01/21/01 PROP, GROUND CLEARANCE
The V530 2 blade Russian prop is 2.4meters in Dia. That's about 94.5" dia.
The MT, and WW 3 blade props are 2.5m (98") or 2.6m(102") in dia. Both
diameters are available from both manufacturers. Hoffmann has a 2.4m(94.5")
and a 2.5+m(99") version. The 2 blader is shorter than the 3 so my numbers
were correct.
The confusion comes to be like this. Many airplanes are designed for a
given 2 blade prop and when it is converted to a 3 blade, the blades are
usually narrower and shorter to give comparable performance with less noise
and vibs. An example is a Bonanza. In the case of the M14P engine, the V530
2blade prop is not of the optimum diameter and blade area for best
performance. It is a short prop for the Yaks that give good ground
clearance. A wilga prop is longer as Jill pointed out a while back. The 3
blade props work better, produce 25% more thrust, shake and vibrate the
airframe less, look better and remove the fabric from the airframe at a much
slower rate than the 2 blader. Those big square tips produce terrible,
nasty vortices that really pound away on the airframe.
V530 94.5"dia
MT 98" or 102" or 107" on special order
WW 98" or 102"
Hoffmann 94.5" or 99"
Also this:
One question was how did I come up with or where did I get the thrust
numbers I have been quoting. Well, several places is the answer. It
depends on the engine/prop combo.
First of all, I should define static thrust. This is a good way to compare
the pull available with a given engine/prop combo. Static Thrust = max
thrust the engine prop combo can produce with the airplane not moving
forward. This is usually measured with a pull dynomometer (there are
literally hundreds of types of dynomometers). A pull dyno is a very
accurate sensitive pull scale. The airplane is tied to a hard point or tie
down anchor with a long nylon strap with the dyno in the line. With a pilot
on board, power settings are selected up to full throttle high rpm with all
other combos in between. These numbers are logged and the max thrust is
determined from the readings. This type of test can be adjusted for given
day and temp conditions.
So, my sources for these thrust measurements are MT, Curtis Pitts, and ME.
MT did these tests, Curtis did these tests and we did them here. For the
M14P 360hp with the V-530 prop, MT got 1325lb static thrust many years ago.
Curtis got 1320 on his dyno in 1996. We got 1320 on our dyno in 1999.
Having 3 seperate instruments operated by seperate people yield the same
results makes the number VERY believeable.
On the 360hp M14P with a 3 blade MT 2.5m(98"dia) prop, MT got 1550 we got
1560. Pretty close.
We have not tested an M14PF 400hp with the MT3 blade prop as we have not had
one available yet(we will soon...
. So, I took the values MT had onrecord. Seeing that their #s and ours have been right together, it is a
reasonable assumption to accept their test values.
To compare these thrust values to those for lycomings, I was able to again
use numbers found by MT, Curtis Pitts, and our own test results. Curtis did
these tests years ago for the hartzell 2 blade props and in 1998 or 99 did a
static test on the 3 blade black composite hartzell in the prototype flame
job S2C. MT has done all these tests too. We got a chance to do these
tests as part of some Prop STCs we are working on for the S2A and S2B,C,S
airplanes. We needed to test both the 2 blade Hartzell and the new MT 3
blade props to compare the results. We all found that the typical 200hp lyc
with 2 blade hartzell made around 400 to 440lb of static thrust. We found
the 260hp lyc with 2 blade hartzell made about 800lb static thrust. The new
MT prop we want to use on the S2A or Eagles or any other acro plane with
200hp lyc made about 470lb static thrust. Not much gain but the prop is
soooo much lighter that the stress on the crankshaft is much less with the
MT. The new MT for the S2B,C,S with the 260hp lyc made 965lb static thrust
as compared to the 960 of the 3 blade Claw from hartzell but 165lb more than
the 2 blader. This showed that the new MT and the Claw had virtually the
same static thrust yet the MT is $12k and the Hartzell is $25k. 2 for 1
price savings!!! This STC is nearly complete.
From all of the above, I was able to show that the M14P/3 blade prop combo
has nearly 2 times the static thrust as a stock S2B with a 2 blader on it
and almost 4 times that of an Eagle or S2A!!!! These numbers prove out with
flight tests. Also from these data sets, it is not hard to approximate the
static thrust of other engine prop combos. Like 250hp 540 lyc with a fix
pitch prop. A guess would be approx 600 to 650lb static. So, a Model 12
with an M14P 360hp/3blade MT combo makes 2.5 times the thrust of a
250hp/fixed pitch powerd airplane. This MORE than makes up for the
200-300lb extra weight. On the other hand, putting a PW R-985 450hp engine
on a Model 12 raises the empty weight by 300-350lb. The Static thrust with
a 3 blade hartzell (only prop for R-985 remotely accepted as acro prop and
lightest), is about 1750lb static thrust. So, 1500lb airplane with 1560lb
static thrust vs 1850 lb airplane with 1750lb static thrust. This shows the
M14P combo to be a better choice.
_______________________________
Scott
SR#174
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