bidrseed2

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Hi all,

Will speak to wren on monday about this, but thought id pick others brains as i dont think this is a engine problem.

Ive got a brand new Wren SS. Ive had 7 flights now. First 4 were fine. But on the last three the spool up time in the air from idle to full is over 10s, sometimes 12s.

On the ground, its about 6s. But on take off today , just seemed a bit longer.

Ive noticed that the spool time is probably longer in turns than straight and level, but this pattern could not be consistently repeated.

My boomerang previously had a Wren 54 mk2, which would pull 2/3 of the fuel of the SS at full throtttle. There were no issues there with my fuel setup.

I do have the UAT at the front of the avionics bay in the boomerang, and the fuel pump at the rear. That 25cm ( or 8-9 inches ) from the Main tank to the UAT and then the same again back to the pump.

This arrangement worked with a less thirsty engine, but could this be unsuitable for the SS?

I have spoke to wren a few weeks ago after the 2 first odd flights and they suggested looking at where the rpm lead was going in case it was getting interference, distorting the rpm signal. Ill post pics of my setup tomorrow.
If that failed to help, the next suggestion was to return the ECU to get its data checked. Id obviously like to explore other options before grounding the airframe.


Could the distance from main tank to UAT, and back to pump be respnsible, especially with increased G forces in the air??
Seems strange it worked great first 4 flights, but now is misbehaving when i havent changed anything.



( Setup 2l main tank, large tygon tube, jetronic clunk and BVM UAT.)





It seems st

747drvr

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Make sure your thermocouple didn't get bumped farther into the engine . If the FADEC senses high temps it will slow down the acceleration. Just one easy thing to check . The SS is a great engine and really brings my Baby Boomer to life !

Marc

jason

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As Marc said, it may be the ECU holding the accel back because of high temp. As well as the thermocouple in too far it could also be the compressor drawing in hot air from the rear of the engine. Have you got a former on the hatch that isolates the rear of the engine?

Jason

Boomerang1

Mine has had no changes in spool up times, on the ground or in the air, & my guess would be about 3 seconds although I have not especially timed it.

I have a 50 oz Dubro tank, a 20 oz Dubro tank & an Intairco UAT but they are all bunched close together & close to the pump. It's then a very long way back to the engine, probably 500mm.

I also had problems with the RPM lead picking up interference from the glow plug & starter leads but in my case the engine did not start properly. I just ran them apart & it's worked fine since then.

The info that it worked great for 4 flights & then problems suggests something has changed, fuel filter partly clogged?

Wren have probably encountered this problem already & have the answer waiting for you. - John.

siclick33

You don't necessarily need to return the ECU to get the data checked. If you have the PC lead (available from Wren) you can download it yourself and look at what is going on. It will easily tell you obvious things such as if the pump is being limited by overtemp etc. If you need more analysis then you can email them the data.

This saves you ripping apart the model and may give you a quicker answer.

Wayne22

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The shorter the lines from the tank/uat to the pump the better...they pump better than they suck....could be a few other issues at play though....

bidrseed2

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Thanks for the suggestions guys,

I looked at the thermocouple. The instructions state it should be 4mm. I reckon its possibly slightly more, maybe around 6mm at the most. Would that be significant?

I have done the mod with the hatch to prevent hot gases. Ive included a pic, in case mine looks insufficient, but i cant see how i could restrict it further.

Oddly enough however, the hatch does seem warmer after flying than it did when i had the 54 mk2 in there.

I just did a test run on the ground with and without the hatch on and got the following.

Idle max EGT during spool up max EGT at max rpm

600 680-720 640 without hatch
640 750 670 with hatch

I wouldnnt have thought this was significantly different.

Ive got air screens on the top of the hatch, so id dont suspect im starving it off air.

Heres some pics of my setup in case im missing something.

davo580

i got the mw54 mk3 at first i had the thermocoup in to far 4mm as spec by wren but here in aussie with the different temps here it was too much as was reading temp to high therefore rpm would not go to full for a long time.. then had to open air intakes more to let more air in because eng taking to long to ramp up .. done all that flys great now.. oh foregot to mention i backed the thermo to only 1.5 mm

David Gladwin

The fact that the time changed after 4 flights is odd but the fact that the poor engine works at all in such awful (aerodynamic wise ) installations is a tribute to our little engines and the aerodynamic tolerance of centrifugal compressors. If this and similar installations were for a fan or axial flow engine the results would be just a series of stalls or surges such is the degree of turbulence at the engine intake.

I have actually had a Wren 44 stall in a a Savex L39 (which is not TOO bad for an unducted installation,) doing tailslides. there was a sort of burping noise (from the engine !!) on throttle up and the engine would not accelerate at all. I shut it down knowing that the EGT was probably rising. The second time this happened I had enough height to idle the engine, dive the airplane to re-establish good airflow to the engine, and then slowly accelerated the engine out of the stall and after that it ran fine.

Any jet engine need a smooth or at least reasonably smooth air inlet and anything else will not help performance, in terms of thrust or operating temps. at all . The installation shown has a virtual flat wall in front of it, the rear of the tank, further compounded by a FOD screen (in itself no bad thing ) but some of these unducted installations really are awful, in fluid flow terms ! Its made worse by another flat screen former (without any holes visible to allow any air to pass through it) at the rear of the engine The turbulence of the air as it meets the engine intake must be dreadful. The temps. rise with the hatch installed are another clue that all is not well.

Please, Birdseed, dont take it personally, I am really directing my remarks at ALL such installations. "designed" totally without any real consideration for engine intake requirements , and there are a lot around but in many instances, inefficient though they are we get away with it.

Tale a look at the book "Jet Engines" by Klaus Hunecke. Chapter 3 deals with intake theory and practice , in many of the totally unducted installations used by so many jet modellers EVERYTHING is wrong !



Regards,

David Gladwin.

bidrseed2

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David,

Not taking anything personally, i appreciate all help i get here.
I understand what youre getting at with the poor air inflow. My dilemma is partly because, and i assume, other boomer drivers are not having these slow spool up times.
It works for others so im assuming theres something different about my setup.

To further deepen the mystery, on flights 3 and 4, after startup and just at the time i put the hatch on the engine ran down and quit.
The reason was "speedlow".
but it started ok next time around and flew successfully.

Ill speak to wren tomorrow, change the UAT position, and use a felt clunk instead of the orbit clunk as the latter is relatively straight forward to do.

felker14

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Had same problem with my Boomerang/Wren SS had to run my glow lead on opposite side of fuse going up to front.

bidrseed2

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Felker,
Ok im very interested!

Youhad to run your glow lead on the opposite side, relative to how Ive done mine, or opposite to rpm or thermocouple???

felker14

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I had glow lead on 1 side with everything else on the other side.

jason

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I would pull the thermocouple out so its in half the length it is at the moment. I would also get the lead from Wren and see what the max temp was during the accel, my guess is that it is slowing the accel down due to high temp.

I doubt it could be anything to do with the tank install if it gets to full power quickly on the ground.

Jason

digitech

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Jason it looks this is one of the ones that will have a surge or lack of air while flying.
my best guess is that it has not enough fresch air when flying and starts sucking in hot air while accel.
he said him self as soon he takes off , at some distance he has lack of power..

@ do you have big enough air intakes?>
the ones on the side are NOT enough.
you need to make naca ducts or screens directly above the Inlet.

bidrseed2

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I have air intakes on top of the hatch. Quick sum, its about 50 sq cm.

i spoke with wren today, they said that as the EGT gets near 50C of the max EGT limit, it will back off the pump.
On the ground i can get near 750 as its accelerating.

They also suggested it was ok to raise the EGT to 850 from its current 800c max.
I think they also said i could back out the thermocouple slightly, as jason is suggesting.


Having read the boomerang thread, i wonder if its worth shifting my turbine back slightly, to reduce the chance of reingested hot air.

digitech

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true a typical of 3mm is ok for the termocouple.
and yes i alsways had the exhaust outside the airframe