4-blade Pawnee Performance?

On Nov 20, 1:05*pm, "bumper" wrote:
No answer on your questions . . . however, I do know there was at least one
incidence of the 4-blade prop making like a Frisbee. One minute it was
dragging the Pawnee through the air, the next it was on it's own and
accelerating away from the plane.

Guess it's important to re-torque the prop bolts?

bumper

That brings to mind a local story. There was a group of retired
geezers rebuilding an Aeronica 7AC Champ. The engine had just been
hung on the mounts and one of the group decided to see how it look
with the prop. He hung the prop on the crankshaft flange collars and
went looking for the prop bolts. Another of the group tried to rotate
the prop out of the way.

The engine which had set for years burst into life as the impulse
magnetos hadn't yet been wired to the ignition switch or the throttle
cable run to the cockpit. The engine revved to redline and then ran
out of gas in the carburetor float bowl which launched the unbolted
prop through the hangar wall.

All geezers present were by then face down on the floor or hiding
under old cars. The story came to light when various members of the
group showed up at the airport restaurant and were forced to explain
the bandages covering cuts received while diving for cover.

On Nov 20, 2:01*pm, bildan wrote:

As a prior Aeronca owner I take exception to your abuse of the name,
good story though.

Andy

I owned one too. The "standard" wood prop hub had a female tapered shaft
with woodruff key slot. There was a single captive prop nut used to secure
the hub to the engine crankshaft that had a male tapered shaft with key.

I can see how it could easily launch the prop if that nut wasn't screwed
down (120 ft lb torque as I recall).

bumper
"Andy" wrote in message
...
On Nov 20, 2:01 pm, bildan wrote:

As a prior Aeronca owner I take exception to your abuse of the name,
good story though.

Andy

Before buying the 4-blade prop, I carefully read the manual. It requires
constant re-torquing of the mounting bolts and if ever oversped by only a
few percent, it is to be removed from service. (We all know that all of
our tow pilots will immediately confess to such an error, right?) Our
club tried one for a while, but removed it in the interest of safety.
Climb with a heavy ship on a hot day was very marginal.

It was very quiet because that tow plane spent most of the summer in the
hangar...




At 01:45 21 November 2008, bumper wrote:
I owned one too. The "standard" wood prop hub had a female tapered
shaft
with woodruff key slot. There was a single captive prop nut used to
secure

the hub to the engine crankshaft that had a male tapered shaft with key.

I can see how it could easily launch the prop if that nut wasn't screwed

down (120 ft lb torque as I recall).

bumper
"Andy" wrote in message
...
On Nov 20, 2:01 pm, bildan wrote:

As a prior Aeronca owner I take exception to your abuse of the name,
good story though.

Andy

On Nov 19, 10:59Â*pm, "noel.wade" wrote:

 Can anyone out there with experience behind both 2-bladed and 4-
bladed
Pawnees give me an idea of what the performance losses are?

My club has debated this off and on, but I was recently at a field
with a 4-bladed Pawnee and was much impressed with the lower noise.

Lower noise is the point. If you have problems with local residents,
or might have in the
future, then lower-noise tugs will help your Public Relations effort.
The very fact that you
have done something, rather than nothing, will help in itself.

In the UK we have had some air tow operations closed down after local
objections about
noise, although winch launching continues at these sites.

I fly from Lasham Gliding Centre in the UK, West of London. Although
Lasham is in the country (“sticks�, “boondocks� ?), it is not truly
rural because many locals who work in the Big City value a quiet time
in the country, particularly at
weekends.

Over the years we have had noise complaints from individuals and, more
difficult to deal with,
from organised groups. We have been air towing from Lasham since 1950
and most of these
people will have moved in since then. However, with over 200 gliders
on site we are very
busy on soarable days, and even on those that are not particularly
soarable, with instructional
flying.

Many years ago, we therefore agreed a programme to reduce tug noise.
This included fitting
better silencers to all tug engines (on the other side of the pond you
call them mufflers) such as
the German Gomolzig series (www.gomolzig.de). Then we fitted four
bladed props to all tugs
to which this was possible. Cost-wise we staggered this over several
years. Lasham owns
three 180hp Robin DR400, a 230hp Pawnee and a 180hp Super Cub. All
now have four
bladers except the Super Cub, which is noticeably more noisy than all
of the others. For some
reason our engineers found it difficult to fit a four blader to the
Cub, although I am aware that
they are fitted elsewhere (advice welcome!). Anyway, that is what we
did, and we get very
few noise complaints today. In addition, our tug pilots are briefed
to avoid flying over the
local villages below 1500ft AGL. If, on rare occasions now, if we get
a complaint from one
place, we simply switch the tow pattern to another direction.

Conditions with little wind and a low inversion are the worst case for
noise on the ground, and
in our area Sundays are more sensitive than other days.

There was even a time where one of the villages asked the club to
attend a village meeting to
explain the noise situation. Although this was a tough one, the fact
that we were able to say
that we had made modifications both to propellors and exhaust noise,
and have a map of
“avoid areas� went down well. We have few complaints from that area
now.

Problem is, I've got club members claiming that it'll cost over $10k
to do the job, and we'd give up 25% to 30% in performance due to the
extra blades.

In my experience as a tug pilot both before and after the noise
reduction programme, the
performance reduction is much less than this. Others who are better
qualified will no doubt
produce figures, but I would suggest between 5 and 10%.

However, if it prevents local inhabitants trying to close your gliding
operation down, then it’s
worth it!

Ian Strachan
Lasham Gliding Centre, UK

"For some reason our engineers found it difficult to fit a four
blader to the Cub, although I am aware that they are fitted elsewhere
(advice welcome!). Anyway, that is what we did, and we get very few
noise complaints today."

My club (not too far from Cambridge!) got planning permission for its
current site conditional on tugs being fitted with silencers (mufflers)
and four bladed props. This was nearly 20 years ago so there were no
approved mods for our then tugs - a Super Cub and a Citabria. Using a
four-bladed ground adjustable prop. tests were done with both. The
Citabria was fine and subseqently fitted with a 4-blader. The Super
Cub's tailplane and elevator etc. was reported as encountering severe
vibration with the 4-blader so it was decided a mod was not appropriate
and it was sold and a Rally acquired instead.

At the Black Mountains GC, Wales, UK we have been using a 235 Pawnee with a
4 blade prop and silencer for at least 16 years to my knowledge. it will
tow a K21 or Duo Discus of a 350 yard downhill grass strip 900ft amsl

The initial acceleration is good, but it is thirsty and it dosent like
long climb. most of our tows are ridge tows to just over 1000ft agl and
there isnt much else we could use anyway



Pete

have a look at www.blackmountainsgliding.co.uk

At 22:59 19 November 2008, noel.wade wrote:
Can anyone out there with experience behind both 2-bladed and 4-bladed
Pawnees give me an idea of what the performance losses are?

My club has debated this off and on, but I was recently at a field
with a 4-bladed Pawnee and was much impressed with the lower noise.

Problem is, I've got club members claiming that it'll cost over $10k
to do the job, and we'd give up 25% to 30% in performance due to the
extra blades.

Can anyone corroborate those numbers?

Thanks, take care,

--Noel