Thrust from Suby 2.5

[Dewie]

Hi All
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[Dewie]

Also check out reply #2 in this thread.

[asra]

Good morning, just to show appreciation for all the contributions, some further feedback.

[Waddles]

I have a 2.5 fitted to my gyro running a 3 blade 76" standard cord Revolution prop through a Autoflight 2.4:1 box.I tether to my truck via the tail wheel axle and use a calibrated load cell to measure the thrust. At 5350 engine RPM with some cavitation I get about 600 lbs of static thrust. Still working on it. Engine is stock and the EMS is a modified (in the usual way) one from a mid nineties EJ 22.Waddles

[Mark Horan]

Here is a little mystery about thrust.I have been flying g6517 around quite a lot lately. The back end of our property climbs up the gourock range, part of the Great Divide, and we are on the eastern side. When the westerlies really start going at this time of year, the air gets a little "lumpy", lots of ups and downs rudder fluttering and clamping to the seat, another few weeks and the winds slow down a little and flying becomes "plesant" again.Lately, I have noticed that I seem to have to use more revs to keep the machine straight and level, but put it down to the bad air. All the guages are in the green and fuel usage is normal although on take off the climb out had seemed to be down on what I was used to.Saturday I did some maint on the machine, oil, filters etc. I have two

[tonydenton]

good enough for me mark, i,ll be changing mine on the week end. my symptoms are word for word of your post.thanks i"ll let you know how it goes.

[Tim]

There must have been some other reason for the increase in performance Mark. If the engine is developing the same rpm"s then the thrust would be the same.You were right in changing that filter though - it sounds like it was on the verge of causing problems (other than potentially injuring your dog).

[Mark Horan]

Tim,I agree with you 100%.!!If the engine is developing rpm then the the thrust should be same. This is the reason why I put this up on a post. The only thing that inreased performance was renewing the filter, admittedly the new filter is not the same Ryco type that was on it before, I replaced it with one with an aluminium body and a 20 micron filter.The difference is that prior to the change, I needed to run the revs at around 4600 + to keep S&L and now this has reduced to around 4200 (tandem rosco, 28 ft rotors, 76 in prop, 2.2 ltr subie, 95kg bum in the front seat). Full throttle is 5300 before and after the change.Some time before I bought the machine, the previous owner and I were doing a cross country flight and although It was a bit of a crappy day there was a lack of performance (headwind) which needed a fairly high power setting to keep enough distance from the ground. the return flight (tail wind) still needed the same revs, but we were able to achieve slightly better alt.Speaking with the owner a few days later, the problem was remedied by replacing the filter. At the time I was confused that a prop running at a certain rpm could have a variation in thrust because of a fuel filter. If the same situation was in a motor in a car, you would notice slower pick up and big hills would slow you down, but there is much more grip between the car and the ground than there is between the prop and the air.I would have thought that if the motor on the gyro was down on power, then it would show up as lower rpm at full power (unless the pitch is set too fine), did I just answer my own question?Any Ideas?Mark.

[birdy]

but with a prop on the back, the engine still spins to max revs and so one would imagine that the thrust created would be the same, but it obviously doesnt!Like Tims says, unless the erpms are down, its not go"n to make a difference.The difference is that prior to the change, I needed to run the revs at around 4600 + to keep S&L and now this has reduced to around 4200 If all else is equal,

[Mark Horan]

Well birdy you is thinkin the way I was thinkin, but after I did somthin, it changed!maybe its the weather, could be the air is thin....kinFiIknow? :-[Mark.

[Mark Horan]

Another question,how come if a fuel filter is cloggy in a car, you can usually get max revs out of the motor in neutral but once you load the motor up by driving it, you notice poor performance?

[birdy]

Another question,Im not falln for that one. I know you know. I recon iv figured your filter prob tho. The new metel one is more streem lined, and its not causen so much disturbed air infrunt of the prop. If your testn gyro performance with any comparisons, you gota be sure your flyn in identical air with each test.DA can make a big difference.

[Ken Watson]

Revs and hp are not necessarily related. Years ago we ran our race bike YZF1000 on a dyno specifically to compare fuels. With ELF we gained 4hp at max revs which was the same max as with avgas. ELF 162hp Avgas 158phIn the mid range at identical revs the opposite was happening, avgas more hp. Now the problem is trying to relate this to pitch. I think the engine may struggle with the pitch in the mid range. I think the relationship could be similar to the rear wheel. If it hasn"t got the grunt, it doesn"t want to turn it. Fuel burn rate alters hp substantially. By burn rate I mean combustion time. Ken

[birdy]

I think the relationship could be similar to the rear wheel.If it takes 10 HP to turn a prop at 1000 rpm, itll take 40 hp to get it to spin at 2000rpm. [160 hp to 4000rpm...... not the 20hp one would instantly assume.]The "load" ona bike wheel aint the same.

[NiqueNaque]

Birdy,,You might be simple but you"re no dummy.Just as a note for all you aero folk, the thing to remember about aircraft is that everything works to the square law, which means if you want to double your speed you have to quadruple your power inputs. The reason for this is that the formula for friction losses in air related motion is:Fl = {Cd x v^2}/2... it"s not too different from calculating hydraulic losses in sprinkler pipe, something I have to do every time I want to do a fire system design.Fl = friction lossesCd = Co-efficient of Dragv = velocityNow this is a very simple formula, as Cd is affected by air density, temperature, reynolds numbers, moisture content, etc, etc... there are many things to consider in aircraft engineering which require in depth research and some degree of experience, But the gist of it is that if you make it go twice as fast you quadruple the friction losses and so quadrauple the power, although it does not usually work out this way, usually because it is a proportion of the number squared.Hope this helps,Nic.