Thrust from Suby 2.5

[NiqueNaque]

Gearloose,I"m in two minds as to whether you"re trying have a shot at me... so to save me any confusion - are you?Yes, I agree that the tree is lying sideways. The principal of moments works for this means of measuring the thrust of the propeller. It is the same principal as using a bell crank on your joystick to transfer the forces from the joystick up to the torque tube at the rotor head, to parallel the concepts for you:The thrust on the prop is cranked around the axle of the main wheels onto the scales via the beam that you attach to the keel. The mast and engine mount attachment is the short leg of the bell crank, the keel and beam is the long leg of the bell crank.Hope this helps.Nic.

[asra]

Hello NicThanks for the clarification, I did actualy understand it the first time.There is nothing quite like the way of the bush when it comes to inovation, I know what is like to have to improvise as I am stuck on Horn Island at present with very few resources except a working computor with a good internet connection at last (after a year of 15K type data rate complete with time outs).However i have seen things done in the bush to a very high standard also but usualy associated with a bit preparation determination and money comensurate with the requirement.I guess what I am saying is that if measuring thrust is important on a repeating basis, why is it that there is not a standard for the process rather than the number of cows Birdy can pull from the bog.Also perhaps a process that involves a higher safety factor in the event of something cutting loose at full power.As a novice scale tech I can sort of suggest that bathroom scales dont quite cut it when it comes to accuracy or repeatability.So to save time,,, If you had the gear for either choice of using a load cell or similar device with proper restraints or pushing a tree over against bathroom scales, and you still chose the bathroom scales I might been having a shot at you. All that good maths you quoted should tell us that the right gear could produce long term savings in stress and hull losses as well as usable data for thrust trends.Cheers and thanks for the interaction

[Tim]

can anybody think of a few theories as to why teh same type of Arplast prop spinning at engine rpm of 5500 is not even close to the ROTAX thrust??Back to the original question. If any two engines are spinning the same prop at the same propeller speed then the thrust would be identical.We need to know what was the re-drive ratio in the Rotax and what RPM"s did it pull? Also was the 2.5 at full throttle?

[NiqueNaque]

Gearloose,Understood - you"re not having a shot. I am not advocating pushing over trees as I do say the machine must be properly restrained.Agree that bathroom scales are woefully inaccurate, however, you can check their calibration by putting a 20 litre bucket on them and then filling it with... 20 litres of water, the reading should go up by 20 kg, if not - bad scales. In the scenario I am painting you are looking for the change in the reading, not the actual point reading, and if the scales follow 20 lts = 20 kg they should be good enough.As to bush mechanics, I was quite impressed when that show of the same name was shown on ABC TV, no end of bright ideas there, talk about clever!Tim,Trust you to point out the obvious... probably the subi can run the prop with blades at a much steeper pitch with wide open throttle.Cheers,Nic

[asra]

Hi Guys Some how we seem to have lost the GYROFAN.Tim just reading the posts, I dont think we are looking at the same prop but a similar one. I would like to hear from Gyrofan a little bit more on the situation.I am guessing that the pitch settings are a little different, the bigget the prop the more dramatic the result, I once altered the pitch on an old Steerman with a Continental 220 hp, 2 degrees pitch change bogged the engine right down and darn near unflyable. followed by cautious minor incremental change it all came good.So if our mate Gyrofan wakes up I am keen to hear this all is resolved with positive outcome.CheersGG

[asra]

:P :P Gyro-G mates,When you guys get going, you really keep me so busy I can"t even read through everything..Thanks, all your inputs are really helpful.

[asra]

Have been thinking..and thinking...and thinking.What is the ignition timing generally used at the 5400 - 5600 rpm mark??We are still way off the mark of the 400 - 450 kg...

[Tim]

From all the experience, which are the most suitable props to run with the 2.5 Suby?

[bosca]

When measuring thrust we need to take into account pressure altitude,temp,humidity etc. Kym.

[NiqueNaque]

GF,Can you post a picture of what you are doing?Be mindful of the time limitations for running engines at WOT and full power, almost all engines have a time limitation at full power. Make sure you know what this time limitation is before your testing programme starts.As to timing** this may not work on a 2.5** but it works a trick on an EA 81 / 82 with a points type distributor. All that you do is loosen the dist. bolt enough to turn it with a little resistance, enough that it won"t turn without you twisting it. Secure the machine so it will go nowhere and will not spring back into position after full power is applied. You must make safe and work from a safe position well clear of the prop and other bitey things like Leo the digital dog [ Ask Allan ]. You then start the engine, warm it up to operating temp and then set it for full power - WOT, at WOT you twist the dist and watch the engine tachometer. You adjust the timing for peak revs but make sure you have the prop pitched up so it prohibits the engine from over revving as this will do the engine some crucial damage which unfortunately will reveal itself at a critical moment, eg - shortly after you"ve committed to take off or on climb out, never a good time for engine failure. , see: http://www.asra.org.au/smf/index.php?topic=1714.0You could set it for peak RPM at cruise by setting the throttle for cruise revs and then adjusting the distributor to the best revs near cruise, say 3800 RPM.You should check the precision of the position by running the engine revs slowly up and down between cruise and WOT power, eg, from 3000 to 5500 and back down 3000 and up again several times. The engine should deliver the same revs each time at WOT, if it does you have some precision, if not there is some drift in the timing / injection which points to a potentially serious problem with your ignition / fuel injection system. If it is consistent and stable then all is well, if not, get it investigated and fixed. You could be looking at injector blockages or calibration, crank angle sensor failure, things like this will affect the RPM at WOT precision.Another thing to watch for is drift in the engine RPM at the WOT setting, if you"re in a workshop or out in the feild on a no wind day with consistent air delivery to the prop and its load conditions do not vary then, if the engine RPM is varying there is an issue of concern in this.. I would expect around 50 ERPM drift but would be concerned if it was 100 or more. If the engine surges up and down I would be gravely concerned as it points to a serious fuel delivery or ignition problem. You must then stop the engine, make safe and lock the dist into place, it helps to mark the location with a texta so you can be sure it hasn"t moved when you tighten up the bolt.Hope this helps.Nic

[tonydenton]

tim, so doesnt the push the trailer and car test say it should be 2400kg plus of thrust. after all pushing a certain weight or pulling at scales should give the same reading. dont get me wrong my thrust test was gyro strapped in to my trailer pushing up against a 3 tonne forklift [ forks raised] moving it all along a cement floor. i ve always wanted to ask someone but it has come up perfectly here. i"m more than happy with my thrust. shouldn"t my reading be 3500kg ???????????

[NiqueNaque]

Tony,Go on, tell me you"re foxing us here.. go on, eh, eh.." so doesn"t the push the trailer and car test say it should be 2400kg plus of thrust."You"re just testing me to see if I"ll bite and I did!!You can"t trick me like that! All the gyro has to overcome is the frictional resistance in the wheel bearing to move the car / trailer along...Cheers,Nic.

[asra]

Thanks guys,Info is starting to point into a direction.

[asra]

The "cleaner air" theory is not quite right. The air going over the fuselage from a tractor prop hit all sorts of obstacles on the way down throwing the thrust all different ways.Plus the actual drag of it running along the smooth fuselage skin.A Cessna 337 (push/pull) climbs better on the rear engine on single engine ops.The gyro in the picture below looses over 100 pounds of thrust from tractor(250) to pusher(360).

[asra]

Good morning Does make sence what you are saying as far as the drag during a thrust test.