Prop Modification
 

I know this is a sacred cow but I'm going to ask anyway. I would like to get my hands on some first hand information from the dark side. I have a copy of Jeppesen's book "Aircraft Propellers and Controls" but I would like to delve into some historical information about experimental modification for speed enhancement. For example some years ago the Rare Bear team put a prop on their plane that was made from a mulit-engined plane's propeller (and Electra or a Neptune or something like that). This is such an obvious performance related item that it is impossible that this hasn't already been done many times - sometimes with disasterous results I'm sure. I will seek out a copy of Jack Norris' book recommended by John Huft but I would like to get into individual experimentation details as well.

Does anyone have such personal information or reference documets, etc.?

Bob Axsom

Talk to the experts
 

Bob,

You might call somebody who designs props professionally for the best info. They might give you some very good references, or even better a dose of their own experience. Hartzell's engineers were kind to me when I went looking for data.

The only hard prop data I have is for the P-51's propeller (paddle blade and the tapered blade), and I have full data sets for the Hartzell F7666A blade used on so many RV's, along with the map interpolation routine.

And, as you mentioned, this is a very touchy area. Lots of lessons learned the really hard way. Be very careful.

Oh yea I'll take the F7666 info
 

Hi Bill, I love a copy of the routine to map & interpolation. I have some of some of the data already (Hartzell sent me). If you could email me (gmcjetpilot@yahoo.com) the routine, I'd appreciate it. I made a spread sheet to manipulate the data, but its clumsy. Thanks, if you'd be so kind send me what data you have as well. It's fun playing with the numbers. Clearly turning slower is more efficient.


PS: Bob there is no prop mod that will harvest more speed than planting a new cowl with round inlets and pressure plenum. :D A new prop will never gain 6 mph.

Seriously there are rarely major breakthroughs in prop efficiency, just small gains measured in fractions of a percent. The best off the shelf prop is the 72" BA prop. Those Hartzell guys are pretty smart and have optimized the 7496 and 7497 blades for us RV'ers. The 7496 might be a tad faster but the 7497 has less or no limitations. The BA props are 3.5 mph faster than the F7666 which is amazing. Remember I said no prop would give 6 mph, but they came darn close. That is really outstanding and it came from optimizing for an exact airframe and engine and mission. They basically found more thrust! (see table below) 1.7% increase in speed means 5.6% more HP or equivalent thrust. At 8000' you are 75% power, so 180 hp is 135 hp. Your prop is only 80% efficient so you really only have 108 hp of thrust. The Holy Grail is to get that 20%. Well a BA Hartzell on a RV, must be around 85% efficiency. In theory you can get up to 90% efficiency; I doubt its really practical. 100% will never happen.

If there is ONE THING I want to communicate is a PROP is not designed in a vacuum, the airframe and engine are intimately connected with the proper design of the prop. One size fits all does not hack it. That is why the Hartzell BA prop might as well have been called the RV prop. Any one that knows a little about props would be impressed that the BA prop took such a jump over the previous HC-C2YK/F7666, which is no slouch either. The F7666 blade is way faster than almost ever other fancy prop out there. The WW200RV prop is only 1.5 mph faster than the F7666 bladed Hartzell (but 2 mph slower than the BA prop). Not bad for a prop that was originally designed 30 years ago for Mooney's. The prop must match the drag of the airplane, the HP the engine makes and the altitudes it will fly at. It's pretty darn complicated because there are so many interrelated factors. Prop efficiency varies with changes in airspeed, altitude, rpm and hp to name a few factors. (table below)

As you know thin blades are better at high speed since the props are going near supersonic, at least near the tips. Of course thin metal blades and harmonics don't mix. The new BA 7496 blade was revised to the 7497, which is a little thicker, and therefore has less limitations.

One thing that bothers me about all GA props, fixed or c/s, lousy roots, which are needed for structural and practical reasons. Look at a P51, its all airfoil right up to a huge spinner (pic below). Most props near the root have all the aerodynamic qualities of a baseball bat or two-by-four. To get around the blunt part of the prop would require a new cowl, spinner. (Cafe Foundation tried it on a Mooney (pic below). It looks weird.)

By the way the blunt part of the prop blade near the spinner is why rectangular cowl inlets have issues. The prop is just beating the air to death near the cowl's inlets adjacent to the spinner.

Bob there are practical limits to dia and RPM. With a direct drive we are stuck with higher 2500-2700 rpms during a race or anytime you want to go fast. If you could gear-down the prop RPM and put bigger paddles (wider chord or more dia) you can increase efficiency. Ground clearance gets in there at some point.

In general the faster you turn the prop the less efficient it is. The trade-off is between more RPM for more HP and less prop efficiency. In general higher RPM means more thrust and more speed. Dave Anders found that 2,900 rpm works best. In general you make 2-5 HP more for every 100 RPM, depending on altitude. This means adjusting the prop gov. The Con is you're exceeding Lyc red line. The other Con could be high RPM harmonic limits with electronic ignition; the higher you go over red line the worse it is. Still many race guys run way over 2,700 rpm. Will the prop fly off? Probably not for a 30-40 minute race. That's your choice. By the way some Hartzell's are rated to an ultimate of 2,900 rpm! Still a harmonic limit is a different story. Probably wise to not mess with it. May be a composite prop would be better if you want to spin at 2,900 rpm? However composites tend to be thicker and less efficient. Its all trade offs.

prop efficiency
 

"A new prop will never gain 6 mph."

Going from 200 mph to 206 mph would mean a 3% speed increase which would require 9.2% more efficiency. If you went from an 82% effficient prop to a 90% efficient prop, that would give it to you.
When Tom Aberle changed from his 2003 two-blade, with which he qualified at 221 mph at Reno, to his new three-blade, he set a new record of 241 mph. And that was at 250 less rpm! That was an efficiency increase of 40%! Then in 2006 he set a new record in the Gold at 252 mph with his new four-blade prop. That was an efficiency increase of 48% over the two-blade! With the new design concept used for these props, Jack Norris, in his new book on propeller design, says that now the upper limit on prop efficiency is 95%. The two props flown by Tom Aberle were running at least 90%! Not only do they convert horsepower to thrust more efficiently, they are also much more quiet!

Props and cowls
 

George gets close to an issue that I believe is key - the interaction of dead air in front of the cowl and the prop.

If pressure recovery happens in front of the cowl as is the case with either oversized round or rectangular openings, there is an area of dead air in front of each opening. Each blade will surely stall twice per rotation as it passes through this area, then struggle to reattach flow before whacking dead air again. There is a current trend towards bring cowl opening even closer to the blades which makes it worse.

If the cooling system is designed for internal pressure recovery as described by Chris Zavatson on his Lancair, no such area of stagnation occurs in front of the cowl openings.

The formula guys have it figured out. Extend the prop out as far as possible to eliminate interference with the airframe, long diffusers for pressure recovery, and a prop like Paul's that is effective right down to the spinner. I'll bet there is more than 10 knots in such a mod.

It was a P-3 Orion prop, and the sound it made with those massive stubby blades at full race power was truly awesome. It would certainly put a quiver in your liver, you felt it more than heard it. No other racer on the course has had (in my 12 years Reno experience anyway) such a distinctive acoustic signature.

Elippse, please check your private messages, thanks

Let's just explain the calcs
 

Slow down. :D. You're throwing numbers around left and right. 40% increase? Well I'm not buying. From the 221 and 241 mph numbers that would represent in about 29% more efficiency. That is not likely either. The speed I believe came for more than a prop change alone.

What props are you talking about anyway?

I seriously believe he (had to) also add power or lower airframe drag. Even if your Reno friend was running 90% prop efficiency to gain all 20 mph, that means his old prop was 60% efficiency, assuming the prop made all the difference. Seriously with respect, the numbers don't fly. May be he flew BETTER that year? What where the temps? Winds? Any other airframe or engine mods? (likely if he's a racer) Your data is just not enough to draw conclusion.

Three blades tend to lose speed from two blades in lower HP aircraft (like under 300hp). The only ones who say three blades are faster on little piston planes are salesmen of three blade props. You don't even say what prop, plane and engine we are talking about? :o

The only way to tell the efficiency between two and three blades is side-by-side real world flight test. All the test I have seen for RV's, says three blade is a disadvantage, but you are right they're more quite than two blades. Even the theory says three blades is slower.

No offense, I don't believe the data or you're missing some data. Let me show you my calcs.


My Calcs for RV Hartzell BA 7496 *verses* Hartzell F7666
The speed increase was actually 205.4 to 208.9 or 3.5 mph. That is 208.9/205.4 = 1.017, lets call it 1.7%. Because speed increases with cube root of HP......... 208.9 = 205.4 * ( BA-HP/HC-HP)^0.33. HP was about 75% on 180 HP, I assume the HP or SHAFT HP to the HC-HP (F7666 blade) prop was 135 HP. Solve for BA-HP = 1.052 * 135 hp = 142 hp. So its like finding 7 hp due to the prop being more efficient. All this is a ball park est. The fact is it went 3.5 mph faster and I think that is about or like adding 7 HP. That is FANTASTIC and realistic and believable. 20 MPH gains from props alone is not any of those things.

I have Hartzell factory supplied (*theoretical*) data for the HC-C2YK/F7666-2 and -4. I don't have time to calculate the F7666's efficiency for Vans flight condition, but its in the 78% or better range. The BA prop is about 5% more efficient, meaning it turns more of the SHAFT HP to THRUST (HP). (*Theoretical because it does not include affects of airframe, but the data is from Hartzell engineering.)

So the thrust for the F7666 was 135 * .78 = 105.3 hp. The thrust for the BA (7496) is 135 * X = 112.3 >> X = 0.83 or BA prop is 83% (efficient for this ONE CONDITION). Efficiency can be higher or lower depending on conditions: (HP, rpm, airspeed, altitude).

What am I missing?


As far as 90% efficiency? Really, I find that incredible and hard to image, but it's theoretically possible. If there are props running at 90% on a RV, I would LOVE to see it. DO you have pictures? It's just hard for a prop at 2,700 rpm and +200 mph to be super efficient. On many planes the prop turns 1900 rpm, of course with a gear box, which is another thread.

The cowl affects efficiency, so unless you change the installation you may never get to 90% on some planes. I can't emphasize enough that installation (and airframe drag and engine) affects the prop. Look at the pics I posted. You need something aerodynamic behind (or far away) from the blades. That is the biggest point I'm trying to make, the airframe affects the prop and 90% is hard to get in the real world. The installation of ANY prop on a RV will cost efficiency apart from the props specifications alone. They interact. Also high speed (racing) at high RPM tends to be at lower prop efficiencies. The second point is controlled test to isolate and compare prop performance, since we often are talking a few MPH difference.

Most good RV props, like a Hartzell BA peak in the 85% range, and that peak will not be at top speed flight. Fast does not mean efficient. Controlled test as Van's Aircraft did cut the myth & folklore. There are no magic props, just the best compromise for that airframe, installation and engine. For a RV the Hartzell BA or Sensenich fixed props are about as good as you can get.

Jack Norris kept saying that you want a smaller area at the tip not a blunt ended prop.

He said the WW RV200 was the smartest design out there and that they really got it right. He kept talking about Mach at the tip and how the Hartzell people ALMOST got it right with the Blended prop, note the smaller tip and then he went on to explain that Hartzell was compensating for somethin'

He had picts of the radical Elp.. Sp? from Reno but those are only made for fixed pitch. IIRC Jack Norris sounded like he knows Aberle.

Jack also mentioned Rare Bear and IIRC said something about, the plane makes so much power that the prop was what they could get???

Jack also says that there are three pages in his book that sum it all up but he would love to sell you his book.

Maybe I'll attend the bakersfield banquet where Jack Norris is to speak at in Jan.

that's what I heard FWIW

somebody post some picts of these props we're talking about. especially the Elip from the Reno racer.