopl, but that could have just been with a lack of rudder input. Another poor decision to continue.

At the far down wind end I thought I another loop would be nice, however at the top (might have slowed too much with the throttles backed off and it was slightly cross wind).

This is where it all unravelled and at the top of the loop it rolled and yawed into a very unpleasant spiral dive. It was quite a long way out and high so tricky to see in detail but looked like it was having nothing with the throttles up so I chopped the throttles and prepared for a very long walk with a couple of bin liners. At that point it stopped rolling (in the dive) so I levelled and got the power back on...circled and landed. Rubbish landing and bent the U/C pin/oleo block and retract mech, but nothing else.

The mossi is quite heavy and with a thick wing, but I can't see why it would yaw at the top of the roll so much even with the engines out of sync at the low end?

My initial thought was one engine had quit, hence leaving it in the dive, getting the air speed back and then returning to near full power to see if I could get her back to the field.

Can anyone explain the why it yaw/rolled at the top of the loop?

Lessons learnt

  • Don't mess about at the field unless the problem is fully fixed
  • If its not 100% don't fly
  • If it does something odd, don't give it another go just to see it will do.It might be worse the second time
  • Back to the shed and modify so that it can be mechanically adjusted to be in sync
  • Listen to Jon more
Andrew Ray25/05/2019 10:54:34
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700 forum posts
19 photos

If you were too slow over the top, most likely the wing dropped as it stalled and as that happened the fin caused the model to yaw as it dropped. A yaw could cause a wing to stall and drop but that would imply that an engine had cut and it doesn't sound that it did as you circled to land.

Danny Fenton25/05/2019 12:44:30
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9092 forum posts
3937 photos

Well done for flying Chris, not sure about the yawing but i can only assume it is speed related?

Mine is all prepped to go again. Mine is/was shiny, a coat of ronseal, hard as diamonds (matt) rollered on has transformed it.

Cheers

Danny

Jon - Laser Engines25/05/2019 21:19:27
4765 forum posts
179 photos

my guess is loss of airspeed combined with a slight difference in engine thrust. Even if both engines were exactly identical with exactly identical props giving exactly identical thrust, torque would still throw it over.

Its also possible you suffered a lean fade on one engine and your reduction in throttle saved the day and kept it running. At the first sign of any engine issue is instantly throttle back to half and more often than not this will save an engine from a lean cut.

 

Nice work on the save though. It all to easy to throw in the towel before its hit the ground and very often the model can be rescued 

Edited By Jon - Laser Engines on 25/05/2019 21:20:24

J D 825/05/2019 23:08:58
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1226 forum posts
74 photos

Some years ago at an air display I was chatting to a former Mosquito intruder pilot. Dam fine aircraft he said. Packed a heavy punch and was fast, got you in and out of the hot spots real quick. [ his language was a lot more colourful ]

We were watching mosquito RR299 doing its display . " Bloomin fools" he said " asking for it flying a mosquito like that at low level" .

A few years later crash it did. The investigation concluded that a temporary reduction of power of the left engine resulted in a yaw followed by loss of control into a spiral dive.

Chris Walby26/05/2019 07:00:39
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952 forum posts
228 photos

Thanks guys for all of the helpful advice and I agree with your analysis.

In preparation to flying the Laser Mosquito (my much needed experience of running glow and flying twins) I build the pair of 70's into a SG Dual Ace. Jon's advice has been very helpful throughout the entire process, but especially with twins and the general concept of always having a plan B in case something goes pear shaped. With that I mind practiced on the simulator with various engine cut scenarios.

Just as well as my electric BH Mossie suffered a loss of a motor on a low fast pass and survived by more luck and me not doing the wrong thing!

JD 8 I have not seen the RR299 footage for quite some time, but the way mine rolled in is very similar, just I had a lot more height on my side. My passion is for the wooden wonder and there are some interesting comments from Mosquito pilots about its flying characteristics, but equally its sting if things don't go well. There were quite a few losses on take off if one engine lost power. The additional drag of the prop and low air speed combined with the urge to apply more power from the good engine would end in disaster.

A reminder to me that it is not the Dual Ace by a very long way and that the greatest enjoyment often needs a proportional amount of effort from me (and respect!).

Jon - Laser Engines26/05/2019 21:50:49
4765 forum posts
179 photos
Posted by J D 8 on 25/05/2019 23:08:58:

Some years ago at an air display I was chatting to a former Mosquito intruder pilot. Dam fine aircraft he said. Packed a heavy punch and was fast, got you in and out of the hot spots real quick. [ his language was a lot more colourful ]

We were watching mosquito RR299 doing its display . " Bloomin fools" he said " asking for it flying a mosquito like that at low level" .

A few years later crash it did. The investigation concluded that a temporary reduction of power of the left engine resulted in a yaw followed by loss of control into a spiral dive.

The accident report for that makes interesting reading. In essence, neither carburettor was set up correctly and this left the engines vulnerable to lean cuts under reduced G. Suddenly loosing 1400hp from one side of your aircraft is certainly going to ruin your day

Chris Walby27/05/2019 06:22:44
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952 forum posts
228 photos

Thanks again for everyone's input, I think I'll go for a combination of too slow over the top (tip stall) made worse by the engine imbalance at low throttle setting and a dob of cross wind as well.

The numpty on the sticks will have hopefully learnt his lesson which leaves some advice for the mechanical modification to do.

throttle servo linkage.jpg

The current arrangement is quite thin wire and barrel on the end of the servo arm. What I would like is a clevis, but I am not sure the best way of fixing the small diameter wire to a threaded stud for the clevis.

Idea's anyone please

I'll take some better photos and post

PS its the servo on the right behind the wires wrapped around the oleo (removed before flight!)

 

Edited By Chris Walby on 27/05/2019 06:25:51

cymaz27/05/2019 06:51:03
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8575 forum posts
1157 photos

Chris, sounds like the loop was too big. How do you prep the engines before flying? With my multi ones each engine is individually run up, tacho and pick up-checked. Then all run up together, that way you can spot a problem or discrepancy in any engine. Do not rush this and anything you’re not happy with, now is a good time to fix it at home.

When setting up the throttles I placed a small length of wire into the barrels and shut the throttle. Then set all the throttle arms at the same angle using a bevel gauge. Are your throttle servos moving at the same speed? Are they at the same position at 1/4, 1/2, throw etc.

Its a pain to get right but great to see

Edited By cymaz on 27/05/2019 06:52:50

Chris Walby27/05/2019 09:18:06
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952 forum posts
228 photos

Thanks Cymaz, I warm each one up individually and set the main needle WOT, then run both up and check they are in sync.

My problem is that I have each servo on a separate channel with a couple of mixes in to get throttle and throttle cut to work, Somewhere in there I have messed it up and the Aux channel does not follow the throttle for the initial 0 to 35% although it catches up by 40%. Adds to the excitement of taking off and I didn't think it would make much difference in flight...I hadn't anticipated doing an inverted stall test (top of loop!). I'll do what Jon suggested (teach me!) and set them on a single channel and spend more time mechanically setting them up. Live and learn.

20190527_082124.jpg

What I need is a simple and elegant solution that results in a clevis so I can tweak it by 1/2 turn to get them spot on.

PS I like your idea of a pin to set the throttles at the same place and that might get me close enough.

cymaz27/05/2019 10:02:36
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8575 forum posts
1157 photos

Can you get Bowden cable down the tube? Then solder some threaded links on.

Or even....two of these

Chris Walby10/06/2019 07:47:44
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952 forum posts
228 photos

Sorted the TX out and have since found a much more elegant way of doing the mix, but that's for another day.

Yesterday was all about having some real fun (although still getting used to this war bird) with the wide sweeping low pass. Climb out and perhaps a cheeky slow roll.

Tried a big loop and just kept the power on until well over the top with no issues.

"Black Rufe" on a low pass (credit to Dave for the photo)

img-20190610-wa0000.jpg

Landing was pants, but that's just me as it sent the warning message out and I just backed off the throttles too quick, caught out by something that lands short ever weighing this much wink

Once again thanks to all that have given advice over the build and flight.

Danny Fenton10/06/2019 08:08:54
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9092 forum posts
3937 photos

Looks great, well done Chris

Cheers

Danny

Nigel R10/06/2019 09:39:26
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2968 forum posts
471 photos

Chris,

Mossie is looking good!

Did you ever return the throttles to a single channel and have a go at the mechanical setup?

cymaz

I like the idea of locking the throttle barrels and lining up throttle arm & servo arms. Going to pinch that for my twin build when the time comes to do engine setup.

Chris Walby10/06/2019 09:59:20
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952 forum posts
228 photos

Once I found out what I had done with the mixes in my TX and got rid of the throttle cut feature that was screwing things up (could not inhibit the knob on DX8g1) I decided to leave it as it is.

I'll need to take the cowls off and while I am doing that I change the linkage to adjustable clevis set up. Winter project!

I think I'll try Jon's suggestion of setting it up with WOT at 100% servo travel, then sub trim at shut is good advice (not that other is wrong) as I am a bit nervous of putting anything solid down the carb inlet and closing the barrel against it. Not an issue with a gentle finger, but if I forget/mess up and drive a servo against it, it may do it some harm.

PS the twin set up is super easy on my DX8g2 (with no mixes) + they are running in sync now so not an issue. Just spending more time fixing the U/C, but that's mu fault!

Jon - Laser Engines10/06/2019 12:00:45
4765 forum posts
179 photos

Looks really awesome in that photo. Now i want one again

Chris Walby10/06/2019 12:46:06
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952 forum posts
228 photos

Jon, If you don't have a pair of 70's you could drop the lead/tail wheel lump and squeeze a pair of 80's in. The bulkheads have to be moved back anyway, so it would only be the tanks needing a bit of a tweak. Don't get me wrong the 70's are more than okay, just best if kept 3/4 to full throttle to keep her hustling along.

Don't forget there's a try before you buy offer from me just in case your on the fence....wink (Free with any hurrie maiden).

Jon - Laser Engines10/06/2019 12:48:55
4765 forum posts
179 photos

stop it! you are a bad influence. i have a matched pair of 80's and could swap the gear from my current 70 powered twin into the mossie....but i must resist crying

Wont say no to a twiddle of the sticks though

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