I really don't know ??, there are too many variables here for me to work out.
We don't have any fitted with Hirth engines so i don't know about the engine,
the speeds are in MPH and we work in knots (even though Australia is Metric) and
i have never used or seen a powerfin propeller. I am going to try and convert
all your figures and see what i get.
I am also going to post this on the eGroup bulletin board for other owners
worldwide to have a look at as there may also be another owner out there with
the Hirth. Finally i have forwarded this to the factory in France, as they have
more exposure to different configurations.
A big question before we chase ourselves in circles is, how did you set up the
flaps and ailerons, i have found the wrong flap setting in the "F" will upset it
considerably !!, it needs to be set up properly to fly at it's best. The other
thing i have heard about the Hirth's (and don't quote me it is third hand) is
that the HP figures quoted are not close to the true achievable performance....
take 5 HP of it ?? i would like to know the answer to that one also
Thanks Michael
"Magrini, William J" wrote:
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Michael,
I am running a Hirth 2706 on an Xair-F
I have a Powerfin F prop with a 3.16 reduction. My 68" 3 blade prop is set
at 14.5 degrees pitch to give me 6000 RPM at climb and about 6200 max RPM.
The plane weighs 256 kilos with 7 gal of fuel and full gross or 450 kilos,
with one 210 lb. pilot and a 215 lb. passenger.
I calculated the CG as follows:
Nose = 13.634 kilos
Right = 125 kilos
Left = 117.3 kilos
13.334 x 146 = 1991/255.9 = 7.77 cm. Forward of datum. So my nose is light
due to the light engine, but for some reason, my overall weight does not
seem light. Is that acceptable?
My 13 lb. 18 amp battery is behind the seat as far forward as possible. I
guess I could get a lighter battery or mount it behind the engine.
My speeds are alot higher than Xair specs show. The plane climbs at about
500 fpm, at about 50 to 55 mph which, I guess is okay for a hot day, and a
heavy load. Cruise is about 60 to 65 mph indicated at 5800 to 6000 RPM.
The plane stalls much higher than the 30 mph indicated in the factory specs:
about 46-48, no flaps. The trouble I'm having is that:
1. The flight numbers are higher than indicated by the manufacturer.
2. When I throttle back to 5500, I quickly drop speed to around 55 or so and
I loose altitude. I need about 5800 RPM to maintain level flight, and about
6000 RPM for a slight climb.
3. The engine overheats if I maintain 5800 RPM or better. I mean really hot.
550+ CHT
EGTs are O.K. at about 1100. I have an air dam on the front cylinder (as
recommended by Hirth because the plane is in a tractor congiguration) and
both cylinders read the same.
I talked to Stuart at Powerfin and he says I need more pitch to the prop, in
order to be able to have more thrust at lower RPMs. He says myt static
thrust is good, but cruise thrust is probably inadequate to overcome the
plane's drag at cruise speed - especially when the plane is fully loaded.
He says I'll have to cut the prop from 68" to 66" to allow for more pitch
and allow the engine to develop full power. The Hirth 2706 is rated at 65 hp
at 6200 RPM. Maximum torque is at 5700 RPM. So the engine runs at a lower
RPM than the Rotax 582. I asked Stuart if he thought I needed a lower ratio
gear box (like 2.59) and he said he didn't think that would help.
Questions:
How does this compare with your 2 seat performance on the Xair at
gross weight using a Rotax 582 engine? I know you like the 618, but how
does the 582 perform at Gross?
Do you think the prop is running too slow because of the 3.16 gear
ratio? What ratio do you run with the 582. What prop pitch.
What are your recommendations? (I can't afford a 618 right now.)
