# Prop tip speed in cruise vs. hp & torque. Is bigger better?

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#### TFF

##### Well-Known Member
What is going on is load to the engine. It can only produce so much power at a setting. Your two different diameter props will be pulling different amounts of pitch at the same engine load. One is swinging more meat so it can't use as much pitch. The smaller diameter can pull more pitch. The engine load being the same. What you don't have is a baseline to know witch way the prop needs to be.

#### djschwartz

##### Well-Known Member
We do not have speed data yet for our airplane as it is the first to fly with this engine / wing / prop combination. Should be flying by July.

I was wondering how much the engine performance would effect the speed. Lower hp and torque at the slower rpm.
Torque is irrelevant to such estimations.

If you assume all other factors, aircraft weight, drag coefficient, propellor efficiency, are unchanged, then speed varies as the cube root of power. That is a simple calculation. The problem is, of course, that when comparing two different power plants, assumptions like "propellor efficiency is the same" are not likely to be valid. And if there is a significant difference is power and/or propellor efficiency then the aircraft will likely be operating at different drag coefficients under the different flight conditions.

This is what engineering is all about. You have to actually go through a fair portion of the design cycle for both alternatives in order to have enough information on which to base an educated and reasonable comparison, especially when you are basing your overall design on a major component for which there is not a lot of practical data available.

Dave

#### Swampyankee

##### Well-Known Member
I got the impression that the OP does not yet know how fast his plane is going to fly with either prop ? Which means that any calculation of "true" tip speed is a matter of speculation.

My understanding of the question was whether a larger diameter prop turned at a lower RPM would be better than a smaller diameter prop turned at a higher RPM ?

To this question I believe the answer is : The bigger prop should always win - all else being equal. It should be quieter, more efficient and produce more thrust. One is usually limited by ground clearance.
As long as the tip speed does not get excessive, yes, greater diameter is better. A 250 mph aircraft is traveling at about 365 ft/sec; with a 750 ft/s tip speed, the resultant speed (it's just the Pythagorean theorem) is about 835 ft/s. This is great enough to worry about compressibility, so the prop designer should really be careful with airfoil selection. A propeller designed by an aerodynamicist who specializes in propellers with support from a good structures guy should be around 85% efficient (ignoring losses due to that airframe that exists solely to mount the engine that spins the propeller).

#### KeithO

##### Well-Known Member
I think that part of the complication of the decision for this OP is that the larger prop is designed to "look" like a prop that was used on considerably more powerful engines. Thus modern props tend to have narrow tips, whereas the prop he is referring to has wide tips.

OP, which prop is shown in the image below ?

#### TXFlyGuy

##### Well-Known Member
That is the 84" propeller. The new 96" (3/4 scale) is being developed for several reasons, one being cosmetics. The other reason is the larger prop will pair up with the higher power engines better. That 84" propeller was actually designed for the requirements of the 180 hp Suzuki engine.

Also note the wing in the photo. That is the old design. Our wing has a span that is 4' greater. Not only is it faster, but it actually will fly slower.

#### rv7charlie

##### Well-Known Member
There's a good article by Barnaby Wainfan in the Feb Kitplanes about prop momentum theory (what's really being discussed). FWIW, I suspect that the increase in diameter will make little difference in prop efficiency in the 200+ mph range, and it's not likely to be a positive change. Many other homebuilts in the same speed/HP range use props much smaller than 84" and seem to be quite efficient at those speeds. *IF* you were going from 3100 rpm to 3600 rpm (+50 HP), the actual speed gain, assuming props optimized for each rpm/hp and, lets say 200 mph for the lower HP, would be around 15 mph at the higher HP setting. (If I did the math right; ratio of speed increase is the cube root of the ratio of power increase.) There's no way that losing 50 HP will result in an increase in speed; that violates the laws of nature. Now, if you were talking about the other end of the speed spectrum, the 12" increase in prop diameter might well improve your climb rate, even with the loss of HP.

Charlie

#### TXFlyGuy

##### Well-Known Member
RV7Charlie - We are sure that climb will improve. The top speed (and cruise speed) will not change much, if any. The airfoil of the two props is similar, but not exactly the same.

#### Jan Carlsson

##### Well-Known Member
THE optimum diameter with 1630 RPM 210 HP and 225 MPH 4blade is 81"-84"
THE optimum diameter with 1890 RPM 260 HP and 241 MPH 4blade is 78"

A better RPM for the 96" prop would be 1310-1320 RPM

but it all depends, Pm me all data on the plane and what you want from it, and we will see.

Tip speeds must include the helix speed, as said it is the old Pytagoras guy. what did he know about propellers?

No reduction in performance have been seen until Mach 0.90-0.92
even if the airfoil degrade at .8-.85 or earlier depending on mostly thickness, some part inboard the tip need to be up in the critical tip speed, not just the tip that have no area.

But in this case the tip speed is not an issue, at 1310 rpm 96" and 240 MPH is 650+ ft sec
The highest tip speed isn't always the most efficient on a propeller, it depends on speed and power, along with the rpm

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