Gangster 75 Build Blog

[Peter Jenkins]

The sunny afternoon tempted me out to give the ST90 a run. So, here's the first photo of the Gangster in flying trim minus the cowlings as I wanted access to the engine and fuel pump at this stage.

First problem, which took a while to find, was that the Dubro filling valve had jammed in so no fuel was reaching the engine. Initially, when I choked the engine, I saw a bubble travel up the fuel line. Unfortunately, it took some time to realise that there was nothing other than a bubble! So, defuel, tuned pipe off, wing off, manifold off, old tank hatch opened, battery and packing out, and then Dubro valve out. You can just feel my frustration in those words!

I didn't have a spare Dubro valve but did have an old MacGregor valve so installed that. Everything else re-installed and the model fuelled up. The ST manual said needle vavle 3 turns open and go from there. Unfortunately, this resulted in the mixture being too rich so decided to turn the model over, place on the build stand and start the engine upright. It wasn't until I'd closed the needle valve and churned the engine over that it ran! So, open the needle, no choking, apply starter and no go. Choke for 2 flips, and she starts!

The running in process is run full throttle for 5 minutes (this is a ringed engine remember) and then gently lean the mixture by 1/8 turn till pinching the fuel tube does not result in an increase in rpm. I had a tach with me so once I had full throttle established, I checked the rpm. I was surprised to see 9,000 rpm. I was using a 14x10 APC as a friend who'd had a similar setup had suggested a 14x12 would be a good match for the pipe and to expect 7,800 rpm. I ran the engine at this speed, lots of oil spewing out, for 5 minutes and then, as I'd not filled the tank more than halfway, the fuel was exhausted. As it was now close to cup of tea time, I packed up but it took an age to clean all the oil off!

Things I learned:

• Most of the noise was coming from the propeller end with hardly any apparently coming from the end of the tuned pipe.
• The engine held speed without wavering
• A 14x10 is clearly not big enough for the engine. I have a 15x10 which I will try next time out and by then the 14x12s should have arrived.
• The pipe must be close to its ideal position as the rpm drop is likely to be around 1,000 with a 2 inch increase in pitch thus putting me in the right ball park for my friend's experience.
• Oh, and a glow engine running rich is a messy beast! Well I knew that but after flying electric and petrol for so long it has just reminded me why I moved away from glow!

[Peter Jenkins]

Posted by Peter Jenkins on 28/01/2019 14:39:37:

Posted by Nigel R on 28/01/2019 10:45:50:

Looking good Peter

re: the above, are you looking for a header/pipe joiner? If so, 19mm ID silicon tube is available on ebay, £3.60 for 1/2 metre here:

https://www.ebay.co.uk/itm/Silicone-Vacuum-Vac-Hose-Pipe-Tube-3mm-4mm-5mm-6mm-7mm-8mm-9mm-10mm-Available/290742298928?hash=item43b1976930:m:mukwgwKXqWPSCPCt7EqavOw:rk:6f:0

Thank you Nigel, that's very useful. Might just get some of that.

And I did! £5.28 for 1 mtr delivered is excellent. I tried it in lieu of the Teflon joiner today. It's a snug bit to both the manifold and slighter tighter on the pipe but I could use the spring clips rather than the small jubilee clip to clamp to the manifold. Unlike with the Teflon joiner, there were no oil leaks from this join! Good call Nigel R!

[Peter Jenkins]

This afternoon, I completed the running in process on the ST90. It produced an interesting situation when I started to throttle back to half throttle with the engine being reluctant to run and showing all the signs of excessively rich mixture which was at odds with the full throttle running. So, I guessed this might be the slow running jet set far too rich at the factory and checked it. It was 4 1/2 turns to fully closed so I re-opened it and closed it by 1/2 turn. The result was that the engine would now run at half throttle. I leaned it a bit more and gradually got the slow running to improve significantly.

To cut to the quick, I managed to get a slow and reliable idle of 1,800 rpm with an instant response to snapping open the throttle. Holding the model vertically with the engine idling at 1,800 rpm resulted in .... a nice even idle! The pump is clearly doing its work. I didn't have enough hands to open the throttle and check the engine accelerated but will do that as part of my pre-maiden checks.

Yesterday, I noticed there was a significant oil leak from the engine/manifold junction, so I tried cutting some greaseproof paper gaskets and the oil residue reduced. So, I shall make a proper gasket and that might make the join oil tight.

I had also run the pipe without the final exhaust pipe section and that had resulted in a lot of mess on the model. Today, I fitted the final pipe and arranged it to point to the left of the model. This resulted in a much cleaner model. I will turn this ejector pipe to pointing vertically downwards before I fly the model in the hope that will keep almost all the oil off the model.

My 2S LiFe balance extension leads turned up today, so after de-rigging the model I removed the battery hatch, drilled and shaped a hole to take the balance plug through the wing mount bulkhead and can now balance charge the 2S Life Rx battery without removing the hatch.

Finally, I re-fitted the cowlings and now am waiting for delivery of a pair of 14x12 props. Once they arrive, I'll re-check the needle settings and then set about optimising the pipe length. The picture below shows the cowlings in place. Note that the engine is mounted as per Mr Reeve's instruction on the centre line of the engine mount bulkhead. That's why there is such a downward curve on the top cowling.

Oh yes, it looks like my pilot has decided to become unglued at the back of his head. He was glued to the "head rest" with canopy glue. The join with the cockpit floor also looks like it is not too far away from giving up! So, while the wind blows and the rain rains, I shall have to have the canopy off and re-fix the bally pilot!

[Jonathan M]

My 2S LiFe balance extension leads turned up today, so after de-rigging the model I removed the battery hatch, drilled and shaped a hole to take the balance plug through the wing mount bulkhead and can now balance charge the 2S Life Rx battery without removing the hatch.

Excellent transferrable-dodge Peter! I'm just now moving over from NiMh to LiFe and was wondering how folks solved that one. Is the balancing end of the extension lead fixed in place rigidly and exposed to the elements, or does it snug away when not in use? Picture?

Jon

[chris Bond - Bondaero]

Why would someone with 2m electric pattern ships want to mess with old style dirty IC

On your heels Peter with my IC Joker.

Can you let me have a link to the pipe support supplier pls.

 

Edited By chris Bond - Bondaero on 07/02/2019 08:54:34

[Peter Jenkins]

OOh! That looks nice Chris! The pipe clamp is from a US supplier, Ohio Superstar - follow this link.

Jon - the lead comes into the wing bay so is out of the elements. The normal switch harness has the usual plug in port for charging and checking battery voltage of course. As the wing needs to be off to transport the model, the balance lead is easily accessible and terminates close to the charge plug. I'll stick in a photo in due course.

[Nigel R]

That looks great Peter.

Starting to think I want to get one of my OS SFs in a pattern build now!

[Jonathan M]

That makes perfect sense Peter - where there's a quickly removable hatch or normally take wings off a bigger model. I suppose I was trying to visualise a solution for a smaller model (like my 60" Middle Phase) where I don't usually have to unscrew the wing bolts to transport or charge. Necessity (howsoever self-inflicted!) being the mum of invention, this now prompts the devising of a wee sliding hatch beside the main switch/port for the for the balance-lead extension.

Look forward to full HD footage of your big Gangster in the air!

[Peter Jenkins]

Today, the 14x12 APC props arrived. So, after balancing and drilling out the hub to 8 mm, one was bolted onto the front of the ST90. My colleague who had one had said I'd need 22 1/2 inches from piston centre to the pipe baffle so, having measured this distance twice(!) I cut a slightly smaller than measured amount off the manifold. I also took the opportunity of installing a gasket made from an old business card(!) between the engine and manifold to try and staunch the oil leak from there.

With the manifold bolted back into place, I put the wing on and then mounted the pipe and pushed it so that it was almost touching the end of the manifold pipe. This should reduce the amount of silicon tube that can pulse and alter the effect of the shock wave travelling up and down the pipe.

So, I'm all set for a final run with the target prop. I'm told I should be looking for around 7,800 to 8,000 revs. We shall see.

[Peter Jenkins]

Posted by Jonathan M on 07/02/2019 14:52:32:

That makes perfect sense Peter - where there's a quickly removable hatch or normally take wings off a bigger model. I suppose I was trying to visualise a solution for a smaller model (like my 60" Middle Phase) where I don't usually have to unscrew the wing bolts to transport or charge. Necessity (howsoever self-inflicted!) being the mum of invention, this now prompts the devising of a wee sliding hatch beside the main switch/port for the for the balance-lead extension

Jonathan, you probably don't want this photo now, but as I took it, I thought I'd post it to show the extension of the balance lead from the LiFe pack in the old tank bay and the position of the switch which you can just see the back of in this photo.

[Peter Jenkins]

This morning's weather was dry but breezy so I decided to check out the performance of the engine with the 14x12 prop fitted. As I said earlier, I have reduced the piston centre to baffle distance down to 22 inches having been told that 21 1/2 inches was the required distance.

The first thing I found following start up was that the engine wouldn't accelerate beyond 1/2 throttle so clearly the main needle was set too weak. I opened this up by a turn and then re-set the needle. It settled 1/2 turn richer. The idle, on the other hand was too rich and that needed almost 1/2 turn towards lean.

However, the performance was pretty good with max rpm being 8,600 and idle settling on 1,900 for a sustained idle with crisp response to opening the throttle. I was quite pleased with that outcome. I do have 3 other props I can try once I get the maiden out of the way which looks like being next week. According to the APC propeller performance charts, 8,600 rpm on a 14x12N prop is around 10 1/2 lbf. Given the empty weight of the Gangster is 9 1/4 lb and fully fuelled it will be around 10 1/4 lb, the theoretical calculation gives me a 1:1 power to weight ratio. We shall see what happens in practice. Incidentally, 9,100 rpm with the 14x10 prop is quoted at almost 12 lbf.

Following cutting down the length of the manifold pipe, I fitted the tuned pipe as close to the manifold as I could. As I said earlier, the quick clips are now usable with the snug fitting silicon pipe and the pipe rests naturally on the rear support as you can see below:

Edited By Peter Jenkins on 09/02/2019 19:25:20

[Peter Jenkins]

The other issue I wanted to check out was the distance from the bottom of the exhaust pipe to the ground. By raising (lowering) the manifold pipe end, I could reduce the pipe's downward cant and, as you can see below, the the end of the curved exhaust pipe is about an inch clear of the ground. The end of the pipe can be rotated to that it points sideways thus increasing the clearance but with a slight amount of thrust which could affect the yaw of the aircraft. A downward facing exhaust may also affect the longitudinal trim. It may be that neither effect is noticeable from the ground - we shall see!

And now some pretty shots!

The pilot, incidentally, has broken loose at the rear and his head has parted company from his headrest. I suspect that the rough running when running the engine rich is to blame but, for the time being, I've left him alone! I will remove the canopy and re-glue him, once the maiden has been successfully concluded.

[Jonathan M]

Looks great! Wishing you fair weather for the maiden

If you discover that you really need more than 1" clearance (heavy landings?) then you could always fit bigger wheels or a taller U/C, or else simply rotate the pipe 45° to point in the correct direction for equivalent down- and side-thrust when opening up full revs.

I can now see why you needed the extension lead for the LiFe balancing-plug. By the way, are you using high-voltage servos or a BEC to step down the voltage for normal ones, or not bothered by this?

[Ron Gray]

@JM - you obviously haven’t seen Peter fly, he doesn’t do heavy landings!

[Peter Jenkins]

Jonathan, the only thing I did when I decided to go the Life route was to swap out the JR NES591 servos I had installed for the ailerons. JR servos always warn not to exceed 6 v which is what a fully charged 4 cell NiMh produces. A 5 cell NiMh does them in! I settled on Futaba 3003 servos for the flying surfaces and 148 for the throttle. The fully charged Life 2S produces about 6.7 v and in my Majestic has not proved a problem with Futaba servos.

As for the pipe clearance, the main thing will be to avoid heavy landings of which my 2 mtr F3A birds are also intolerant! The East solution is to rotate the pipe outlet with bigger wheels as my next choice - or both!

[Peter Jenkins]

Oh Ron, that's tempting fate! 😭

[Jonathan M]

Posted by Peter Jenkins on 10/02/2019 09:53:09:
...I settled on Futaba 3003 servos for the flying surfaces and 148 for the throttle. The fully charged Life 2S produces about 6.7 v and in my Majestic has not proved a problem with Futaba servos.

Cheers for that Peter - good to know as I'm got sets of Futaba 3003 and 3001 servos in each of the two models I've bought 2s LiFe packs for.

[Peter Jenkins]

A fully charged 5 Cell NiMh has around 7.4 V I think. I have a 5 cell in another airframe and that has Futaba servos all round and has never caused a problem. JR servos, unless HV, should never be used with a 5 cell NiMh as they will burn out.

[Peter Jenkins]

Well, this morning I managed to fly the Gangster for the first time. Had 2 flights and the airframe came through it without any damage. Several points have emerged:

1. The CG on the aft position quoted by Mick Reeves was almost spot on. It might be worth moving forward a smidge but that's being pernickerty.
2. The engine set up appeared fine with no hesitation or heart stopping moments when overshooting. However, while the mixture is a tad on the rich side the power appeared adequate for normal sport flying but I need to get the pipe length shorted a tad more and possibly try a different prop. It has a 14x12 that pulls 8,600 max revs. I'm minded to try a 15x12 and according to the APC performance chart, if that pulls 8,000 revs it will provide about 2 lb more thrust. I'll see how it goes.
3. The control throws are not right. Only full rates gave me the sort of feel I am looking for. That means the ailerons are too small and need more throw and ditto the elevator. The rudder appears OK at the moment but as I progress with trimming that might also need increased throws. I increased the size of both the elevator and rudder as I felt they were too narrow. Should have extended the ailerons while I was at it but thought they looked about right.
4. A little more right thrust is required to get her to track straight vertically.
5. Knife edge needs sorting as the aircraft pitches to the u/c one way and canopy the other.
6. Spin, both upright and inverted is fine.
7. I normally have a lot of expo on max rates, which I only use for spinning, so having to fly on max rates was uncomfortable with the amount of expo I'd put in. I will increase throws and re-set the low rates and see how that affects the handling.
8. The handling is a tad odd. While I'm used to flying 2 mtr F3A airframes and didn't expect the Gangster to be in this league, it still felt slightly hesitant on occasions. Will have to see if this changes with the changes to the control throws.
9. Otherwise, the aircraft rolls well, tried 4 sec slow rolls and they were pretty decent, and it will fly large loops but you do need to work at keeping it flying straight - more side thrust will help here. However, it's not an unlimited vertical aircraft and given the power to weight ratio I'm not surprised.

Hope to make a few tweaks and fly it again shortly. Will report back on how the tweaks are doing.

[Nigel R]

Good stuff Peter!

#3 What about doing gap sealing on the ailerons, might just speed up the response a little bit? It doesn't appear that the wing has a large taper so (in my opinion, etc.) I wouldn't expect a really quick roll from the design. What sort of roll rate are you expecting? I'd have thought something like this a maximum of about 1 to 1.5 rolls per second would be about it?

#5 The knife edge pitching sounds odd, I've never had one go one way left and other way right. Any idea what causes that behaviour?

#9 what is the lateral balance like? It should be pretty close with the inverted motor, but maybe one wing panel is a little heavy?

 

Nimh cells - would usually expect them to top out around 1.5V when first charged but they relax after a while, to around 1.4V. Straight off the charger a 5 cell pack might even be up at 7.5V.

Edited By Nigel R on 13/02/2019 08:55:44

[Peter Jenkins]

Hi Nigel

Yes, gap sealing was on my list of tweaks for the ailerons and that is now in place. The roll rate on my original low rates was very slow - the sort of rate that I'd set for doing very slow rolls. I've now upped all rates and reduced the expo on aileron and elevator. I've also zeroed the trim on aileron and elevator and adjusted the turnbuckle links so that the controls are where they were. Might need to give them another tweak after flying but have also reduced the trim steps back to 1 from 4 so that will help in fine tuning the trim.

I've also added about 1/2 deg more to right thrust and this has had a very minor effect on the alignment of spinner to cowling but you have to look hard to spot this!

I've replaced the 18 oz tank with a 12 oz one as I had used less that half a tank at the end of the 2nd flight. No point lugging around weight you don't need and I reckon I will only want about 12 to max 15 mins in flight.

Ref KE behaviour, this is primarily down to CG position and the slipstream. My electric contra prop equipped F3A bird is virtually identical on either KE and will also snap left or right in exactly the same way. No torque effect or rotating slipstream to jigger things up.

The static lateral balance was fine. I did not check it out in flight though. That will be tomorrow's task. I perform a vertical dive power off and then pull hard to level flight and look to see which wing ends up low. You need to do this at least 6 times to make sure you are getting a repeatable result of course.

I'll post again after the next set of trimming flights.

[Peter Jenkins]

Well, today's experiments came up with the following:

• in it's current configuration (pipe length still too long) the 15x12 prop dropped rpm to 6,900.
• replaced it with the 14x10 and that produced 9,100 and gave a better vertical performance than the 14x12
• the increased right thrust was, on the whole OK but there were times when the aircraft still pulled left in the vertical (I'm still pondering why that happened and it may be due to the difference in vertical speed and how the prop swirl is hitting the fin)
• the up/down thrust (instruction said set at 0 deg) is correct. Going from 1/2 power to full power produced no pitching until the speed had risen and then there was a slight climb which is to be expected.
• the vertical down line (this is a 0 -0 set up) produced a very slight pull to the canopy that can be cured by a touch of down mixed to closed throttle - it is so small I decided to leave it alone.
• whether the gap sealing or increased throws helped more on the ailerons is a moot point. However, at normal speed (1/2 throttle) the low rate roll is now too fast but on a vertical with a full roll the rate produces a relatively slow roll - something that getting the pipe to its optimum length might improve as currently the speed drops off quite a lot
• the elevator in low rates has too much deflection so I will turn this down
• the snap/flick roll isn't! This will need a very much greater aileron deflection to produced the required roll rate so I think we'll leave that as a manoeuvre that does not suit this set up. I think the big span wing, with the associated aerodynamic damping provided, is a major cause of this.
• Forgot the check the lateral wing balance when flying- a job for another day.
• the inverted spin is now better as both more elevator and more rudder movement have increased the rate of spin and it looks like a proper spin. Recovery remains almost immediate on centering the controls.

Overall, the higher speed provided by the 14x10 prop was beneficial. The Gangster is now a very pleasant aeroplane to fly although there are still a few tweaks needed to get the best out of her.

I will not cut another 1/2 inch off the manifold length and see whether that brings the pipe more into the power game. That may get the 14x12 prop revs up although it might make landing a tad more difficult with the slightly higher residual thrust at idle compared with the 14x10. With the 14x10, and an idle of 1,800 rpm, the aircraft floats a long way on round out. Only solution for calm days is to keep the approach as flat a possible and the speed down.

As a comparison, I flew my 2 mtr Majestic (DLE 35RA power) to compare with the Gangster. Well, Mr Paysant Le Roux's design, and the power of the DLE, resulted in a comprehensive victory for the Majestic over the Gangster but then again, the Gangster was not designed to be a specialist aerobatic machine.

[john straw]

Peter,

I have recently acquired a Gangster 75. Could you tell me where the CG is please.

Thanks in advance.

[Peter Jenkins]

Hi John

Great choice! I think I either went for 4 1/2 or 5 inches back from the LE at the fuselage. I would start with 4 1/2 and if that's OK try 5 in aft. Having read my notes on the first flight, I might have used the 5 in setting but I note that I didn't specify!

This is a scan of the CG and flying notes in the Gangster instructions:

If the ailerons on yours are as per the plan, then I don't think it will perform a decent flick roll. Apart from that, the aircraft is benign and will do all the usual club aerobatics.

Do post a picture and let us know how you find flying it.

Peter

[john straw]

Hi Peter,

Thankyou for that.

I'm looking forward to the next suitable day to fly her.

Will keep you posted.

John.