Improving Performance By Modifying PROP TIPS by Dick Hess, EAA 93406 135221st Ave., NE

Redmond, WA 98053 Anyone who has been in EAA for a few years has seen a lot of different designs for wing tips. They are bent up,

bend down, swept back, carved out and twisted. Some are purported to lend tre-

mendous performance gains to their aircraft. My own airplane, a VariEze, has wing tips after this fashion developed by Richard Whitcomb of NASA. In my never-ending quest to improve the performance of my bird, it seemed only natural to extend the concept to my propeller. Anything that improves the performance of a wing will also work on a propeller... O.K., so I'll stop short of adding flaps! Hartzell has been building their Q-Tip props for some years now, quite successfully, which has always intrigued me. Nonetheless, I wanted to start off with something simpler. I ordered a 58x62 propeller made of yellow birch. But when it arrived it was such a beautiful piece of woodwork with that glossy varnish and white painted tips, I had no

40 APRIL 1987

heart for carving it up. I flew the airplane for about 120 hours and thought no more about it.

Then came the fateful day when, while preflighting the airplane, I found

damage the likes of which I had never seen before. My first impression was that someone had clamped the leading edge in a pair of pliers about 1/2 inch from the tip. The indents were straight and parallel and about .03 in. deep on both sides, complete with what appeared to be a machined indent on one side, similar to the grip in a pair of pliers. Further inspection revealed, however, that the indents wrapped around the leading edge with uniform depth, which ruled out the pliers theory. The only theory left is too far fetched to relate. To avoid dwelling on the unfortunate, I look ahead to the future. Repairs called for stripping the fiberglass covering, filling the indents in the wood, refiberglass, repaint and rebalance. As long as I had to do all that I saw my opportunity to experiment with the tips. The original tips had been sawed off squarely in both the planform and frontal views (see Figure 1) , which is one

Figure 1

of the most inefficient tip designs flying, albeit easy to construct. I like simplicity and wanted to avoid adding mass to the blades that might fly off, so I decided to remove material instead. The problem in this direction was that my prop was already pitched to allow a maximum speed of 2800 rpm which is above the red line of 2750 rpm for my Continental 0-200. I was concerned that anything reducing the span or reference area of the blades would allow even higher rpm and also reduce the power available at 2750 rpm. Therefore, I determined to leave the planform essentially unchanged. This left for consideration a family of designs in which the underside of the tip is carved out, similar to the Bonanza wing or the new Cessna 210 wing tip. Now we come to the methodology for shaping the propeller. Engineering professionals everywhere will gag at this, but here is what I did: I decided to "eyeball" it. The airfoil used on the propeller is a flat-bottomed version of the Clark-Y. I held my rotary grinder at a 45 degree angle to the plane of the blades lower surface (in the frontal view, see Figure 2) and removed material up to within 1/32 in. of the upper surface of the tip airfoil from leading edge to trailing edge. This left the planform unchanged. It also left what resembled a shark's tooth at the leading edge. To avoid being labeled a Neanderthal by my associates, I smoothed it to a more aerodynamically pleasing shape, accepting a slight loss of planform area and a smaller leading edge radius near the tip. I then made the cut surface slightly concave to help direct the airflow in the outboard direction. At this point (Figure 3), it was starting to look pretty sexy. Every good aerodynamicist knows that means it is time to leave well

Figure 2

UJ

worker. He had built a simple device on

his lathe that allowed him to balance his propeller in both axes quite accurately (see

Figure 4). It consists of a solid

Figure 5

3000

• NORMAL TAKEOFF AT SEA LEVEL

MAX RPM

2500

NEW TIPS 25 RPM 2000 100 VCAS - MPH

Figure 6

2000

• 2000-3000 FT. • FULL THROTTLE • 980 LBS. GROSS WEIGHT

CLIMB RATE FT/MIN

Figure 3

enough alone, so I sanded the edges

smooth and proceeded to the finishing

stage.

Painting was a breeze, not unlike the kind that blows over the Eastern seaboard every hurricane season. The problem boiled down to my unknowing insistence on spraying acrylic lacquer over enamel which results in a rough, moon-like surface. I finally stripped the entire propeller down to bare wood and fiberglass, then varnished the wood and put a thin coat of lacquer on the

fiberglass-coated blade tips.

The balancing is an important stage. My initial attempt involved fitting a tube through the hub. I achieved a tight, concentric fit (I think) by wrapping masking

tape around the tube to make up the

difference in diameter. I then balanced

the ends of the tube on two leveled carpenter's rulers. I added paint to the propeller tips until it was apparently ba-

lanced with the prop in both the 12 o'clock and 3 o'clock positions. A quick flight test showed that while the balance was reasonable, the vibration levels left something to be desired. The purpose of homebuilding, they tell us, is education, and this was the right time to start asking questions. The answers came from Ingvar Svensson, a local Mustang II builder and co-

aluminum hub 2-1/2 inches in diameter with a flange on one end and a 5/16 inch threaded hole through the axis of rotation. The bolt inserted in this hole has a hemispherical cup ground out of the bottom end such that the propeller and hub assembly may be balanced on this bolt by placing it on a pin head, the pin being a 1/8 inch steel rod with a rounded tip that is clamped vertically in a vise. Adjusting the bolt in or out allows coarse or fine balancing, based on the height of the balancing point above the center of gravity of the assembly. With this set-up I could immediately see that my prop was out of balance in both axes, and repairs were rapid. Subsequent flights have felt smoother than when the airplane was new. I can't point to any accelerometer data, but I can safely say that it is easier to read the fine print on a sectional chart! I have performed limited flight testing to compare the modified propeller with the baseline performance. I recorded engine speed versus airspeed during a normal take-off, and cruise speed versus engine speed in level flight. Full throttle climb test were performed at the same airplane gross weight and density altitude. For these, the engine was leaned to maximum rpm then set up in

a full power climb at a constant

airspeed. Once stabilized, I used a stopwatch to time the climb from 2000 to 3000 feet. The results are as follows. The static rpm dropped from 2175 to 2150 (as closely as I can read it from a 2 inch gauge, see Figure 5). This indicates increased drag from the tips at higher angles of attack. Whether this is pressure drag from separation or induced drag due to increased static thrust cannot be determined on the basis of this data alone. The climb data is more revealing though. The average of the two climb tests indicates an increase in climb rate of 150 feet per mi-

200

NEW TIPS

1000

100 VCAS-MPH

SO

150

nute (see Figure 6). Using the approximate equation: dP = (climb rate increase x gross weight)/550 Also, dP = (thrust increase x airspeed)/550 where dP = increase in horsepower output

and adding the appropriate scale factors, a climb rate increase of 150 fpm

at a gross weight of 970 Ibs. at an airspeed of 88 mph yields a thrust increase of 19 lbs., or a power increase of 4.4 horsepower. A double check on these numbers

can be made by means of the cruise

speed chart (Figure 7) which showed speed increases of 3 mph at lower engine speeds and 2 mph near the top

end for a given rpm. I used the equa-

tions

p = BHP x RPM/2750 x Np

Also, P = (drag x speed)/550 - (Cd x 0.5 x p x V x S)/550

where P BHP 2750 Np Cd p V S

= = = = = = = =

horsepower output rated brake horsepower red line RPM propeller efficiency factor airplane drag coefficient air density in slug/ft true airspeed in ft/sec. wing reference area in ft SPORT AVIATION 41

which is a simple variation on the former equation, assuming a linear relationship between engine speed and power output. All values in the equation are known except Np and Cd. By assuming Np = 85% for the original prop, Cd is estimated at .06873 for the test condition at 2200 rpm and 139 mph. Since Cd did not change by modifying the propeller, we can use this value in the equation with the airspeed obtained with the new prop, which yields Np = 90.6%, or a power output increase of 4.48 horsepower, which is reasonably close to the 4.40 obtained from the climb data. Repeating this calculation for the test condition at 2600 rpm yields an increase in Np of only 3% which is expected due to the reduced loading of the blade tips at higher airplane speeds. The theory behind all of this extra performance is that the shape of the tips forces the core of the trailing vortex slightly outboard, effectively increasing the blade reference area by increasing the span. The extra area increases the thrust while the extra span helps keep the induced drag low. Notice that the results are different

200

VT AS MPH 3 MPH

100 2000

2500

RPM

from merely increasing the blade pitch, i.e., substituting a "cruise" prop. A cruise prop puts out more thrust at a given rpm which causes the airplane to fly faster at that rpm, which would give results similar to that shown in Figures 5 and 7. But a cruise prop also takes more power to turn at any rpm because of induced drag, which limits the power available for climbing and top end speed. My airplane not only climbs faster, but the maximum level speed and

rpm have also increased from 190 mph true at 2825 rpm to 196 mph true at 2900 rpm as shown in Figure 7. Once again, the reason this experiment was so successful on my propeller was that it was so inefficient to begin with. A prop with rounded tips in the planform view would probably not benefit much by any such modification, unless the blades are already pitched too much for the application. But if your prop is sawed off squarely like mine was, you may wish to experiment with it. Feel free to write to me if I can be of any assistance. Good luck!

About the Author Dick Hess has been a practicing aerospace engineer for 10 years, with experience in aerodynamics, flight testing and automatic flight controls and, as he has mentioned, he has built and flown a VariEze. He is a member of EAA Chapter 26 and of the Flight Research Institute.

EAA Membership Honor Roll This month we continue our recognition of persons who have qualified for the EAA Membership Honor Roll. When you receive your new or renewal EAA Membership Card, the reverse side of the attached form will contain an application with which you can sign up a new member. Fill in your new member's name, enclose a check or money order and return to EAA Headquarters and you will be recognized on this page in SPORT AVIATION — and there is no limit to how many times you may be so honored here. Introduce your friends to the wonderful world of EAA . . . and be recognized for your effort. The following list contains names received through the months of February 10. CHARLES ROBERT ROBBINS

EARL J. SKELTON

Sheboygan, Wl

Lake Worth, FL

RICHARD J. PARELLA

LARRY TALOVICH

DAVID C. GROFF

JEFF AKERS

NEIL BIDDERS

JAMES A NICKOLOFF

Yakima, WA

Monroe, LA

CHARLES A. SMITH

DAVID A. MIHALIC

RAY BENTLEY

Vacaville, CA

Scott Depot, WV

Modesto, CA

Apple Valley, CA ROBERT F. MENARY

Southborough, MA

Gatlinburg, TX

Wimberley, TX

Northbrook, IL

CHARLES MORGAN

SIDNEY G. CLARK St. Louis, MO

TOM STEELE

WILLIAM NIEDHARDT

RONALD G. SCHROEDER

Middletown, OH

Madison, Wl

Menominee Falls, Wl

JOE G. MYERS

MILTON A. SELF

Hamilton, OH

Torrance, CA

Rio Rancho, NM

Beaumont, TX

MARK W. KUNKEL

KEITH JOHNSON

Pullman, WA

WILLIAM CHARLES ERRAIR

San Francisco, CA

BOB CREITZ Tulsa, OK

Tavares, FL

JOSEPH C. GUGEL, JR.

JON ROADFELDT

THOMAS B. MCGAHEY, JR.

Nutley, NJ

Roseville, MN

Lakeland, FL

JERRY DALE POCKRUS Justin, TX

JOE HASS

WILLIAM BRACKETT

Grass Valley, CA

Athens, OH

ROBERT L. SHELLMAN

JAMES A. MADEWELL

EDWARD J. STASIEWICZ

JAMES E. CASH

Mequon, Wl

TORE NOREBERG

Wichita, KS

EDWARD O. LESKE

ROBERT M. GARDNER

DAVID E. HENSHAW

Nebraska City, NE

Windsor, Ont., Canada

JERRY J. MULDER

Red Deer, Albt., Canada

Chicago, IL

JAMES GEORGE

WILLIAM C. STAVANA

WILLIAM E. MICHAEL Winchester, IN

Middletown, CT

STEVEN L. SCHMITT

STEWART D. PARTINGTON

Princeton, IL

Kanata, Ont., Canada

DOUG FISHER Holton, KS

ROSANNE AMBLER

ED ALDERFER

Detroit, Ml

Vineland, NJ

WILLIAM T. VOGEL

Detroit, Ml

42 APRIL 1987

Cortland, OH

Long Beach, NC

Falun, Sweden

EUGENE P. BRADY

Melbourne, FL

HARRY G. HIGGINS

Tallahassee, FL

Sidney, OH

Mayfield, KY

BRIAN P. GORMAN FRANCIS A. ULIANO RODNEY K. HOCTOR Ft. Worth, TX FRANK LOEB

Spring Valley, NY