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Ed will have to let you know about the
A models. The 72" on the short leg RV4 got broken when the partner
slammed the throttle closed in the pattern on a still-cold engine,
the engine quit with the prop vertical, and he then did a very
high speed wheel landing on a wet/muddy strip. He hit a puddle and
the nose tipped down just enough to get a tip in the dirt.
When I flew the 76" on the same short leg RV4, I was very careful
to make tail low takeoffs and 3 point landings. It's been a long
time, but IIRC I probably had less than 8" clearance with the
tailwheel on the ground. I only made a couple of test flights
using that (borrowed) prop; I never intended to put it into
normal use.
6"-7" on a trike *sounds* like you could get away with it with
decent technique, but the safest thing to do might be to ask your
type group who has the least clearance, and what surfaces they fly
from.
Charlie
On 10/20/2021 5:50 PM, Stephen Izett stephen.izett@gmail.com
wrote:
Thanks for the real prop knowledge and feedback Charlie and Ed.
Ground clearance is another area I’m left wondering
about after hearing what Charlie has played with.
I think I’ll have just over 6.7 inches when the 72”
MT goes on. I wasn’t game to go for 74”.
What clearance does an RV-xA have with a 72” prop?
Steve Izett
When I had Tracy's 2.17
PSRU, I swung a 67 dia x 72 pitch prop. Climb and
cruise were fine but takeoff (especially from a short
runway on a hot day left something to be desired).
When I got Tracy's 2.85
PSRU, I had a 76 x80 wood prop build by Performance
propellers. I flew a few times with it as such and
while performance was much better rpm was only around
5800. Plus the prop was only a couple inches from the
ground on my RV-6A Nose gear aircraft - was concerned
about landing of grass strips with possible mole holes.
So I had the diameter reduced to 74 Inches. That was
what the doctor ordered.
There was a
considerable difference in aircraft performance
particularly during take off. With the new combination,
I had to tap the left brake to keep runway alignment (if
I cobbed full power from a standing start) until
airspeed reached approx 40 mph at which time the rudder
authority became adequate to hold the nose down the
runway. Starting acceleration was considerably better
with the new combination, it literally push me back in
the seat. Climbout improved up to around 1700 fpm
previous had been around 1000 fpm.
I had thought I would
probably have to give up a few knots on the top end, but
it turned out I actually increased airspeed by approx 4
mph. So it was really a win win for me. I could get up
to around 6200-6250 WOT at cruise, so apparently at high
airspeed the prop/gearbox unloaded the engine to pick up
a few more HP.
It appears that for
takeoff there is nothing better than the volume (mass)
of air your prop is able to push behind it. So even
though the prop turned slower with the 2.85, the much
larger prop and the engine increase from a take off rpm
of around 56-5700 rpm to 6000 and the increased torque
pushed a much larger volume (mass) of air.
I really like the
change the 2.85 gear box made possible - oh, yeah, now
the prop also turned in the conventional direction.😄
Ed
------ Original Message
------
Sent: 10/20/2021
9:01:32 AM
Subject: [FlyRotary]
Re: Ag Ops
Hi Steve,
I can't claim any expertise, but I do
have some experience with various prop diameters
on RVs. Van started out recommending 68" dia
props on the 2 seat RVs. My 1st RV4 had a Warnke
72x72. With 160 HP, it had 1200-1500fpm climb
and would cruise at 170 kts @ ~9.3 gph rich of
peak, ~75% power for a Lyc 160. The cruise
matched Van's numbers, and climb was at least as
good, if not better than Van's numbers. After my
partner in the RV damaged the Warnke, I replaced
it with a 68" from another mfgr and the plane
never performed as well in either cruise or
climb. At one point, I tried a 76" dia IVO.
Climb was incredible, but the plane would hit
a wall at about 145-150 kts (known problem with
IVO's blades; they don't have enough twist for
high speed operation). Another data point: Van's
current catalog lists a Hartzel 74" dia c/s prop
for the 2 seat a/c and the f/p props are in the
72" dia range.
My calculations on optimum dia &
ratio for the Renesis are similar to yours. If
you start with the Lyc-standard 2700 prop rpm,
with the realization that 74" is not too large a
diameter (for tip speed issues) at 2700, then
the optimum ratio would be determined by your
desired max engine rpm. With a controllable
prop, you can give up a little bit in diameter
& get back low speed thrust by going to
flatter pitch & more HP. But if fixed pitch,
I try for the largest dia I can safely fit on
the plane. With a typical fast 2 seat homebuilt,
gear leg length, not tip speed, will almost
always be the limiting factor even at 2700+ rpm.
The bigger the diameter, the better the low
speed mass flow (thrust), and there will be
little to no penalty at any cruise speed under
the 180-190 kt range (well above 200 mph).
The above assumes a properly designed
prop, of course. One reason to desire that
2.4-2.5:1 ratio is that it gets prop rpm where
the prop carver is used to working, and we're
much more likely to get a usable prop on 1st try
if he's in familiar territory. If you tell him,
say, 200 HP @ 2700 prop rpm & cruise at 75%
will be 170 kts, he'll likely be able to get it
right, 1st try. If, on the other hand, you're
telling him the same HP & speed, but you
want, for instance, 76" dia & 2300 prop rpm,
he's going to be guessing on how to carve the
prop.
Bottom line: I fit the largest dia I can
safely swing, regardless of f/p or c/s (because
low speed thrust will always improve with
diameter increases), knowing that ground
clearance will be a problem long before tip
speed becomes an issue. If you haven't already
bought the 72", I'd be asking them for a 76" if
they'll sell you one and you can safely swing it
on your plane. And be sure to tell them the
actual rpm range where it will operate. Their
'stock' 72" for a Lyc will be sub-optimal
turning in the ~2000 rpm range with a 2.85
drive, unless you intend to really flog the
engine hard all the time & pour a lot of
fuel through it.
FWIW,
Charlie
Hi
Andrew
Continuing to fine tune the
Renesis cooling system and aircraft.
Completing a new exhaust system
after a leaking gasket warped a flange.
Last flight temps max’d at 91C
(196F) on takeoff with 20C (68F)OAT and we
got her up to 189knots but only 2600 feet
due to cloud.
Learning that increasing the IAS
by ~10knots in climb has a significant
effect on cooling.
Also in the process of changing
the current 66.5” Airmasterto for a 72” MT
prop.
I only read after going with our
gearbox/prop combination that Tracy
recommended only using the 2.85:1 with props
of >=74”
For those with expertise in this
area, is it true that we should choose:
1. Largest prop dimeter with
acceptable clearance
2. Target cruise tip speed of
~0.8-0.85 speed of sound for best efficiency
If this is correct then with a
72” prop and a chosen engine redline of 7500
RPM (4 port Renesis) a better gearbox ratio
would be ~2.5:1
The 2.85:1 would be better
suited to a 74-76” prop at that redline.
Hope you have a great harvest
and get back in the air soon Andrew.
Steve
Hi Steve, hows the
Glasair going? I been away from
flying for awhile, hopefully back
into it in new year. Did start mine
recently after it been sitting idle
since feb 2020, So happy I
persevered with the mazda, it
started & ran so sweet I
contemplated doing a couple of
circuits but for the legalities. No
MR and I need AFR first.
Neil, not
sure I agree with your rotor setup
proposal, ( not that mine is
better) can get some weird
aerodynamics with intermeshing
rotors.
Whole thing
of nurse cart & drone need to be
a package, rotors will never be
stopped during refill so drone needs
to land on roof of truck or trailer
for refill from underneath to keep
blades clear of dumbasses like me.
Truth be
told, I dont think we can get
anywhere near the efficiency of a
ground rig or Ag plane, we easily
average 80 ha/hr each machine +
nurse cart, going to need a swarm of
drones to get near this. But could
be a fun project.
Andrew
Good
to hear your voice Andrew.
My son-in-law
in Kojonup would be very
interested in what you
guys are talking about.
Cheers
--
Regards Andrew
Martin Martin Ag
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