Background & Objectives

When planning my RV-8 I knew right from the start that I wanted a constant speed prop. Any aircraft with the speed envelope of an RV (50-230 mph) would be operating under a considerable compromise with a fixed pitch prop. Therefore I planned on installing the Hartzell compact hub constant speed prop that Van's offers it's builders right from the outset. My Hartzell has performed admirably and I really have no complaint with it, but... if you have an experimental aircraft that you've built yourself you know about that little voice in your head that keeps talking to you. "Gee, if I just made this change I could (substitute any of the following here: make it go faster, make it more efficient, make it more functional, make it look better, etc. ad nauseum)". That little voice has been talking to me about propellers. My engine/prop combination is probably the single most common RV power train there is, and I'm sure there will be many builders interested in what can be achieved with a prop switch, hence this section of the site.

My interest in upgrading my prop began by reading an article in the December 2000 issue of Custom Planes Magazine by Larry Olson titled "Constant-Speed Pitch Distribution". In this very interesting article Mr. Olson explains that even though a constant speed propeller changes its pitch to suit the condition, there is more that must be considered in assessing efficiency. Although the whole blade does indeed change pitch, the twist of the prop at any point on it's blade can only be optimized for one airspeed and rpm. Therefore inefficiencies are introduced under any other condition. To be sure, a constant speed prop is a quantum leap in efficiency over a fixed pitch model, but this twist profile (aka pitch distribution) concept made sense and got me curious. After a bit more reading I learned that the Hartzell HC-C2YK-1BF that Van's sells for the O-360 family, and that I have on my plane, is optimized for airframe speeds slower that the typical RV cruises at and therefore the whole blade is not providing optimum thrust at all stations under that condition. Hmm.

Also, while I have the standard 8.5-1 pistons in my O-360, I do have electronic ignition, Unison's LASAR system specifically. Hartzell's recent position on higher compression ratios and electronic ignition systems pointed out that with either higher compression or electronic ignition systems there would be a greater power pulse being generated that introduces additional resonance modes. In addition to observing the NO CONTINUOUS OPERATION FROM 2,000-2,250 RPM restriction, they explained that there was another vibration zone at or above 2,600 and that time spent in that range should be minimized while at high power settings if you have either high compression pistons or electronic ignition. Following is Hartzell's "position letter" on this issue obtained from Brad Huelsman at Hartzell Propeller in Ohio:

 

The Hartzell  propeller Model HC-C2YK-1BF/F7666A-2 has been vibrationally approved  per FAR23.907 on the standard  production Lycoming Engine Model O-360-A1A, and similar models, rated at 180 H  at 2700 RPM with a restriction to avoid continuous operation between 2000 and 2250 RPM.  The propeller vibration characteristics and stress amplitudes on a reciprocating engine installation are primarily mechanically generated by the engine.  Any modification to the standard engine configuration to include high compression pistons, electronic ignition, FADEC, tuned induction and exhaust, and turbocharging or turbonormalizing have the potential to adversely effect the propeller vibration characteristics and stress amplitudes.  Hartzell  Propeller, therefore, does not endorse any such engine modification unless the specific engine and  propeller configurations have been tested and found to be acceptable vibrationwise.

The Lightspeed electronic ignition is not certified for use on any aircraft engines so its use is limited to the experimental/amateur built market. Hartzell recently conducted a test with the propeller model HC-C2YK-1BF/F7666A-2 installed on a standard Lycoming O-360-A1A engine, except for a modification to equip it with the Lightspeed ignition in place of one magneto.  The results of this test show an increase in the propeller vibratory stress amplitudes within the 2000-2250 RPM range currently covered by the  operating restriction noted in the first paragraph, and additionally above 2600 RPM with high power settings.  Based on this data, continued safe use of this propeller on O-360-A1A and similar engines equipped with Lightspeed electronic ignition would require the following:

1. Continuation of the current restriction to avoid continuous operation between
2000 and 2250 RPM.
2.  An additional restriction to limit operations above 2600 RPM to takeoff.  As soon as practical after takeoff the RPM should be reduced to 2600 or below.
3. The propeller blades are life limited (with electronic ignition or higher compression) to 20,000 hours of operation.

Well, there are just some times when I want to go fast, and to do that it's balls-to-the-wall which means continuous operation at 2,700 rpm. While I will of course observe Hartzell's recommendation of this extra restriction, coping with this vibration/resonance problem can be added to my quest for an optimized pitch profile. Now as long as I'm motivated to deal with these issues, which means looking at alternative propellers, wouldn't it be nice to shed some weight as well? And so began my quest for prop alternatives. Before beginning however, lest I get distracted, I thought it would be a good idea to establish some clear objectives to be achieved with a possible prop swap...

• Increase performance:
  If pitch profile is optimized for the RV speed range, and with the latest thinking in blade aerodynamics, I would expect the following performance increases:
  • Increase of at least 5-10 mph at 75% cruise or above.
  • Increase of ~200 fpm climb at 130 mph below 8,000 ft.
  • Reduction of noise and perception of "smoother operation".
• Eliminate vibration concerns - Since composite materials tend to absorb vibration by their nature then virtually any composite prop should eliminate the resonance issue experienced with the Hartzell. While I don't have much need to spend time in the 2,000-2,250 range, I am more concerned about the newer advice against high power high prop rpm operation. The simple fact of the matter is that our engines put out more horsepower at 2,700 and when you want to get to your 10,500 cruise altitude at gross weight you want to keep it spinning to get there as quickly as possible. I don't want to have to pull the prop speed back, and thus the power produced, for fear of shortening my prop's life.
• Lower weight - Every RV builder wants the lightest plane possible to enhance it's flying qualities and maximize its carrying capacity. Most of the composite prop options would allow me to lower the empty weight of my plane by 10-30 lbs. This reduction would also shift the CG aft a bit which in my case would be just fine, I'm still forward of the aft CG limit with a 240 lb. pax and 40 lbs. of baggage in the aft baggage compartment. RV-8s are nose heavy when flown solo to the point that many examples cannot be trimmed to approach speed when solo. A lighter prop would go a long way toward providing better balance here.

Now that I'd established my objectives the search began. With these objectives the first decision I could make was to go with a composite prop instead of an aluminum alternative. After all, the vibration and resonance issues raised by Hartzell are largely due to the natural properties of aluminum which can resonate somewhat like a tuning fork. Any composite prop would be inherently more damp and therefore less susceptible to sympathetic resonances.

Initial research into the composite prop options indicated that at this time (fall 2002) there were four alternatives. They are from MT Propeller, Aero Composites, and Whirl Wind Propellers. Below is an overview of the three options. For comparison, the Hartzell is 55 lbs plus the Van's spinner assembly which adds another 4 lbs. Therefore all three options would be lighter than the Hartzell's system weight of 59 lbs. Since then Whirl Wind has developed another significant option, a prop designed specifically for RVs, the model 200RV. Click here to see a comparison table.

The first decision was to go with a two or three blade design. In doing my research I learned that there are inherent advantages and disadvantages to each. As part of my investigative process I exchanged e-mails and had phone conversations with each manufacturer. While at AirVenture 2002 I visited with the Aero Composites and MT Propeller folks at their booths, Whirl Wind did not display.

I was initially attracted to a 2-blade design for the simple intuitive reason that there is less blade area and therefore less drag. We have all heard that 3-blade props improve climb, it's a popular retrofit for Cessnas and Bonanzas, but since I was more interested in improved cruise speeds and not necessarily improved climb (RVs climb pretty darned well already!), this became my initial direction. As I learned more and more from various sources however it turns out there's a very small difference in efficiency with the 3rd blade, something on the order of .5% loss due to the increased area. There are also offsetting factors in favor of a 3-blade design. When I distill down what I've gleaned I get the following summary.

3-blade props:

Admittedly this is a fairly simplistic lay summary of some fairly sophisticated engineering and aerodynamic principals, nevertheless this is how it sorts out from my point of view. Deciding on a prop however is not as simple as just looking at the features and benefits, there are a lot of other important considerations such as the reputation and support of the manufacturer, weight, and cost. Every manufacturer uses different techniques which can alter the fundamental balance of characteristics. For example you can see from the table that the Whirl Wind 150 is the lightest prop of the group yet it is also a 3-blade design. The Hartzell is a 2-blade with less drag than a 3-blade, yet it will likely not be the fastest on an RV because of the pitch profile. So, many factors must be looked at simultaneously in selecting the right option for your application.

Assessing the manufacturer is an important consideration as well. As important as a prop is, I want the manufacturer to be credible and to be there down the road should my prop need service or parts. From this standpoint clearly MT comes out on top with a long history of high-performance props and certified designs. Clearly their track record is best, they have the most complete testing (required for certification), and an established network of service centers. Whirl Wind and Aero Composites are newer companies but with significant technical expertise in each case. I believe Whirl Wind has been around a few years longer and does have more props in the field. I was impressed with the fact that the 150 design has been in "test period" for a full two years before being released for full production just a two months ago. They sent eight props out for installation on various aircraft, including several RVs, and had the pilots return the props periodically for inspection. The 150 was put into production and offered for sale only after this period was completed. They report there were a few relatively minor changes (none to the blade or hub) made to the design as a result of the test period data.

In doing my research I learned some interesting things from all the vendors, but especially from Whirl Wind: Whirl Wind blades are dynamically balanced, not just matched by blade weight. The reason for this is that the moment of any imbalance is more important than the outright amount. Once the blades are dynamically balanced the prop is assembled, then statically balanced. Builders should still have their props dynamically balanced in the traditional method, but the measures taken by Whirl Wind ensure that the prop won't contribute to any imbalance, rather it will be in the engine assembly.

So, which one to choose? All three of these props have pitch profiles suitable for the RV so that's not a point of differentiation, and each manufacturer has certain characteristics that are appealing. In the end though it was the light weight, the fully finished spinner assembly, reasonable lead time, apparent quality of both the design and construction, positive reports from their beta pilots, the candor of Jim and Patti Rust (proprietors), and the overall value that caused me to pick Whirl Wind.

With my initial decision made and my order in it was time to collect some baseline performance data and think about testing methodology. Testing is important because if I can't make an accurate comparison of any performance differences then I'll never know if switching props was worth it.

Installation & testing

12/29/02 saw a break in the weather here in the Northwest so out we went to collect the "before" data on the Hartzell. I chose to do the testing near gross weight because A) it would allow me to take a passenger to assist with data collection, and B) most performance figures are shown at gross weight in order to be conservative and show worst case. Frequent backseater Randy Griffin was drafted for the task and we planned for the test to be close to my plane's 1,800 lbs gross weight. Fuel was noted for each test and gross weight at that time computed. Contents of aircraft (people, equipment) were weighed post-flight to verify weights. In all I think this is fairly sound data.

Test equipment:

Prop Tach 3 optical tachometer, guaranteed to be accurate within 1 rpm.

Radio Shack Sound Level Meter with several scales as well as A and C weighting.

Heuer analog stopwatch.

Tanner Racing digital race car scales guaranteed accurate to within 1 lb.

Scope creep
As with many projects, the scope of this one has crept. Instead of simply installing and testing the Whirl Wind 150 against the Hartzell, I have entered into an agreement with Whirl Wind to expand the scope of the test to include a two-blade model as well — a non-counterweighted version of their model 200C would also be included in the test. Other new models under development could be included as well, which turned out to be the case with their new 200RV.

Hanging the prop was made quite a bit easier by the light weight. Picking up this prop compared the Hartzell is truly a shock. The spinner parts including the backing plates and fill plates behind the blade roots, are very high quality pieces. The precision of both these fiberglass parts and the the hub assembly is truly impressive. There is a front bulkhead in the spinner that engages the front flange on the hub -- this can only work with very precise manufacturing.

Installation & testing of Whirl Wind 200C
For background on the performance of the 200C, there was an article comparing the 200C to several other aerobatic props run in the June 2001 issue of Sport Aerobatics magazine that you might find interesting. Download it (Word doc) by right clicking here.

The 200C was installed on 3/16/03. On 3/24/03 the weather broke and we were able to launch our prop testing mission. The testing went well and we feel like we have solid comparison data depicted in the tables below.

Test results

Testing was performed for five parameters: weight, climb performance, max. cruise, top speed, and noise. Test methodology is detailed in each section.

WEIGHT
Test: weigh each propeller assembly complete with spinner using digital race car scale.
Notes: this might seem an obvious test, but I wanted to verify the exact difference in weight and not rely on manufacturer's claims. Reductions are all relative to Hartzell baseline.

Since weight is an important parameter I wanted to get accurate data here too. I calibrated the scales twice and also measured other known weight items and it all crosschecked accurately. Stainless steel spinner screws weigh nearly a pound (laying under the prop in the pic) so if you ever weight a prop don't forget them. See the Interpretation and Conclusion comments at the end for comments on the impact of the weight and CG change.

CLIMB
Test: time climb 2,000' to 5,000'
Conditions: W.O.T., full rich mixture, 2,700 rpm, maintain 120 mph (Vy)
Notes: RPM measured with optical prop tach, time measured with stopwatch, ROC then computed from time data. This is the most difficult test to fly correctly which is why I thought more runs would yield a more reliable result. In retrospect I wish I had taken more runs with both the Hartzell and the WW 150.

This is the most difficult test to fly correctly, airspeed control is critical to the result and difficult to do. This is why I thought more runs would yield a more reliable result. In retrospect I wish I had taken more runs with both the Hartzell and the WW 150.

TOP SPEED
Test: note IAS and TAS
Conditions: W.O.T., 1,000' msl 
Notes: RPM measured with optical prop tach. Results consistent with previous top speed measurement of 217 mph TAS taken on 65° day -- lower OAT will yield higher power.