Your both right, good question, but.....
avaviat said:
The speed of sound.
Turbofans are in a sense really fast props, and they work best on fast planes.
I used to build a few electric model airplanes... my "fast" models (45Kts) used direct
drive 15,000RPM (and faster) props. My "slow" (4.5Kts) planes used gear
reductions to spin large props at ~3000RPM. Both flew beautifully... but if you
swapped powerplants (fast prop on slow model and viceversa) the slow model could
barely climb and the fast model couldn't climb at all...
Michael White and avaviat, you are both right.
jcoloccia, its not a stupid question, it has been thought of, but the design challenge is
just too great, and the law of physics just conspire to "get medieval on our buttock".
First avaviat, a Jet is in no way comparable to a prop. First they are ducted. The
tolerance between the fan blade and cowling is very small, basically Zero. They use
frangible rub strips and the blades do rub. A turbine blade is razor thin and made of
titanium mostly. The twist, the number of blades, "blade loading" and many factors
spell apples and oranges. Last it is not just a FAN. Directly behind the fan are
"stator vanes" and more blades. Way more going on to discuss here. Forget that
analogy, props spinning out in the open and turbine blades in a "duct" are two
differnt animals. The are like the same as your girlfrend's cat to a Tiger. Both have
fur.
Second avaviat, the tip speed is a little over 0.40 mach on your 15,000 rpm 51mph
model plane, which is about 1/2th the tip speed on a RV doing 200 mph with a 74"
prop turning 2,500 rpm. Your model's prop tips are well below supersonic. I assure
you if it was supersonic it would suck. Tip speed is a function of not only RPM, but
prop diameter and the aircraft's forward speed. A 74" dia prop is not a 7.4" prop.
avaviat, you are right, putting a slow prop on a fast plane or a fast prop on a slow
plane is bad news. Props are designed around a factor called "J" or the advance
ratio, V/nD, the ratio of velocity (aircraft speed) verses the prop RPM and diameter.
A prop that is efficient for a particular airframe or planned designed speed, will not
work well on another plane.
Also, model airplanes are not a fair example to a real airplane, because of thrust and
drag are out of "scale" to a full sized plane and prop diameters are small. However
your point is correct a prop for one plane is not good for another. It however does
not have too much to do with tip speed. If you change aircraft drag, HP or target
speed you change the optimal prop design.
What what about jcoloccia's question. A direct drive prop for a Subaru. The
punch line is
supersonic tip speed. Over 0.90 you are in trouble.
Michael is very correct, as a prop tip speed approaches about Mach 0.9 efficiency
goes to heck. In fact Mach .85 is about all you want to get near with you prop tips.
A real fast RV at 2,700 can bump up to that 0.85M tip speed.
You can google this info. It does not mean fast turning props going mach 0.90 don't
work, but I can tell you a 68" 2-blade or 3-blade prop turning 4600 RPM on a plane
going 200 mph would have ugly efficency. The tip speed would be well over Mach-
1 by a large margin. That is unworkable. On take off at 6000 RPM it would be much
higher than that. This is like sailing with the anchor still out. What about a smaller
dia prop, or more blades, thinner, wider or curved?
Remember the unducted fan Boeing came up with in the 80's:
http://en.wikipedia.org/wiki/Unducted_fan and this is cool
C-130 Herc
The slower the tips the better for efficiency. Don't get efficiency and thrust mixed
up. Slower also means less thrust, but more thrust for given HP. So turning slower
will make you go slower, but more efficiency for HP you are putting into the prop
(prop HP). There is a trade off.
What about turbo props or those "unducted fans" on high speed turbo props? They
turn the blades slowly (see below). The problem of blades (props) and high forward
speed is it gets complicated, because the "vector" or tip speed from rotation and
forward speed work against you.
For sure RPM over say 3,000 to 3,200 gets to be problematic for any normal plane
(prop size and airspeed) similar to a RV. At some point, forward speed, prop speed
and thrust have to balance. I mean a solution to tip speed is just not turn it or make
the prop real small? However you will not go very fast with Zero RPM (zero thrust)
or small diameters.
High speed turbo props turn their blades real slow, like 1700 RPM. However these
blades are designed for this RPM and forward speed, not an after thought. Props
have only one RPM and speed they are best at.
This is why Eggenfellner is going
to have problems with his new slow turning re-drive. Running a prop designed to
run at 2,500 rpm at 1,700 rpm is not going to work well. He is loosing
efficiency unless he has a custom prop made for that RPM, fwd speed, HP etc.....
All parts have to be designed to work together. Change one thing affect 10 other
things.
**************************************************
Factors affecting the prop besides the design of the prop itself are:
*RPM
*Air density
*HP available
*Aircraft drag
*Airspeed (climb/cruise)
Factors affecting performance due to prop design only:
*Diameter
*Number of blades (more is not always better)
*Blade design (thickness, plan&tip shape, twist, airfoil, chord vs station, area)
(Blades affect the Ct and Cp, coefficient of thrust and power)
**************************************************
There is nothing wrong with gear boxes. Lycoming and Continental have geared
engines. I think most of the radials have planetary reductions. There is nothing
wrong with turning the engine fast and prop slow, but for RV's you don't want to
turn your prop much over 2,900 RPM. The little Reno formula racers turn 3,000-
4,000 rpm, but they have smaller props and they are not going for efficiency, they
are going for speed; so they don't care about a little loss of prop efficency if they can
get enough additional HP out of their poor little engines to justify it the prop losses.
However there is a limit, and when you get to MACH 0.90 you are really making
noise not thrust. The Subaru needs to run 4600-6200 RPM. That is way to much,
unfortunately.
Most props run somewhere around 75% to 85%. That means 1 HP from the engine
gives you say 0.80 HP worth of thrust. More correctly it is equal to [(Thrust *axial
speed) / (resistance torque * rotational speed)].
A prop with 0.83 efficency in slow cruise may only have 0.79 efficiency in another
flight condition. So even an efficient prop can not be efficient over a wide range of
RPM's and speeds. This is worse for our 6000 RPM prop, that needs to work for
100mph to 200 mph and rpms from 3000-6000. A Normal prop only has a range of
about 500 RPM and +/- may be 100-140 MPH. Trying to get a prop to work over
3000-4000 RPM, over a wide speed range would be a hard trick. Unfortunately a
Subaru does not make much HP at 3000-4000 RPM.
Moral of the story, a prop has to be customize to the exact application and most
important flight operations. That is why the Hartzell Blended Airfoil is so ideal for
the RV, because it was designed specifically for the RV. Most other offerings are
generic, not exactly for the RV's: HP, speed, drag, altitude and RPM's. That is why
alternative engine set-ups suffer a little, due to lack of prop availability. I like Tracy
of RWS, he uses a fixed prop and deals with the loss of T/O and climb performance
for light weight and simplicity, not to mention lower cost.
NOTE: Prop efficency improvements and changes are measured in 10ths or even
100ths of a percent. To get 1% change is fantastic. That is why I doubt many claims
from prop manufactures who never tested their prop on a RV but somehow gained
tremendous efficiency all of sudden?
With all of Hartzells tweaking on the C2YR/F7496, BA Hartzell, they only got 3.5
mph more than the venerable Hartzell HC-C2YK, the previous standard prop for
180HP/200HP RV's. THAT IS AWSOME!! That is a 1.7% gain in speed. That is
some serious squeezing. They did it right. First the HC-C2YK is no slouch and is
faster than almost every "fancy composite prop" out there (except the WW 200RV
which was only 1/2 mph faster). However the HC-C2YK was made for the Mooney
and Comanche. A good match to RV's but not quite optimized.
Van's Prop
comparison
Its hard to get every bit out of a prop. However the Hartzell BA is the fastest prop
by 3 to over 8 MPH, compared to other brands. Hartzell addresses the experimental
market and offers props specifically designed for the RV, Lancair, Rocket and other
kits. I don't know of any other manufacture that actually tests and designs
specifically on and for the RV, except Sensenich.
Prop design and analysis is complicated, but I am sorry to say a direct drive prop on
an engine capable of running from 3000 rpm to say 6000 rpm, from 50 to 220 mph,
is a design task yet to be solved. Something could be done, but its beyond the scope
of what you would put on a little single engine piston plane, with 160-180HP. Its
too easy to make an engine that produce a 150-250 HP at 2,700 rpm direct drive
engine, Lycoming. (Sorry I had to say it.)
G