Engine thrust line

[Keith Berriman]

I am flying a Seagull Harrier 3D on an Irvine 46 .inverted to suit the front cowl. It is a reasonable set up but how the engine peaks out when flying model inverted is amazing. Using Laser 5% I am looking to turn the engine sideways as it was in a Wot 4. My query is is the thrust line of the engine critical against the wing thrust line looking at my options I may be a little higher than the inverted engine set up. Trust I have explained my query correctly

[Peter Jenkins]

I'm slightly confused. You say you mounted the Irvine inverted to suit the front cowl but that side mounting it will raise the thrust line. Are you going to remove the cowl completely? If not, then your engine thrust line must be the same if the prop shaft is still in the centre of the cowl.

From your description of the engine peaking when flying inverted, your tank position is probably the cause being too high when upright which is why the engine leans out inverted with the tank now low. Just rotating the engine 90 deg to sidewinder should raise the carb and you may well end up with the needle valve at the same height as mid tank - that will solve the leaning out problem when inverted.

If the tank centre line is still above the needle valve when the engine is side mounted then you will need to lower the tank height. I cannot see how you can get the prop shaft to match the cowl and end up with a different thrust line whether inverted, side mounted or upright for that matter.

Have I misunderstood your question?

[Keith Berriman]

Hi Peter. Thanks for the responce in the inverted set up looking at the side view from spinner nose point down passed the engine bearers to a line down the model side all is in line when mounting sidewinder the engine is 6mm higher than the line down the model. I take your point re tank and inverted as when I go sidewinder I need to lift the tank as high as possible to get a decent line to carb centre.

[Allan Bennett]

In theory your engine thrust line should be the same whichever way you mount it. That may mean you having to re-drill the holes in the firewall for the engine mount to ensure that the propshaft centreline is the same as it was before.

As Peter has pointed out above, changing the engine orientation does change the carburetor position relative to the fuel tank, so it might improve things without having to move the tank.

Do you do the full-throttle nose-up test before flying, to ensure that the mixture is rich enough to cater for vertical and inverted flying?

[Peter Jenkins]

An alternative solution, if it's possible, is to leave the engine mounted inverted and move the tank so that it's mid point is now level with the carb. That way you only have one job to do.

As Allan has pointed out above, you will need to re-drill the engine mount holes and put blind nuts on the other side of the firewall to secure the mounting bolts. You will need to remove the fuel tank to do this. Unless you are very lucky with the design, this will be a fiddly job but one that will be made a lot easier by using locking forceps (you can find them here). They are an invaluable aid in many situations where space is limited.

That's why I suggested that you consider re-locating the tank to a better position.

[Keith Berriman]

Hi Peter and Allan. In the inverted position the tank line was central to the carb in sidewinder set up the tank top is just in line with carb line and have been able to lift the tank a little. Not much will be done now until the weekend to test the sidewinder position

[Biggles' Elder Brother - Moderator]

Leaving aside the mechanics for a moment - yes moving the thrustline makes a difference.

In normal flight the thrust line being 6mm (that's quite a lot actually) higher will create a downward couple because it will change its position relative to the centre of drag. As you go faster this effect will become more noticeable. You may be able to trim it out, or you may not (especially at full throttle).

In 3D flight the problem will be that the thrustline will presumably have a different relationship with the "vertical" position of the CoG - so the aircraft may not hang straight as it were - it may even develop a sort of pendulum effect and oscillate.

Only testing will show how severe these effects are - but be prepared for some possibly radical changes in behaviour and handling!

BEB

[Keith Berriman]

Bit of an update to all. Double checking the spinner on the engine against the cowl cut out for spinner and using fixing holes in cowl against the fusalage it was spot on. So this afternoon was test flight time and all went well, engine started, good tick over and top end sweet off we go. Good take off and plenty of nice smoke trail as before in my Wot 4 but it was down on performance. On landing and going out to pick up model after engine cut operation it was only then I noticed the model is some what heavier that Wot 4 or Acrowot that I fly on other 46 engines. Looking at the open frame structure it was suprising. It looks like I will need at least a 53/55 two stroke.

Happy Days

[Peter Jenkins]

Well now I'm confused. Was the performance OK with the engine inverted? If it was, I cannot think why turning the engine to sidewinder config should cause a loss of power.

You say that in the inverted engine case the tank centre line was in line with the engine carb. That is a good position and should not have caused a difference in mixture when inverted. I had thought that the tank was higher than the carb and that would give rise to peaking out or running lean when inverted.

You say that there was a good smoke trail - without seeing how good I'm tempted to wonder if you might be running a bit rich. Perhaps you can let us know if you feel you have got the engine running at peak speed and then backed off the needle valve to drop rpm by 300-400 revs.

Have you weighed your 3D model and compared it to the Wot4/Acrowot weights?

[Keith Berriman]

So I trust I am not going to go to far of track. I have weighed the Acrowot I fly on OS 46 at 6lb14oz great turn of speed if needed. I have weighed the Wot 4 I used to fly on the Irvine 46 at 5lb 3oz and the Harrier comes in at 5lb 11oz surprised me. What could be wrong ?? I changed fuel earlier this year to Laser 5% ( Yorkshire man deep pockets and short arms ) but never flew the the Wot 4 with this fuel. The OS has had no noticable difference in performance so one last option is to get to the field and put some Irvine 5% back in the tank and see what happens

[Denis Watkins]

RC, 5% synthetic fuel will fly a 46 model however you please to fly it, and will not take a 3D model from one state to another, where it will not fly briskly.

Do all the usual tuning checks, the plug, is the element grey, then put in a shiny new one.

Pressure leaks, front bearing leak, backplate loose.

Is the elbow near the carb nipple blocked with fluff inside, etc etc

Edited By Denis Watkins on 24/05/2018 09:58:46

[Stuart Z]

RC,

Given you have changed fuel, have you tried the rpm reading with a tacho?

Might help with overall tuning to see what you are getting with both fuels.

S

[Keith Berriman]

To bring my query to a close. Today just a little too windy for most flyers I took the option to have an empty field. Set up the Harrier filled up with my Irvine 5% fuel and tweeked the needle in a little and away we go. All back to normal nice speed into wind crisp exhaust note an powered across the sky as I was used to. Fetched the Wot 4 out new ASP 46 filled that with Irvine 5% but that would not peek as it did previously on the Laser fuel. Conclusion fly the Irvine on Irvine fuel and others on Laser 5%

[Peter Jenkins]

I've always used 10% nitro fuel on my Irvines (40/46 and 53) and they like that.