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Prop strike yesterday (and propeller specific noise level)

You don’t need a 3B prop to absorb the 250-300HP of a typical SEP, constrained to the typical SEP prop diameter.

In some cases, like mine, you get a bit extra prop clearance.

1B props are basically no good because the thrust axis is precessing / wobbling (hard to explain what I mean).

Administrator
Shoreham EGKA, United Kingdom

Why do many Rotax aircraft have 3 blades?

Maoraigh
EGPE, United Kingdom

According to Google it is to smooth out the installation. Rotax run at 5300 RPM with gearing so any vibrations using two blades might cause maintenance issues?

Oxford (EGTK), United Kingdom

So yesterday we finally managed to get the first test flight done. Goal was to verify the performance of the new prop as well as to break in the engine with a 2-30 flight at high power regimes.

The new 3 blade prop.

Montreux and the Chillon Castle

View direction Zermatt and Matterhorn

Aletsch Glacier

While the flight was quite pleasurable and a nice memory why we actually go through all the hassle, the airplane will need more work.

We found that the propeller only delivers 2500 RPM at take off. This is obviously not enough. We also found the airplane has lost a good 10 kts of speed on the average, over the 2 blade prop. This needs to be investigated. Also the prop appears to generate some vibrations we need to sort out, I sent Peters article on balancing props to the people in question.

So while the airplane was airborne now, we are not out of the woods yet. But it was still a nice flight.

LSZH(work) LSZF (GA base), Switzerland

Looks wonderful

Most props would benefit from balancing; anecdotally 3B ones more than 2B ones.

Adjust your governor for the absolute spot-on redline, otherwise you are losing takeoff performance. The last few % of RPM makes a big difference to HP. But the maint shop should have done all this before you flew the plane!

Now you have no excuse for not coming to our fly-ins

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Now you have no excuse for not coming to our fly-ins

I have to get my medical back and my SEP. Planning that for September. Until then I am totally booked out with work and other absences.

Peter wrote:

Adjust your governor for the absolute spot-on redline, otherwise you are losing takeoff performance. The last few % of RPM makes a big difference to HP. But the maint shop should have done all this before you flew the plane!

Absolutely.

LSZH(work) LSZF (GA base), Switzerland

Horsepower is roughly in proportion to RPM in aircraft engines, so 2500 RPM is about 7% down on power.

I had a reason to decrease my static rpm to 2550 from the certified value of 2700 rpm, related to the slow control response of MT electric prop and overspeed during go arounds. In my case (unrelated to conventional hydraulic props) it was an overall improvement. In making the change, I consulted with a friend who is a CS propeller designer and manufacturer, and he indicated that the negative effect would be on the order of adding 50-100 ft to my takeoff roll. He also agreed that the certified 2700 static rpm setup was not wise with slow electric actuation, and was likely done to get the published ground roll as short as possible as opposed to being good practice with that type of controller. Otherwise it make no difference to performance once the plane is above some airspeed below stall speed.

Last Edited by Silvaire at 07 Jul 15:15

Lovely day for it

Horsepower is roughly in proportion to RPM in aircraft engines

You can make 2700rpm with 10HP if the propeller goes flat pitch on breaks, obviously not lot of thrust or power

In Mooney M20C, it has variable pitch but you should still see 2650rpm on static, there is a backstop mechanism that prevents propeller from going flat pitch on low power (basically acts like fixed fine pitch when aircraft is not moving), if you see less than 2650rpm on static you need an adjustment to prop fine pitch backstop setting in the spinner, or adjustment on governor flyby weights springs, or both propeller spinner & governor spring will be required.

On fixed pitch C172 fixed pitch, 2300rpm on breaks is more than enough, the propeller is not efficient with ASI = 0kts and won’t give 160HP but it should go near 2700rpm by 40kts though

Last Edited by Ibra at 07 Jul 14:39
Paris/Essex, France/UK, United Kingdom

Typically constant speed props/governors are set in their certified configuration to redline rpm at full throttle at lowest pitch, with the plane stationary. This does vary a bit, but is generally true. Yes, both the governor and the low pitch stops can control static rpm.

Obviously static rpm with a fixed pitch prop at full throttle and zero airspeed depends on whether the prop is optimized for climb or cruise, but in any case to maintain adequate takeoff performance it needs to be above the minimum value specified by the TCDS.

2300 static rpm is roughly 85% power at full throttle for an O-320, as used on a C172. Aircraft engines are in a mild state of tune that maintains roughly constant torque and therefore linear power over the operating speed range.

Last Edited by Silvaire at 07 Jul 15:23

Maoraigh wrote:

Why do many Rotax aircraft have 3 blades?

Some Carbon 3b props are today much lighter than traditional 2B wood made ones, and indeed reduces prop clearance, and this is really usefull on some ultralights.
As said by Robert, Rotax are reducted engines and vibration reduces the lifetime of the reducer.
A 3B carbon prop from eprops is 1.6Kg, it’s a Variable pitch “on the ground” – you an modify pitch with a tool while on the ground but obviously not in flight, and it’s an uncertified. This kind on propeller gives a lot less vibration and can adapt on your flight profile, and works great with Rotax.
It can easily replace a 2B VP that are 10Kg heavier, and in some case just wonderfully improves WB, but can also require to put some weight to adjust.

Their last prop (from eprop – I have no interest) is a light 3B VP (2.5kg), looks very efficient and promising. It’s been used for the speed record of the blackwing 635 (202kn at FL100), and Blackwing do claim it’s also more efficient than then original 3b MT. A jump on their youtube channel shows how to assemble it…

This been said, 3 blades props don’t always go better. I had an exemple on a TB10 I have used for some time. I was regularly flying on 2 TB10s, 1(old) with a 2blade and 1 with a 3blade, and the 2B one were consistently faster, over 5 knots.

Last Edited by greg_mp at 08 Jul 08:33
LFMD, France
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