4 place glide ratio

[bestbearhawk1231]

Hi Guys, out of curiosity does anyone know what the glide ratio is for the 4 place?

[jaredyates]

I'm getting about 10.5:1.

[Battson]

What speed are you using Jared, best climb?

[bestbearhawk1231]

Thanks Jared, was that with the engine at idle or engine off?

[jaredyates]

That is with the engine idling and the prop control all the way back. The engine idles at about 650 RPM in that case. Obviously if you were stuck at low pitch but still windmilling, your glide would be shorter. I'll see if I can measure it sometime to compare the difference.

[Battson]

That is with the engine idling and the prop control all the way back.

With respect, at engine idle the prop control has no effect whatsoever. The low-RPM end of the governor's range of control is normally much higher than idle RPM, so the governor keeps the blades on the fine pitch stops.... unless you have a prop which needs oil pressure to fine pitch, or a full-feathering model.

[kestrel]

I'm pretty sure that my governor will bring the rpm below 1,000 rpm. ...and it seems likely that at 80-90mph or so, that an idle engine would be higher than 1,000 rpm. I'll try to check this if I get to fly in the crappy weather this weekend.

[Battson]

I tried closing the throttle while maintaining best glide speed, then working the prop control, I saw little / no change in RPM. Try the same experiment. Mine is like Jared's and idles at about 650. Of course, YMMV, we almost certainly have different set-ups.

Also - afterthought - as you say if it's wind-milling really fast, then yes the governor would take control provided there's enough oil pressure for it to defeat the return spring. In faster aircraft this would almost certainly be the case, I expect the high lift / slow speed of the Bearhawk allows the RPM to slow outside the control range at best glide speed.

By the by, most governors need about 50PSI base-pressure in the oil system for their internal boost pump to develop enough pressure to overpower the return spring in the hub. In many engine failure scenarios the governor is unlikely to have much control over the blades.

[jaredyates]

Jonathan, have you tried bringing your prop control back to the stop with the engine at idle in flight yet?

[Battson]

I haven't had a chance to experiment properly. I did do one approach with the governor back below 1800RPM (near full travel) at about 65kts, and after closing the throttle on short final the RPM dropped to 1400. Going to full-fine at that point made no difference to the RPM - i.e. the prop was already on the fine pitch stop.

We flew 10 hours this weekend with travel to the STOL comp, the contest, and fly-in the next day - but all of it was flying for competition or flying with passengers on board so I couldn't muck around. 70 hours on the clock now.....

P.S. the Bearhawk took out 3rd place in the STOL heavy touring category. Best take-off was 34.1m (105ft), but my landings were no prize. I think we'll do much better next time, I should have removed the seats to reduce weight but I ran out of time.

[kestrel]

I didn't think to do any in-flight tests for the effects of windmilling, but purely in terms of the range of the governor: On the ground I idled up to 1,000 rpm and pulled the prop control back. I didn't keep it back, but the prop did to steep and pull the engine down to 800 rpm before I pushed it back in.

[jaredyates]

If I pull the prop back at idle an 50 knots or so, the rpm drops. At the end of the test when I advance the control, the rpm increases.

[Battson]

That is interesting, I don't see that. What RPM are you running at full power?

[jaredyates]

I'm getting 2700 static and in flight. The theory as I understand it is that at idle, the blade is going to be at the low pitch stop, and the governor is in an underspeed condition. When I pull the prop control all the way back, at some point in the travel, the governor will become overspeed and open the oil valve. As long as there is oil pressure, the blades will move off of the stop and RPM will drop. If that is all correct, and yours isn't acting that way, then there could be any number of reasons. Perhaps your governor is designed or configured so that even with the control all the way back, it never finds itself in the overspeed condition, and thus never opens the oil valve. Are you definitely getting linkage travel all the way to the stop? Next time I get to fly I'll see if I can figure out a relationship between the lever position, gliding speed, and the start of the RPM drop. Now I'm curious to see where in the travel that valve opening happens.

[Battson]

Yes, I think my governor is meant to have a lower limit of around 1400-1600 or something, depending on the propeller. Certainly in my case it doesn't appear to have any effect in glide with throttle closed.

Also my prop control set-up doesn't have enough travel to reach the low pitch stop on the governor, just due to the angles involved - but the manual says "low RPM is reached prior to the stop - do not adjust low RPM stop". So I am not too concerned by that, but maybe that plays a part.

I have to say I am surprised to learn your governor still works to such a low speed in such a slow aircraft, I must retract my earlier comment! It is a Hartzell governor?

[jaredyates]

Mine is a Woodward. I tried it out today, and at around 55 knots with the control full forward, I was getting an idle of around 1000 RPM. When I started bringing the prop control back, it started reducing RPM at about 2" of travel, just over half way. In the glide with the prop all the way back, I was seeing right around 650-680 RPM.

As for how the prop was impacting the glide ratio, it was pretty dramatic. Keep in mind that this is a very limited test and a small sample, but I got 6.96:1 with the prop forward, and 9.6:1 with the prop back. The difference between my previous 10.5:1 and this 9.6:1 is mostly due to my deviation from the ideal speed, which was in part because I didn't take the time to figure out what it should have been at today's heavier weight. So all of these are pretty rough numbers, but I think it's safe to say that I was observing a 25-30% reduction in glide ratio as a function of the windmilling prop.

[Battson]

I've done some more reading and a 10% increase in glide distance seems common. In my case, having done more testing in the weekend, my governor cuts out at 1200RPM under all regimes where there's oil pressure, and that takes all the movement I've got. So for me it makes no difference at all. Mine is a PCU5000X governor, and my control geometry means I stop about 1/4" short of the coarse pitch stop.

Jared - thanks for the package with the calendar and apparel!
This year's calendar is really great, some really good photos - especially like the ones of Pat's and Georg's machines, but they are all great (except that questionable orange-yellow one....)