Next Catto Prop - Page 3

RVTrumpet · #21 08-23-2016, 12:48 PM

Super, super cool prop Axel! Can't wait to see it in action.

RVbySDI · #22 08-23-2016, 01:11 PM

Quote:

Originally Posted by gammaxy

I think we are attempting to point out that the patterns in the oil (or dirt trails) do not show how the air is moving.

Think about watching rain streaks on your car windows. If you travel slow enough, gravity moves the rain drops straight down the window. As you pick up speed they get angled back, but you wouldn't assume the air flow is angled in the direction of the streaks until you are going reasonably fast.

Now consider that the oil on the propeller blade is subject to > ~4,000 G's outwards from the prop hub (rather than 1G for the water on your window). Also assume that the air flow (what we actually care about) over the propeller is more-or-less aligned with the chord of the propeller and interacts with the oil film through skin friction drag.

Any reasonable set of numbers I try has the centrifugal forces dominating by such an extreme amount (30-100x) that to draw any conclusion that the airflow doesn't travel approximately in the direction predicted by over a century of propeller theory would be premature from the presented evidence.

Here's the equation I derived for anyone who cares to criticize my math. It's probably impossible to calculate an exact answer, but showing that there's really no contest between the two forces is not difficult.

Centrifugal force magnitude at propeller tip relative to aerodynamic force =
4*(oil film thickness)*(density of oil)/((skin friction drag coefficient)*(density of air)*(diameter of propeller))

I estimated .0041 for the skin friction drag coefficient. There's a lot of small numbers in the denominator and only one small number in the numerator (oil film thickness).

Well, the way my practical mind sees this is that given the facts you describe does it still really matter? That is, does it really matter that the air is acting in that particular fashion BECAUSE there is a huge force (centrifugal) that is overpowering another force (natural air flow)? If that centrifugal force is there in the majority of times in which we wish to determine the effects of a particular blade design does it really matter that this huge centrifugal force is what is sending that air in that direction? No matter what is making the air behave in that way, we still want to design a prop blade to take advantage of that air movement. Do we not?

Kyle Boatright · #23 08-23-2016, 05:14 PM

Quote:

Originally Posted by RVbySDI

Well, the way my practical mind sees this is that given the facts you describe does it still really matter? That is, does it really matter that the air is acting in that particular fashion BECAUSE there is a huge force (centrifugal) that is overpowering another force (natural air flow)? If that centrifugal force is there in the majority of times in which we wish to determine the effects of a particular blade design does it really matter that this huge centrifugal force is what is sending that air in that direction? No matter what is making the air behave in that way, we still want to design a prop blade to take advantage of that air movement. Do we not?

I think the point is it is likely water is moving in that direction, not air.

RVbySDI · #24 08-24-2016, 08:12 AM

Quote:

Originally Posted by Kyle Boatright

I think the point is it is likely water is moving in that direction, not air.

Ok, makes sense, thanks Kyle.

Snowflake · #25 08-24-2016, 08:27 AM

Quote:

Originally Posted by RVbySDI

Forgive the question since I am not an aerodynamic engineer. Heck, I didn't even sleep in a Holiday Inn last night. But whether it is centripetal force or airflow that is moving the air, does that really matter in terms of HOW the prop blade is moving the air? Isn't the fact the air is moving in a particular manner the important issue? Not what is making it move in that manner. Please educate me if this is not true.

That's a valid question, and I agree this might not be intuitive. What's happening is that the air is flowing over the top of the propeller blade from the leading edge of the prop to the trailing edge of the prop. That's how the lift is generated on each blade that acts as thrust.

The oil, however, is much more viscous and is stuck to the prop. Due to centripetal force it is being thrown out towards the tip of each blade, as shown in the photo with the really oily prop. The oil is moving to the tip, but the airflow is definitely not flowing with the oil.

RVbySDI · #26 08-24-2016, 08:51 AM

Quote:

Originally Posted by Snowflake

That's a valid question, and I agree this might not be intuitive. What's happening is that the air is flowing over the top of the propeller blade from the leading edge of the prop to the trailing edge of the prop. That's how the lift is generated on each blade that acts as thrust.

The oil, however, is much more viscous and is stuck to the prop. Due to centripetal force it is being thrown out towards the tip of each blade, as shown in the photo with the really oily prop. The oil is moving to the tip, but the airflow is definitely not flowing with the oil.

Of course, makes perfect sense now. Thanks.

So then my next question: HOW would one ever be able to measure what exactly the air is doing over a prop with such a huge force obscuring any attempts to observe that flow? Is it now just a matter of entrusting the analysis to some group of physicists who have a string of mathematical equations to plug numbers into? Is there not a practical observational method that would account for this powerful obscuring force?

Ok, so that was more than just one question.

Snowflake · #27 08-24-2016, 11:37 AM

Quote:

Originally Posted by RVbySDI

HOW would one ever be able to measure what exactly the air is doing over a prop with such a huge force obscuring any attempts to observe that flow?

That, my friend, is a question they've been trying to answer for ages.

Near as I can tell, the only way is theoretical calculation. Any physical method of observation would involve something that had mass... Oil drops, water, even tufts have mass that would be "thrown" to the outside of the propeller disc.

I wonder if a smoke trail, streamed into the propellor, and photographed from a rotating camera mounted at the hub, would work... No contact with the blade, you might be able to observe the air flow pattern?

AX-O · #28 08-24-2016, 02:06 PM

The prop showed up today.

At this point, I am not sure I should post results since the reason for the design of the prop apparently is not valid.

RVbySDI · #29 08-24-2016, 02:13 PM

Quote:

Originally Posted by AX-O

The prop showed up today.

At this point, I am not sure I should post results since the reason for the design of the prop apparently is not valid.

Huh? Not sure I know what you mean by this statement.

nigelspeedy · #30 08-24-2016, 03:10 PM

G'day Axel,

I like the way you are continuously testing and adjusting your plane to get the optimum performance for your mission. I don't think there is any harm testing the prop and letting folks know the results. If it performs better great, if not try prop number 4 or 5. Its all knowledge, even if it only helps us not try it again. We could argue all day about the theory, why it will or wont work, but flight test is the answer in the back of the book.

Cheers

Nige