Vance Jaqua wrote:
Good hit Paul ! to clarify I will us an exampe Classic
prop eff is thrust hp(thrust times plane speed)
divided by shaft power input. Say in a climb at
constant 100 mph the engine was being prop limited to
an rpm where it delivered 100 hp and you were getting
200 lbs thrust. Then you cranked the pitch down so the
engine could get full revs and put out 130 hp a 30
percent increase - well you would not get this full 30
percent increase in thrust because the efficiency
would be dropped by the increase in disc loading and
you would propbably get 25 percent at most or about
250 lbs. You really do not get excited by the fact
that you are now spending a bit more fuel(with
slightly poorer eff.) - because now you can climb over
that ridge instead of circling back for more altitude.
As for variable gearing you could have had pretty much
the same effect by downshifting to get engine speed up
- but a certified "transmission" would probably weigh
and cost more than a "certified" variable prop. Vance
I think this is what Vance is trying to say.
The "Const Thrust" column is the thrust for an 80% eff. constant speed prop.
The engine is generating 100 HP and prop RPM is 2490 all the time.
The "Theory Thrust" column is for the thrust you would
get at that HP and RPM level with an 80% eff. prop. Not too useful.
Academic only. You can probably ignore this one.
The "From Prop" column is the thrust you are going to get with a fixed
pitch prop as prop eff. and HP increase with speed.
The "Geared Thrust" is the thrust you will get if you have an infinitely
variable transmission and you run the engine at 100 HP and
but the prop RPM is bogged down and the prop eff. is poor. This column
needs iteration so perhaps things are a bit better than this.
In other words the RPM numbers will increase and the prop eff. will
increase slightly.
However as you can see it is no where near as good as a constant speed prop.
At real low speeds the constant speed prop is almost ten times the thrust.
It is however roughly twice as good as a fixed pitch prop at low speeds.
Paul Lamar
The Aircraft Rotary Engine Newsletter. Powered by Linux.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org
DEFDBL A-Z
CLS
HPConstant = 550
HP = 100
OPEN "data4.txt" FOR OUTPUT AS #1
PRINT #1, " Const Const Actual Theory From Prop Geared"
PRINT #1, "MPH HP Thrust HP RPM Thrust Prop Eff.% Thrust"
FOR Vel = 10 TO 150 STEP 10
PRINT #1, USING "####"; Vel;
PRINT #1, " 100";
fps = Vel * (88 / 60)
ConstThr = (100 * HPConstant / fps) * .8
PRINT #1, USING "########"; ConstThr;
ActHP = 50 + (.333 * Vel)
PRINT #1, USING "########"; ActHP;
RPM = 1950 + (3.6 * Vel)
PRINT #1, USING "########"; RPM;
Thethrust = ((ActHP * 550) / fps) * .8
PRINT #1, USING "########"; Thethrust;
angledeg = .6 * Vel
anglerad = angledeg / 57.295
eff = SIN(anglerad)
realthrust = (Thethrust * eff) * .8
PRINT #1, USING "########"; realthrust;
eff2 = eff * 80
PRINT #1, USING "########"; eff2;
gearthrust = (100 * HPConstant / fps) * eff2 * .01
PRINT #1, USING "###########"; gearthrust
NEXT
endit:
CLOSE
SHELL "more < data4.txt"
SYSTEM
Const Const Actual Theory From Prop Geared
MPH HP Thrust HP RPM Thrust Prop Eff.% Thrust
10 100 3000 53 1986 1600 134 8 314
20 100 1500 57 2022 850 141 17 312
30 100 1000 60 2058 600 148 25 309
40 100 750 63 2094 475 155 33 305
50 100 600 67 2130 400 160 40 300
60 100 500 70 2166 350 165 47 294
70 100 429 73 2202 314 168 54 287
80 100 375 77 2238 287 171 59 279
90 100 333 80 2274 267 173 65 270
100 100 300 83 2310 250 173 69 260
110 100 273 87 2346 236 173 73 249
120 100 250 90 2382 225 171 76 238
130 100 231 93 2418 215 168 78 226
140 100 214 97 2454 207 165 80 213
150 100 200 100 2490 200 160 80 200