50cc planes on 23x8 props can't fly aerobatics!

[exciter900rr]

Ok... I'll bite-

I have no "real" math knowledge, but I look at things a bit different.....

The way I see it, the formula you are using must be a "generic" one. I would assume that for a given pitch, the formula assumes that it is:
a. fixed pitch along the blade
b. constant chord
c. symmetrical

I would also imagine that it does not allow for the different airfoils and pitch changes along the blade.

I am theorizing through my knowledge of flying model sailplanes, and designing a couple as well.... I know, that a particular airfoil, at a particular thichness, will seek out a particular speed that it will be efficient at. any less, and it will be inefficient. any more, and it will be overly efficient (create lift).
Since we are using an engine to operate this propellor at speeds far greater than the prop's "stall speed", And the propellors are flat bottomed, and even undercambered airfoils, we are actually creating lift. they are not just "plowing through the air" at a given speed. so the initial pitch of the 23/8 at rest or "as measured" would be far different at 7000 rpm. furthermore, it will "unload" slightly and rev more in the air at speed, so it will in fact produce even more "lift" in that instance... This might suggest that your 23/8 actually acts like a 23/12 in operation..

Once again, this is my THEORY, not to be taken as fact! I have no idea what formulas it would take to prove/disprove this, or how to do the problems if I knew what they were....
This is just what my brain tell me what is going on...

Might I be close to what is really going on?

[exeter_acres]

	Quote:
	50cc planes on 23x8 props can't fly aerobatics!

Please don't tell my QQ Yak... as it flies IMAC and 3D quite well...it must not know....

[thewrap]

I'm putting my first vess on in about a week. Vess 23b BABY!

[Mike Anderson]

	Quote: Originally Posted by exciter900rr
	Ok... I'll bite- I have no "real" math knowledge, but I look at things a bit different..... Might I be close to what is really going on?

I regret starting this thread now . . .

Pitch speed is a simple theoretical forward aircraft speed that is a function of propeller pitch and rotation speed only.

Blade airfoils, shape spanwise change in airfoil & angle of attack all contribute to the power it takes to turn it and the thrust that it develops. All the variables are captured in a single constant for power and another constant for thrust.

Here's the central issue: The theoretical speed to fly a typical IMAC plane straight & level is 60-65 mph. The theoretical max speed the 23x8 prop can fly at is 49 mph. So the plane can't fly straight & level unless it is trimmed with up-trim to do so. But it doesn't matter because IMAC planes are flying mostly vertical so thrust is king, not airspeed - it's flying on it's prop (not so much on its wing).

Time to move on to another thread . . . .

[RTK]

	Quote: Originally Posted by Flyfalcons
	I think your theoretical stall speed is way off.

That is what I was thinking, at 25mph my 50cc'er is still flying well

[Mike Anderson]

	Quote: Originally Posted by RTK
	That is what I was thinking, at 25mph my 50cc'er is still flying well

It could be , if your wing loading is lighter than what I used in my calc's or you are at high altitudes.
Check out this online (sea level) calculator: http://www.cloudchasers.org/page%20t...ll%20speed.htm

Interested in the math? Vstall occurs at maximum lift coefficient, which for a symmetrical airfoil is about 0.8.

Vstall = Square root of [ (Weight (oz) x 3519) / (ClMax x Wing Area (sq in) ] (at sea level)

[Craven]

r2d2piersquare4x8=32=bfr549co2h2oc3pokz1000x4nfbas 933/\[e=mc2]/o\k2zzzzzzzzzzzzzz

[robertp17]

	Quote: Originally Posted by thewrap
	I'm putting my first vess on in about a week. Vess 23b BABY!

What engine you putting it on. I have a 23B as well on my da 50 with the stock muffler and its turning 7100 all day long. Even after an 18 minute flight. The best I can tell thats what some are getting on a pipe so why is mine so high with the stock muffler? Needless to say it yanks my aeroworks 260 around no problem. Nice prop but the thing barks loud.

[RTK]

	Quote: Originally Posted by Mike Anderson
	It could be , if your wing loading is lighter than what I used in my calc's or you are at high altitudes. Check out this online (sea level) calculator: http://www.cloudchasers.org/page%20t...ll%20speed.htm Interested in the math? Vstall occurs at maximum lift coefficient, which for a symmetrical airfoil is about 0.8. Vstall = Square root of [ (Weight (oz) x 3519) / (ClMax x Wing Area (sq in) ] (at sea level)

Please do not tell my plane about this math, it might never fly the same again
According to the math, my plane should stall around 25mph, but it sure seems much slower to me.

[Mike Anderson]

	Quote: Originally Posted by RTK
	According to the math, my plane should stall around 25mph, but it sure seems much slower to me.

Especially when torque rolling, eh!

[ghostrider_113]

How can a sailboat go upwind? How can it sail faster than the wind speed? It has to do with the sail's angle of attack to the airflow. A propeller is not seeing static air when it is flying. The angle of attack of an airplane propeller changes with the airspeed of the aircraft. So the mathematical calculations are maybe good on the ground, but not while flying.

[tommy321]

Hey Mike, Getting back to your analysis...

It's interesting , and I'm trying to figure out where it falls apart (and we know it has to fall apart somewhere since so many of us fly our 50cc aircraft with 23x8 or therabouts propellers).

I can think of two main reasons why it might not be working out:

1. The propeller is probably unloading more than you expect in the air... i.e. the 7000RPM static becomes 8500RPM in the air, you now have a pitch speed of 64mph which is twice your stall speed.

2. These larger propellers are probably more efficient than the 10" propellers that these rules of thumb were based on. My first run-in with these rules was Moto-calc, and I think they came from past experience with smaller aircraft. I bet you that these larger propellers with their thicker airfoils do a better job than the smaller APC and Master Airscrew propellers at creating thrust from rotational energy.

I notice that full scale aircraft design doesn't use pitch speed, but uses advance ratio. Advance ratio is the ratio of forward velocity to the propeller tip velocity. The graph I found online for a Cessna 172 propeller showed it's maximum efficiency at roughly J=0.7 (or in otherwords the airplane is flying at 0.7 times the propeller tip speed). Your average Cessna cruises along just fine at 2300RPM and ~100mph. I'm not sure where to find "pitch" data for a C172 propeller though. (I noticed that the standard C172 prop is the McCauley 7557. The prop is 75" in diameter, I wonder if it has a 57" pitch? - if it is 57"*2300RPM=124mph which is barely twice the stall speed.).

I wonder if as we get to these larger models the 2.5-3 times pitch speed rule just stops working as a good rule of thumb.

Tom