arj1 wrote:
What happens if you measure if with approved organisation and it shows less?
No idea.
arj1 wrote:
What happens if you measure if with approved organisation and it shows less?
You will get an ammended noise certificate reflecting the actual measurement. Which is what we will pursue.
For now, maintenance managed to get the noise class reversed to B which it was before, so for now we are good again.
Mooney_Driver wrote:
For now, maintenance managed to get the noise class reversed to B which it was before, so for now we are good again.
Great! How did they do that? And “for now”…?
I guess you could simply keep using the old noise certificate. How would anyone know? The Swedish CAA saw fit to withdraw the noise certificate for one of my club’s PA28s. Apparently ICAO Annex 16 was changed so that the aircraft didn’t need a noise certificate and by bureaucrat logic that meant it can’t have one – so they demanded we return it. Of course we made a copy of it first and that’s what we show to airport staff on flights to Germany…
Airborne_Again wrote:
I guess you could simply keep using the old noise certificate. How would anyone know?
Err by counting to 3 instead of 2?
For now means that we will eventually have to do a noise measuring flight to determine what the noise really is. Because right now, first of all we still will get punitive noise taxes (not as high as with A) and if we have to show the certificate outside the country, we will face bans and high landing fees.
In the end, the whole thing was a total screw up by all concerned: Me because I trusted others to do my work. Maintenance because they based their recommendation on assuming (probably factually correct) that a 3 blade prop is quieter, MT because they did not find it necessary to tell us we will have a problem when we ordered the prop. The usual, lack of communication and too much trust.
Well, for now we will operate like this and see what comes. Selling off is still an option I consider.
MT because they did not find it necessary to tell us we will have a problem when we ordered the prop
MT ?
The prop shop.
Another bit with causes head scratching.
In the STC, Hartzell limits the use of the prop thus for installations which have the powerflow tuned exhaust we have:
“Avoid countinous operation below 22” MP between 1950 and 2350 RPM".
Well great, that is about the whole power range of 55 and 65%, the tables don’t even offer values for 2400 RPM at 65% low level, let alone 55% and so on. Also there is no information about the impact on KTAS e.t.c. in that STC, which implies there is none, which is contradicted by users on the net which claim 1-3 kts speed loss at constant FF. We will find out. So in fact for most of the spectrum such as 55 and 65% cruise we mostly use, we have no power setting tables at 2400 RPM, particularly low level up to 7500 ft. Also the STC does not indicate any performance factor or bias as some call it.
I wonder how to go about this. My hunch is to level the plane at the desired DA and set the power to an interpolated (or extrapolated) MP, 2400 RPM and mix. Fuel flow would then have to correspond to the % power we aim for, which we can correct to it. Then note down MP and TAS. In other words, we are down to performance test piloting in order to know how to plan flights with this setup.
On the test flight, the airplane did perform massively better in take off and initial climb, but notably worse in cruise, provided the indications are correct.
MTOP at DA 2000 ft AMSL was about 25" with 2500 RPM. IMHO that is on the low side, particularly about RPM. Still, take off performance was massive. (And the mischievous side of me thinks, hmm, why not leave it like this and do the noise measurement first? We could hear each other talk without headsets…)
Cruise however was quite a let down:
At all levels with proper fuel flow for the power we got troughout a 6-10 kt deficit towards POH values. While these are known to be marketing inspired, with the 2 blade we did regularly see TAS of 150 kts flat out and similar and 140 kts in cruise at 65. Right now we are more in the 140-145 kt range Max power and 135 kts in cruise (65%)
I tried to do an extrapolated power table (which still is based on the 2-blade version but at 2400 RPM instead of 2300). If those figures are right, we have a speed bias of about -8%. That would be massive. It would also mean, range will reduce by about the same amount.
Will get interesting to find those things out. I am not sure we will keep this prop, if only half of the stuff now transpiring proves right.
So the rumours that 3 blade props cost speed appears true and worse than expected (I was told 1-2 kts).
I have quite a lot of experience in C172RGs and C182s (both RG and FG) and the three-blade variants of all these models are quite a bit slower than the two-bladed ones. I never bothered to do real testing, so cannot provide accurate data, but I would say a reduction in cruise speed of about 5-8 kts is to be expected. Just yesterday flew a rented C182 with a three-blade prop and was surprised at the rather underwhelming cruise performance as opposed to the two-bladed version I’m used to. YMMV.
Mooney_Driver wrote:
Err by counting to 3 instead of 2?
Does the noise certificate actually say “3 blade prop”? No one at an airport office will have any idea what the p/n of the prop means.
Airborne_Again wrote:
No one at an airport office will have any idea what the p/n of the prop means.
I get ya. Yea, something like that.
172driver wrote:
I would say a reduction in cruise speed of about 5-8 kts is to be expected.
In a Mooniac’s ear that sounds like quite a blasphemic thing to do to a plane which is built for speed. jeez…. But that is pretty much what we saw in the test flight.
