Tech Question - Prince Prop

[hevansrv7a]

I have a Prince P-Tip prop. Prince says it changes pitch in flight to provide up to three inches of change in the good direction to make the climb vs cruise compromise less of a compromise. I also have a GRT EIS/EFIS so I can get reasonably accurate numbers. I have calibrated the TAS with GPS as provided by GRT, but it does not correct IAS.

How, in simple terms, can I test this on my airplane? Is there a line or curve that RPM vs. IAS or TAS would follow if the prop were not self adjusting? Other useful concepts? Any help from the experts would be appreciated. I promise to report the results in this forum. Thanks in advance.

 

[Scott Will]

Howard... the GRT system does not have a correction for IAS. This is because if you have another airspeed indicator in your plane the two would read differently. I, too, have a similar problem. My indicated speeds are on the low side. The reason is because of errors in the static system. Originally it was about 10 kts and now its about 4 kts after I installed the Van's pop rivet head over my flush static port.

Best,

 

[hevansrv7a]

Howard... the GRT system does not have a correction for IAS. This is because if you have another airspeed indicator in your plane the two would read differently. I, too, have a similar problem. My indicated speeds are on the low side. The reason is because of errors in the static system. Originally it was about 10 kts and now its about 4 kts after I installed the Van's pop rivet head over my flush static port.

Best,

Hi, Scott, nice to hear from you. You are right about the static error, I think. I used the Van's kit and before calibration actual TAS was about 4 kts slower than the instrument was indicating. Now that we agree on that, what can you tell me about the relationship between RPM and TAS?

I looked at some data from a C-172 and played with it in Excel. From what I can tell, the knots per RPM goes up a little (about 8% from 2100 to 2700 rpm) as you go to higher rev's. It also seems to go down a little as the air gets thinner (about 4% from 2500 to 10,000). I haven't figured out any formula yet, though. My next step, while waiting for more answers from the forum will be to gather some data from my airplane.

 

[Scott Will]

Here's what I have so far... and I still have to repeat my tests from a week ago but here's a little chart I made at 8500'... RPM vs. TAS

% Power - RPM - MP - TAS
55 2340 18.8 150.4
65 2470 20.3 159.3
70 2590 21.4 164.8
75 2640 22.2 171.7

It's fairly linear. Was messing around with my operating costs, fixed and variable, and was doing a little math like we do at the airlines. With my current hangar/insurance/fuel/oil costs, my most economical speed to fly is right at 65% power.

 

[hevansrv7a]

Preliminary Data manipulation

Lacking anything beter, I computed for the C-172 and for Scott's -7A the relationship between RPM and TAS and then how it changes as the RPM advances. Three things stand out.

One - an RV goes about 50% faster for each RPM. No surprise there. C-172 from about 38 to about 42 versus about 64 to 65 for the RV (Knots per 100 RPM). But this is merely interesting and not answering the question.

Second, Scott's data is "flatter". At 5000 and 7500 feet the 172 Knots/Rev advances from 2300 to 2600 by 4% and 5% respectively while the RV has at most 1% from 2340 to 2640, altitude unstated. This is very interesting and highly suggestive that a FP prop yields a fairly constant relationship between RPM and speed at least in the narrow range we are interested in. While the C-172 and the RV vary a little, it's pretty flat. This is directly on point. Thanks, Scott!

Third, the change in percent power is different for the same RPM's. C-172 goes from 54% to 83% at 5000 and from 51% to 70% at 7500. the RV goes from 55% to 75%, altitude unstated. If this was around 5000, then the power change is flatter, too. Again, this is only interesting.

More to come when I get back in the air.

 

[elippse]

What you are looking for is the effective pitch of the prop. This is the distance, in inches, that the propeller travels forward per revolution. The formula is very simple: TAS in mph times 1056 divided by rpm! On my prop the variation of the effective pitch over a range of rpm and TAS is very little; it runs between 75.5" and 76.5" in cruise. That can be due to small variations in recorded rpm or TAS. It is very stiff and doesn't change pitch at all. However, you can't use this formula for climbing as the prop load is quite different.

 

[gmcjetpilot]

Hummmm I am such a skeptic

I have a Prince P-Tip prop. Prince says it changes pitch in flight to provide up to three inches of change in the good direction to make the climb vs cruise compromise less of a compromise.

How, in simple terms, can I test this on my airplane? Is there a line or curve that RPM vs. IAS or TAS would follow if the prop were not self adjusting?

The way I read your question, how do you know its "self adjusting? You want to know if you can record RPM, TAS and manifold pressure to see when the prop "adjusts" and the affect of the lower pitch on takeoff/climb, to higher pitch in cruise.

I'm not sure you can really definitively know if your prop's pitch changes, by how much or when, by just looking at RPM v TAS.

Comparing other RV's or other planes with different props is cool, but the drag bucket for different airframes may skew the data (slope of your curve). The change in shape or slope of your plot can be engine and airframe issues, not just pitch changing, which should be subtle and gradual.

To get "a line or curve that RPM vs. IAS or TAS would follow", I would swap in another fixed pitch prop or two (on the same plane), than look at the data comparatively. You will see the advantage of the flex? That is what its about. You want to see what if any affect the prop flex (pitch changing) has.

To me performance is what's important in my opinion. If you are happy, than that's all that matters. I wouldn't worry about when or if the pitch changes. Are you getting more RPM on take off, less RPM in cruise while getting greater climb and top speed compared to other props. That is key. Really a PROP comparative test with other props that don't flex would be interesting.


"Almost, Almost Constant Speed Prop"

The theory of how it the prop changes pitch is different from manufacture to manufacture. I have excerpts from Prince and Warnke write ups below.

This is what Prince says: http://princeaircraft.com/html/carbon_fiber_brochure.HTM

"Wood P-TIP Propeller: Before the "Carbon Fiber P-TIP Propeller", this had been considered one of the most efficient and lowest noise propellers available; the only wood/carbon fiber anti-vortex droop tipped propeller in the world. This propeller is a wood, fixed pitch propeller that relies on the forward pull of the propeller to automatically provide pitch change proportional to the amount of force being applied to the propeller disk."​

Warnke props also claims their prop is "Almost, Almost Constant Speed Propeller". This is what they say:

"When you fly behind a Warnke Propeller, you get earlier take-off, faster cruise at lower r.p.m. and lower temperatures in the engine department. Our smooth running, carbon-fiber wrapped propellers are engineered to deliver top-notch performance to utilize all the "umph" your engine has to offer. Unique characteristics such as almost, almost constant speed performance will enhance any aeroplanes flight attributes. Our special props work with your engine to provide a VERY small span of RPM between full power take-off and full power top-end. What you get out of that is more r.p.m.'s at take-off . . . where you need them. Then as the aircraft increases speed and centrifugal forces act on the propeller you get an increase in pitch . . .right when you need it . . . at cruise. Therefore, you get faster climb and higher cruise when compared with any other wooden or composite propellers and less weight than a constant speed propeller."​

OK sounds good, Prince says its the wood prop with scimitar shape and "disk load" or pull on the prop, changes the pitch. They imply the Carbon version does not flex but is thinner so it performs better. Warnke says its centrifugal force that does the pitch changing? Two different things. How do you prove it?

I see how a wood prop with highly swept, curved blades (like boomerangs) might twist on take off (increase pitch) and twist less in cruise. However I don't know if I'd want my prop bending too much. The Warnke story of centrifugal force changing pitch is harder to believe.

People are happy with their Prince prop performance. We need another PROP SHOOTOUT of fixed pitch props, to cut through the hype.

 

[elippse]

I've always wondered how a prop maker was able to get consistent compliance or twisting characteristics in the wood from prop to prop, let alone from one blade to another, to get each blade to twist exactly the same under power/torque/load/ centrifugal force, or whatever. If anyone knows the answer, I'd appreciate an explanation! Thanks!

 

[gmcjetpilot]

Art and magic

I've always wondered how a prop maker was able to get consistent compliance or twisting characteristics in the wood from prop to prop, let alone from one blade to another, to get each blade to twist exactly the same under power/torque/load/ centrifugal force, or whatever. If anyone knows the answer, I'd appreciate an explanation! Thanks!

Wood is a God made composite and it sure does vary, but the answer I think is the artistic ability of the prop maker in matching wood grain and blanks. If you know wood and have a selection of material, no doubt you can control properties. I suspect they take great care when picking out wood so its consistent from tip to tip, exploiting their natural properties. Most if not all props are laminates, thin vanier's glued or bonded together. I'm sure there is some manipulation when making blanks by laying up the wood layers strategically.

Wood based props are definitely not a stamp-it-out-widget thing. They're works of art. With man made composites material (fiberglass/carbon) you can vary the "lay-up" to tailor the stiffness in different directions (like wood quality and grain). From my above post, a wood prop covered in composite will be so stiff that there would be minimal flex. However with composites you can make the blade a little thinner and there fore more efficient. Bottom line the "flex" magic is a bit of a mystery and with out measuring the physical flex in flight than its a guess what they are doing.