4-blade Pawnee Performance?

[noel.wade]

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

[bildan]

On Nov 19, 3:59*pm, "noel.wade" > wrote:
> Can anyone out there with experience behind both 2-bladed and 4-bladed
There are several Pawnees with 4-blade props and a few with 2-blade
props in Colorado. Yes, it will cost you $10k and be somewhat quieter
overhead and much quieter on approach, taxi and takeoff. The main
effect seems to be 4-blade props shifts the prop noise to a higher
pitch which is somehow less annoying.

As for performance, I've towed behind most of them and I'd estimate
that there is a small gain in initial acceleration followed by a small
loss in rate of climb - somewhere around 5% each way.

Mile High Gliding, the dealer for the Hoffmann 4-blade, uses them on
their 230HP Pawnees to tow over gross 2-32's from a 2200' runway with
a density altitude sometimes over 10,000'. Many of these tows go to
10,500 for their "Mile High Glider Ride". There's no way this would
work if there were a 25% performance loss.

> 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

[[email protected]]

On Nov 19, 5:59*pm, "noel.wade" > wrote:

> Can anyone corroborate those numbers?

Numbers? No...

A four blade Pawnee going low overhead at full power is still very
loud. The prop does nothing for the engine noise and the Pawnee has
quite a lot of that. What it eliminates is the 'snarl' from high
speed prop tips that is most noticeable to an observer in the plane of
the prop. If 'snarl' is your problem, maybe this is your solution.

The performance loss is noticeable and annoying, but not a deal
breaker in itself.

Clubs I know who've bought them have not been real impressed. For the
price, I'd like to be impressed. Ymmv.

-T8

[bildan]

On Nov 20, 9:09*am, wrote:
> On Nov 19, 5:59*pm, "noel.wade" > wrote:
>
> > Can anyone corroborate those numbers?
>
> Numbers? *No...
>
> A four blade Pawnee going low overhead at full power is still very
> loud. *The prop does nothing for the engine noise and the Pawnee has
> quite a lot of that. *What it eliminates is the 'snarl' from high
> speed prop tips that is most noticeable to an observer in the plane of
> the prop. If 'snarl' is your problem, maybe this is your solution.
>
> The performance loss is noticeable and annoying, but not a deal
> breaker in itself.
>
> Clubs I know who've bought them have not been real impressed. *For the
> price, I'd like to be impressed. *Ymmv.
>
> -T8

It's interesting to do some highly oversimplified "back of the
envelope" numbers on Pawnee tugs. (The following ignores some
important stuff.)

Assume a 40:1 glider weighing 1000 pounds being towed at its best L/D
airspeed of 60KTS and climbing at 500FPM. 1000/40 = 25 pounds of
aerodynamic drag. 60x25/325 = 4.6HP to overcome that drag.

Of course the tug is lifting the glider at 500FPM or 8.33 FPS so
8.33x1000/550 = 15.14HP to lift the weight of the glider.

15.14+4.6 = 19.74HP which is the rate at which work being done on the
glider by the Pawnee. In other words, only approximately 20HP is used
to tow the glider.

If the tug engine is actually producing 230HP (highly doubtful), 210HP
is being wasted somewhere - most of which is probably just the Pawnee
hauling itself through the air. That wasted energy is where most of
the noise is coming from.

The above suggests that a tug with a highly aerodynamic airframe could
use far less HP which is why 80HP motorglider tugs seem to work so
quietly.

[sisu1a]

> The above suggests that a tug with a highly aerodynamic airframe could
> use far less HP which is why 80HP motorglider tugs seem to work so
> quietly.

A tug like this: http://www.sportaircraftworks.com/oto%20bin/dynamicnew.htm
According to John Roake, turnaround on a 600m launch with this plane
averaged 4-5 minutes (takeoff to landing) and it only took 100 Euros
worth of fuel for 8 of these planes to launch a field of 60 gliders
(no other stats provided unfortunately). With efficiency like that I
bet these planes don't make a lot of noise though...

-Paul

[Darryl Ramm]

On Nov 20, 8:51*am, bildan > wrote:
> On Nov 20, 9:09*am, wrote:
>
>
>
> > On Nov 19, 5:59*pm, "noel.wade" > wrote:
>
> > > Can anyone corroborate those numbers?
>
> > Numbers? *No...
>
> > A four blade Pawnee going low overhead at full power is still very
> > loud. *The prop does nothing for the engine noise and the Pawnee has
> > quite a lot of that. *What it eliminates is the 'snarl' from high
> > speed prop tips that is most noticeable to an observer in the plane of
> > the prop. If 'snarl' is your problem, maybe this is your solution.
>
> > The performance loss is noticeable and annoying, but not a deal
> > breaker in itself.
>
> > Clubs I know who've bought them have not been real impressed. *For the
> > price, I'd like to be impressed. *Ymmv.
>
> > -T8
>
> It's interesting to do some highly oversimplified "back of the
> envelope" numbers on Pawnee tugs. *(The following ignores some
> important stuff.)
>
> Assume a 40:1 glider weighing 1000 pounds being towed at its best L/D
> airspeed of 60KTS and climbing at 500FPM. *1000/40 = 25 pounds of
> aerodynamic drag. *60x25/325 = 4.6HP to overcome that drag.
>
> Of course the tug is lifting the glider at 500FPM or 8.33 FPS so
> 8.33x1000/550 = 15.14HP to lift the weight of the glider.
>
> 15.14+4.6 = 19.74HP which is the rate at which work being done on the
> glider by the Pawnee. *In other words, only approximately 20HP is used
> to tow the glider.
>
> If the tug engine is actually producing 230HP (highly doubtful), 210HP
> is being wasted somewhere - most of which is probably just the Pawnee
> hauling itself through the air. *That wasted energy is where most of
> the noise is coming from.
>
> The above suggests that a tug with a highly aerodynamic airframe could
> use far less HP which is why 80HP motorglider tugs seem to work so
> quietly.

Or lets guess around 30 hp assuming prop (in)efficiency, ~80% best
case?, but who knows in practice. And the wasted energy is where *all*
the noise comes from :-) And as a data point, the ~50 shp engine in my
ASH-26E give up to 700 fpm climb... on a good day.

A more efficient tug could reduce the amount of prop thrust needed and
therefore noise that needs to be made in the first place. If prop
noise is a significant factor, which it seems to be, adding blades
definitely seems to help.

BTW I agree with your earlier post about the apparent differences in
towing behind a Pawnee with a Hoffman 4 bladed prop. Soar Truckee
switched to a four bladed prop and did some good community PR work
around reducing noise, including a positive article in the local
newspaper. See -

www.soartruckee.com/pdfs/Pickle_part_1_0612.pdf and
www.soartruckee.com/pdfs/Pickle_part_2_0612.pdf

The rate of climb with a heavy glider (e.g. DG-1000S with two
occupants) at ~7k'+ density altitude seemed a little lower than a two
bladed prop, but that's not a scientific comparison. Mile High may be
able to provide data on that as well. Hopefully a tow pilot with
experience with these will weigh in.

Darryl

[Bruce]

sisu1a wrote:
>> The above suggests that a tug with a highly aerodynamic airframe could
>> use far less HP which is why 80HP motorglider tugs seem to work so
>> quietly.
>
> A tug like this: http://www.sportaircraftworks.com/oto%20bin/dynamicnew.htm
> According to John Roake, turnaround on a 600m launch with this plane
> averaged 4-5 minutes (takeoff to landing) and it only took 100 Euros
> worth of fuel for 8 of these planes to launch a field of 60 gliders
> (no other stats provided unfortunately). With efficiency like that I
> bet these planes don't make a lot of noise though...
>
> -Paul
>
The European "ultralight" tugs do a fine job. The Dynamic is really
quiet, fast, economical and tows well. The difference in towing behind
something like a Dynamic or Samba versus one of the heavier aircraft is
substantial. With the modern ultralight being a much better match to the
glider.

[bildan]

On Nov 20, 12:12*pm, Bruce > wrote:
> sisu1a wrote:
> >> The above suggests that a tug with a highly aerodynamic airframe could
> >> use far less HP which is why 80HP motorglider tugs seem to work so
> >> quietly.
>
> > A tug like this:http://www.sportaircraftworks.com/oto%20bin/dynamicnew.htm
> > According to John Roake, turnaround on a 600m launch with this plane
> > averaged 4-5 minutes (takeoff to landing) and it only took 100 Euros
> > worth of fuel for 8 of these planes to launch a field of 60 gliders
> > (no other stats provided unfortunately). With efficiency like that I
> > bet these planes don't make a lot of noise though...
>
> > -Paul
>
> The European "ultralight" tugs do a fine job. The Dynamic is really
> quiet, fast, economical and tows well. The difference in towing behind
> something like a Dynamic or Samba versus one of the heavier aircraft is
> substantial. With the modern ultralight being a much better match to the
> glider.

Someone said that the typical airplane engine is an excellent
transducer that converts money into heat and noise with a small,
although useful, amount of thrust as a side effect.

[Bruce]

bildan wrote:
> On Nov 20, 12:12 pm, Bruce > wrote:
>> sisu1a wrote:
>>>> The above suggests that a tug with a highly aerodynamic airframe could
>>>> use far less HP which is why 80HP motorglider tugs seem to work so
>>>> quietly.
>>> A tug like this:http://www.sportaircraftworks.com/oto%20bin/dynamicnew.htm
>>> According to John Roake, turnaround on a 600m launch with this plane
>>> averaged 4-5 minutes (takeoff to landing) and it only took 100 Euros
>>> worth of fuel for 8 of these planes to launch a field of 60 gliders
>>> (no other stats provided unfortunately). With efficiency like that I
>>> bet these planes don't make a lot of noise though...
>>> -Paul
>> The European "ultralight" tugs do a fine job. The Dynamic is really
>> quiet, fast, economical and tows well. The difference in towing behind
>> something like a Dynamic or Samba versus one of the heavier aircraft is
>> substantial. With the modern ultralight being a much better match to the
>> glider.
>
> Someone said that the typical airplane engine is an excellent
> transducer that converts money into heat and noise with a small,
> although useful, amount of thrust as a side effect.
Indeed, consider the Wilga...

Something like a Samba or Dynamic tows smooth and at half or less of the
cost of the fossil fuel converters I am a fan. Last contest I had a
quicker ride to release behind a Rotax914 equipped Samba XL than the
preceeding run behind a Cessna 182.

The Cessna tow cost twice as much though.

[bumper]

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

"noel.wade" > wrote in message
...
> 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
>

[bildan]

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.

[Andy[_1_]]

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

[bumper]

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

[Bob C[_2_]]

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
>
>
>

[Ian Strachan]

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

[George Knight]

>>"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.

[Peter Thomas]

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
>
>