What Size Propellers?

YOUR AIRPLANE IS just about finished and you do not yet have a propeller for it. Joe down the street has a good metal prop but the tips are curled from a taxiing accident. Joe doesn't know what aircraft it came from but he realizes he won't need one himself for a couple of years, so he will let you have it pretty cheap. The offer is tempting for you have just found out what the prices are for new metal props. The money you have set aside may not be enough for a new propeller. Even if it were, can you afford to pay a big price for a new propeller, which may not be exactly right for your bird? What to do?

Don't think that you are alone in this particular type of dilemma. I would estimate that a good 200/0 of the airplanes at Oshkosh were equipped with a propeller, which obviously was a poor match for the engine and airplane. How does a guy select the right propeller for his airplane? Well, let's just see if we can't reason our way through the problem.

Maybe you won't get the perfect propeller for your bird on the first try, but if you approach your problem with logical reasoning, you will at least have eliminated a gross mismatch in your selection process.

START WITH YOUR ENGINE
Our discussion will be on the fixed pitch propeller problem only. The type engine you have installed will dictate the basic propeller approved for it. The engine manufacturer has established for you the authorized maximum and minimum propeller diameter and pitch permitted. If you rely on this information alone, you will not get into any serious trouble. At worst, your bird may be a bit sick in its performance, but at least it should fly well enough to keep you safely airborne.

Propeller manufacturers have listings of their propellers. These listings show the propeller models available for different engine/aircraft combinations. In addition to the diameter, the recommended standard, cruise, takeoff and climb pitch is also given for the different aircraft using a particular propeller. A considerable amount of work and research made possible the information shown in Figure 1. It can be used as a quick reference by anyone who has yet to determine what propeller his engine can handle. With this information in hand you can then start your search for the right model prop. Unfortunately, time did not permit a broader report to include other excellent propellers on the market.

If this information becomes available later, we will pass it along as well. The propeller pitch range shown, in some cases, is the absolute maximum authorized by the propeller manufacturer. In other cases it merely reflects the composite limits for the representative aircraft listed and the authorized limits for that prop Model may really be just a bit greater. However, stick to the numbers shown, or check with the manufacturer, or Propeller Shop, before you do anything drastic.

I guess it is generally accepted that the propeller diameter should be greater for efficient low airspeed operations, and smaller for high airspeeds. This does not mean, of course, that you can get STOL performance just because you put on a large prop any more than you can make a racer out of a Pietenpol with one of Ray Hegy's small props.

One more word on propeller diameter. Have you ever heard of this one? "Keep your prop as long as possible as long as possible!" I'll give you one good argument for longer prop diameters. Next time you get near a nice homebuilt with a beautifully cowled engine . . . just check and see how much of the propeller area is blanketed out by the engine cowl when the prop is in the horizontal position.

IT MUST SUIT THE AIRPLANE TOO
You may have the correct prop for the engine but maybe you can't use it on your airplane. For one thing, the diameter may simply be too big. In other words you can't get enough ground clearance. Years ago this matter of ground clearance was quite an important thing. However, with the tricycle landing gear somehow that problem has been shoved off' into obscurity. Now that I think about it, I haven't heard any comments from the FAA as to a minimum propeller ground clearance for homebuilts. While there are some inspectors who may insist on a minimum clearance, it has become more a personal responsibility of the homebuilder. A minimum of 9" is good insurance. By nine inches, I mean with the tail up in level attitude and at least nine inches between the prop tip and the ground. This guarantees, among other things, that your prop will stay longer longer! There is a beautiful sleek all-wood tail dragger job I know of that could never make a wheel landing safely because it has little, if any, propeller ground clearance. Anyhow, by all means do consider the requirement for safe ground clearance in selecting your propeller diameter.

WEIGHT AND BALANCE IS A FACTOR
Determine from your weight and balance calculations whether you need a heavier prop (metal) or a light (wood) propeller. The wood prop will be lighter by about 10 pounds for the smaller engines.

Considering the higher cost for metal propellers, I wonder why more builders are not using wooden propellers. I realize, of course, that wood props are more delicate and not as well suited to outdoor parking conditions. I guess one reason for the popularity of the metal propeller is, believe it or not, because it is much heavier than a wood prop. Most homebuilt designs are prone to be tail heavy and a metal prop often helps to balance things up. Wood props are nevertheless gaining in popularity again. They are being made by a number of people in EAA circles and can be obtained for reasonable prices. A recent price list showed that metal props for the smaller Continental engines were listing for about $250 and up to $310 for props for a 165 hp mill. The same company's wood props listed for much less than that . . . about $105.

SEE WHAT OTHERS ARE USING
While you are busy searching through Trade-A-Plane and the local airports for that outstanding propeller "buy", continue to gather performance data from the builders and owners of aircraft like yours.

If your airplane is a popular one, there are probably a number of them already built and flying. Ask their owners what size prop they are using. If possible, compare the results as reported by them and form your own conclusions. Remember, though, that some of these gents may not really have accurate data to give you. Accept the information with the same reservations you may have in accepting your neighbors’ claim of gas mileage for his new car.

STATIC RPM AS A GUIDE
Once you have a propeller, the static rpm that your engine can turn up with that a particular prop is a good indicator of whether or not it can get you off the ground safely. By static rpm, we mean the rpm the engine is capable of reaching at full throttle while the aircraft is stationary.

Static rpm is established for all certificated engines. If your engine is not one of those shown in Figure 2, check with any local A&P mechanic or repair shop. What does this static rpm mean to you? Well, if you full throttle your engine and it does not rev up to the recommended static rpm . . . it is quite possible that the prop is too big in diameter or that it has too much pitch, maybe both. It means that perhaps you won't get enough thrust from that prop for a safe take-off. What if your static rpm is much higher than that recommended? Well, you may get off' O.K. but there is the possibility of exceeding the red-line limits for your engine, even during the takeoff sequence.

Lycoming advises that, when using a fixed pitch propeller, the static rpm should be 2300 plus or minus 50 rpm when the engine is rated at 2700 rpm. This will vary slightly depending on the type of propeller. If the static rpm is too high, there is the chance of overspeeding at full throttle level flight. (With a constant speed propeller, static rpm will be the rated rpm of the engine. This is controlled by the low pitch settings of the propeller.)

Here are the recommended static rpms that each engine should be capable of achieving with any suitable fixed pitch propeller.

FIGURE II

MAX RPM OR REDLINE RPM AS A GUIDE
In straight and level flight, with full throttle, what is the maximum rpm that the engine will turn? Is this above or below the recommended rpm redline limits established for that engine Model? If the rpm exceeds the redline limit for your engine, the prop could be too short, or more likely, the propeller could absorb more pitch. To sum this all up. If you pick a prop that is approved for your particular engine . . . one whose diameter still allows safe ground clearance, and the proper static rpm . . . you probably are close to the prop combination you need. You can refine the pitch requirement by checking your full throttle rpm in level flight for the fine pitch adjustment to get the most speed out of the old bird. You may recall from your studies on fixed pitch prop theory that climb suffers with an increase in cruise capability and vice versa. Obviously the final selection between climb and cruise performance is one of your own preference . . . you can't have both with a fixed pitch prop.

Another thing. Each time you decide to make a pitch adjustment on a metal prop that means another trip to the Prop Shop and another 20 bucks or so.

BEWARE THE DAMAGED PROP
If you locate a damaged propeller and hope to have it repaired for use on your own airplane, beware. Do not attempt to straighten the bent blades yourself. It is better to hike yourself off to the nearest Authorized Prop Shop and have them evaluate the propeller's condition. The prop may be bent and damaged beyond repair limits. FAA Advisory Circular, AC 43.13-1 (used to be CAM 18) gives guidance for the repair of metal propellers. In this regard be extremely careful in buying a damaged propeller even though it looks repairable to you. Some uninformed (or unscrupulous) individuals may have partially straightened the blades creating the appearance (innocently or deliberately) that the thing can be repaired, when in fact it is non-repairable and in a critical condition.

A reputable Prop Shop will ordinarily refuse to cut a prop down beyond its authorized limits if they know it is intended for use on an aircraft.

Straightening, cutting, shaping and balancing a propeller is certainly beyond the equipment capabilities of the average builder. It is better to entrust these technical operations to an Approved Prop Shop. In return, you will ordinarily receive a good serviceable prop, a yellow tag, and peace of mind.

LET'S BACK OFF AND TAKE ANOTHER LOOK
This you must realize. The propeller people design a propeller for a particular engine and then, based on their calculations and vibration testing, establish the maximum and minimum diameters that must not be exceeded. They also determine what pitch limits must not be exceeded.

The recent rash of metal propeller failures has aroused the concern and attention of the FAA, the EAA, and the propeller manufacturers.

EAA members in scattered areas have already been reacting to the problem. By the time this article appears you may have read quite a bit on the subject of propeller failures . . . about propellers being cut down beyond limits authorized by the propeller manufacturer and about the game of Russian Roulette being played with unknown propeller vibrational stresses. Take a look at Figure 1 again and note that most of the propeller diameters listed are greater than those typically found on a large number of homebuilts.

There is no doubt about it. The propellers used on manufactured aircraft, as a rule, have larger diameters than those used by the homebuilt types!

At this point the representative aircraft shown in Figure I may or may not be considered as particularly helpful. After all, how many homebuilt are as large as the Piper Cub or are like the Ercoupe? I guess this is exactly where the problem really shows. The size of the homebuilts, their weight, and their (generally) lower drag characteristics, certainly, are different from the larger store boughts.

No doubt this has a bearing on the propeller dimensions and pitch requirements for the best performance.

It would appear that there is a direct conflict between the scientifically inclined engineering set and the sometimes not so scientifically bent homebuilders.

Nevertheless, I don't believe that there is a flagrant or deliberate intent by the homebuilder to disregard the engineering guidance available regarding the propellers/engine/airplane match. How come, then the situation that we have? An informal check of my own showed, that of the many projects I checked, NOT ONE was equipped with a new metal propeller . . .

The metal propellers installed were hand-me-down types off of wrecked or canabalized aircraft, or, at best, were "serviceable" used propellers. It is true that some of them had gone through authorized prop shops and were properly reconditioned. A good number of them, however, were cut down to salvage a bent prop. A few "prop sources" around the country are advertising props as "suitable for a homebuilt". Is that so? What it means is that they have a damaged prop which was probably cut down and refinished. It can no longer be used legally on a store bought aircraft so why not sell it to the homebuilders . . . they will use anything. Well, from some of the propeller accident reports I've read about, it may be true. Not sensible, but true.

But even some homebuilders are playing the same game with themselves as the target. They have a damaged prop, and it will fit the hub. And then the obvious thought that it must be cut down to salvage it. He does so, and winds up with a smaller than desired diameter propeller so, as the final step, he has a bunch of pitch cranked into it. According to his incomplete and faulty reasoning the airplane will be faster . . . but even if that were so . . . what about his prop . . . his engine? How long will the prop last? How long can the engine operate above the recommended maximum redline rpm authorized by the manufacturer?

The McCauley Industrial Corporation has offered the advice that all of the EAA people should be encouraged to use a propeller within the diameter range that is approved vibration-wise on the engine. Too many of them, they said, are using short diameters on 4-cylinder engines which may get them into trouble.

To show you that this business of performing drastic surgery on a propeller is a matter for wide concern, still another well known propeller manufacturer, the Sensenich Corporation, also cautioned that the modification of propellers to suit experimental aircraft design limitations should not be done. The aircraft should be designed originally around the propeller and engine and not vice versa. At the same time, they also recognize that it is not economically practical or feasible to run vibration surveys on each of the many homebuilt designs . . . so where do we go from here?

Our best advice is to go back to Figure 1 and stay within the recommended parameters.