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Prop Clearance


dugald
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Hi All,

I recently picked up my first paramotor - a Fresh Breeze Simonini 122 (one ring cage). I got it second hand so I am spending some time carefully checking it over. It is about 5 years old now and was probably bashed about a bit at some point (although there is no major damage showing).

Last night I was checking prop clearance and it appears that from propellor tip to the top of the cage it has 8cm clearance. From the left side it has 7cm clearance. From the right it has 8cm clearance. But at the bottom it is only 4cm. I tried loosening the engine mounts, lifting the engine up a bit and tightening them again, but it made little difference.

Is this something to be concerned about or do I just have too much time on my hands? 8)

I would appreciate it if someone with the same cage could check theirs so I know if it is standard or not.

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If it has rubber 'silent block' anti-vibration engine mounts these should be changed in case they are sagging (they are prone to deterioration & failure).

The cage may have simply suffered a few heavy landings and might need a bit of careful force to bend it back to even clearance.

The cage is only there to keep things out of the prop though, and those clearances are perfectly adequate to keep the cage and prop separated. Might be a good thing to have more at the top & sides when doing a forward power launch.

Probably a more important thing to check is the tightness of the netting which may be a bit slack after 5 years.

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Thanks for the reply. Yes it uses the rubber cylinder shaped mounting blocks - I will order a new set just in case.

The cage netting was really loose when I got it abut I didn't realise this until recently. I slipped when starting the motor and ended up with the whole thing on top of me (full throttle) and my shoulder went into the prop due to the netting giving too much. I removed a total of 21 inches of line from the netting and it is pretty tight now.

The bottom section of the cage ring is actually built into the fram that the engine sits on so if I was to gently bend something it would be that. However I don't think it will give much - its probably best just to leave it alone as you say.

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I had a bumpy landing a long time ago, on an old H&E 120.

The same thing as yours, the prop was too close at the bottom of the cage.

But looking at the tubes running up and down at the back of the motor (back of pilot), I noticed it was slightly deformed and not straight.

The engines weight had buckled the tubes a bit.

As it was Stainless steel, two of us bent it back straight and I got my bottom prop to cage clearence back!

Cheers

Mike :mrgreen:

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Some set ups have a natural amount of sag anyway. If you are happy that the mounts are in sound condition but it is still sagging then you can shim the lower mounts and use appropriately longer bolts. Lateral differences should be expected in belt driven machines as the belt tension adjustment relies on an eccentric centered big pulley housing which when rotated and refastened allows you to take up the slack. The eccentric rotation means that with the exception of full slack or full tightness the pulley is off center either one way or the other.

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Cheers - that would be useful. I can't see how it could really be different - nothing seems to be bent as far as I can tell. The engine has three mounting points, two lower and one upper. If you drew a line betwen the upper and lower ones, the line would not be exactly vertical - which means that the engine is pointing around 10 degrees towards the ground. Perhaps the rubber engine mounts are sagging slightly as aquitix mentioned.

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I doubt that your engine mounts have become soft or drooped, they normally become brittle hard and crack in my experience. The above motor has done just under 150 hours on the same mounts.

.

Still looks great. 8)

Not sure what FB recommends, but other manufacturers seem to recommend replacement after 100 hours or so. They certainly stretch and deform when fitted as engine & gearbox mounts on classic cars (after much longer though) but I guess a well balanced engine & propeller won't vibrate so hard.

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Thanks for the photos - that machine looks brand new! The tube reinforcement on the bottom part of the frame looks like a good idea.

I have removed the engine and inpected the mounting blocks, as when on the frame taking the weight of the engine, they looked like they had a slight slope on them (the cross section slightly like a rhomboidal rather than a rectangular). This is barely noticable (so could be my eyes playing tricks) and the rubber still seems quite fresh - no cracks or hardening. The bolts also seem just fine (I held them against a straight edge). If the rubber is sagging at all it wouldn't be more than a millimeter, so I'm not sure if that is the problem or not, but will replace them just to be sure. The engine generally is angled a few degrees towards the ground. Perhaps the frame is slightly bent, although there is no evidence of any of the tubing being bent or skewed.

My prop was just inside the cage at the bottom end and just sitting outside at the top, so I added a large washer to each of the lower mounting blocks and this has straightened it up a little. Lower clearance is now around 4.5cm and at the top it is 7.5cm.

Is that a reasonably safe clearance? I am still in training so the chance of some bumpy takeoffs and landings will be quite high!

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Thanks for the photos - that machine looks brand new! The tube reinforcement on the bottom part of the frame looks like a good idea.

I have removed the engine and inpected the mounting blocks, as when on the frame taking the weight of the engine, they looked like they had a slight slope on them (the cross section slightly like a rhomboidal rather than a rectangular). This is barely noticable (so could be my eyes playing tricks) and the rubber still seems quite fresh - no cracks or hardening. The bolts also seem just fine (I held them against a straight edge). If the rubber is sagging at all it wouldn't be more than a millimeter, so I'm not sure if that is the problem or not, but will replace them just to be sure. The engine generally is angled a few degrees towards the ground. Perhaps the frame is slightly bent, although there is no evidence of any of the tubing being bent or skewed.

My prop was just inside the cage at the bottom end and just sitting outside at the top, so I added a large washer to each of the lower mounting blocks and this has straightened it up a little. Lower clearance is now around 4.5cm and at the top it is 7.5cm.

Is that a reasonably safe clearance? I am still in training so the chance of some bumpy takeoffs and landings will be quite high!

With the prop attached push the tip of the prop towards the frame and see if you can force contact throughout the circumference without obviously forcing it too much.

Then with the motor on your back and support yourself in a clear area pull full throttle quickly and listen / feel for any tinkering then check prop tips and frame for contact marks. I have had many prop contacts with the frame on previous motors and in my experience it tends to occur approx at the 3 o-clock or 9 o-clock positions as the motor tends to torque lateral rather than verticle. Make sure nobody is standing near you and wear your helmet.

My motor appears to have a similar difference in top vs bottom clearance as yours which should be reasuring.

The angle of prop ref the ground with the motor free standing is not important. Its the angle of the prop when hanging clipped in whilst in the harness that you need to check. Also get someone to push the centre of the hub (engine off spark plug cap off if electric start of course) and see if it alters the potential angle of thrust dramatically ie pushes your top half to a face plant or your bottom half to a recline position. Its not perfect simulation but enough to ensure your hang position and thrust line are not grossly innacurate. Bring the motor to a group of flyers for a check if unsure. My very first flight on a PPG nearly ended in disaster because I didn't do a hang check pre-flight and the attachments were to close to me creating a very upright position and my break lines went into my prop after take off when I got into the harness. :oops:

The 'tube reinforcement' is a highly technical addition to the bottom of the frame. It is actualy garden hose split and cable tied. :shock: When putting the motor in my van fully rigged I have to tilt it to get it through the side door and the bottom of the frame scrapes on the metal step so I have prevented damage to the frame using the hose. If you want to copy hurry because I am applying for a patent. :lol:

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Yes, it is reassuring to know that it isn't too far out!

I have reinforced my workshop so that I can do hang tests without the whole thing collapsing on top of me, which I have found very useful as the harness settings I had guessed on required quite a bit of adjustment. The position is quite upright with the motor attached via the j-bars but the angle of the prop seems quite level (not pointing up or down).

I tried using a little bit of force to check whether contact could be made as you said and I can't get anywhere near it, so I think it should be okay. I will get my instructor to give it a thorough inspection as you say.

I am cutting up my garden hose as we speak - I chose a nice clean section right in the middle... :lol:

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  • 3 weeks later...

I got a new set of engine mounts and they have made a difference. Close inspection of the old mounts did not reveal any cracking or stress marks but the change resulted in an additional 2cm of clearance, so they must have been sagging a bit.

Clearance at the bottom is now 6cm, which I am happy with - the additional 2cm looks like quite a lot in situe, and the clearance being quite uniform now tells me that the frame probably isn't bent as I feared it was.

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