No simple answer
cobra said:
George, You mentioned that spinning the prop faster generates more thrust due to increased HP.
Cobra there is no simple answer, but there are some general rules that apply to a Lycoming powered RV with a Hartzell prop. Yes, more RPM = more HP = more thrust = less efficiency (sometimes). In general the increase in HP overcomes the lower prop efficiency, at least in (mid altitude) cruise. In climb at low speeds efficiency may increase with RPM! go figure. Prop dia is also a factor.
At 8000 feet, a 100 RPM increase 2600 to 2700 rpm gained 0.75-1.5 mph; the theoretical gain was only 0.0-0.25 mph at 12,000ft. You might think climbing higher will reverse this, where lower RPM gains speed. It might, but the data says higher rpm is a tiny gain or a wash. Really the 2600, 2700 rpm thing for grins and giggles, it is a moot for practical purpose, except for racing. If you are racing and there are no winds aloft than down low is the way to go. Typically for long range you will fly high and pull the prop RPM back to 2400 or less. Less noise, less fuel burn for a slight loss in speed.
This is indeed the advantage of a constant speed prop, setting the best RPM, independent of engine power and airspeed.
CLIMBS:
At low altitudes and slower speed MORE RPM produces more thrust and MORE prop efficiency. In a 100 mph climb, sea level, an increase of 100 rpm makes more HP (of course), more thrust and better prop efficiency and obviously Climb rate increased (but not much). HOWEVER I am not suggesting 2,700 RPM or 100mph climb speed, at least past initial T/O. Most folks reduce there RPM to 2,500 (2,600 max) after takeoff for many reasons, gas burn and noise being two of them, plus wear & tear. Also 100 mph V-climb can be steep in a RV. I like 120 mph initial for better forward visibility (collision avoidance) and cooling if real hot.
That brings up the point, WE don't always fly for most efficiently for other practical reasons, such as picking the smoothest cruise RPM. If you are in-tune with your plane you can tell a 50-100 rpm difference may be smoother.
Also a prop, like an aircraft WING, has its best L/D (lift over Drag), where the angle of attach is most efficient, but we don't fly best L/D in cruise, it's just too slow. Same with a prop. There is an ideal efficiency (most thrust for given power) and than there is the practical power / speeds we want, need or can only operate at (due to limitations in the engine, airframe or both).
When calculating the theoretical prop performance from the data, one parameter change affects 10 other things, they are all interrelated. It takes 14 iterations of the prop data (between J, Cp, Tip Speed Mach) and requires several calculations for Air density, Engine Power. A change in anything like, air speed, RPM, prop dia, air density or HP will affect most of the other factors. Outside factors affecting the prop: airframe (drag) and engine.
The most efficiency or range or MPG's are low low rpms, say around 2300-2350 RPM and at altitudes of 12,500-16,500 feet (approx 65% to 55% power). This is where prop efficiency, airframe drag and reduced power (fuel burn) all come together. Your TAS will not be as fast as 8,000 ft, but your gas mileage will be better, e.g., 15%. Typical MPG on a RV at 8,000ft/2400rpm is 19 mpg but +23 mph at 16,500ft/2300 (zero wind). Does wind and gas to climb to altitude (slower ground speed during climb) justify climbing? Depends on leg length, winds, weight and air temps. 8,000 feet to about 10,000 is a good compromise for speed/range. For long range, especially with favorable winds, 12,500 feet is better. Above FL125 you need O2, which kind of sucks the fun out of climbing higher, pun intended. (Note: depending on you own physiology I don't recommend flying above 8,000 all day especially at night for long periods with out O2. After flying all day at 12,500 feet, I get a mild but unpleasant headache that night.)
cobra said:
Do you know if some upper speed limit, other than supersonic tip speed, affects prop efficiency or is thrust linear to rotational velocity? The Lyc engine rpm limits seem to filter out answers to that question.
The theoretical (efficient) max tip speed, I have read Mach .75 to .92 Mach as a practical range, after 0.92M looses are too high. Yes in a RV with 2700 RPM we are not supersonic, but tip speed, regardless if it's 0.70 or 0.90 mach, is a big factor in effcinecy.
Is thrust linear to rotational velocity? Can't answer, does not compute.
Seriously the short answer is NO, it's not linear, but RPM is a factor. You could have a whole course to explain that, but invite you to google you heart out on the subeject, lots of info on the web. Long and short of it is J, Ct and Cp are not linear. Air density vs altitude, airframe drag vs speed, power vs. RPM are all non linear.
Remember tip speed is a function of prop dia, RPM and aircraft forward speed, but thrust is affected by many factors. That's why those big bladed turbo props going 300 kts turn their props at slow RPMs, but the airfoil, twist and angle of attack are optimized for that application.
P-3 Orion prop
Hartzell's are optimized for a 200 mph single engine tractor airplane with a direct drive 180-250hp piston engine at 1700-2700 rpm. Originally the 7666 blade was meant for typical 60's/70's retracts with 180-250 HP and top speeds around 190 MPH, which is a pretty good match to a RV. Since they are still made and in service for almost 40 years that's not bad. Even the latest and greatest of other brand props can't beat it. It took a new prop blade design from Hartzell to best it but than by only a small margin. They did a good job way back when. BTW the fastest RV, Dave Anders + 250MPH RV-4 had a Hartzell prop with 7666 blades.
The blended airfoil has a blade designed for a 200HP 220 mph RV. In a direct test (Van's aircraft did) between the 7666 vs 7496 (blended airfoil) there was about a 2-3 mph increase at 8000 feet, at the same power. That is incredible. Since changes in props are measured in fractions of a %. This is not as much new technology in prop aerodynamic design but clever use of all aerodynamic characteristics matched for a specific airframe. Prop designers have always played around with different tip shapes (remember the Q tip), blade shapes/areas/thickness/numbers, airfoils and twist distribution. That is why I laugh when prop makers claim better performance. Really it is more limitations from the material being using. Not picking on MT but as a trade off for smoothness and light weight, by using wood fiberglass construction, you trade performance. Wood blades are also a little thicker, which is an efficiency hit. Also multi blades and blade are not as efficient (ever), but for high HP fast planes (250-300kts plus) they are needed:
NOW HERE is a PICTURE of a Multi-Blade PROP
Prop planes are limited to about 0.60 Mach, or about 400 mph at 30,000 feet. Faster prop aircraft are possible, but efficiency is poor.
cobra said:
Isn't the problem with prop rpm related to harmonic vibrations transferred from the engine? If so, different engine designs should have different prop limits.
Well we where talking about performance, but Yes absolutely, Hartzell and Sensenich have targeted the experimental market (RV's in particular). They test their props and even made props just for Lycoming powered RV's. Hartzell has done extensive testing on different engines (Lycoming and Clones) with stock and modified (HC pistons and electronic ignition). For engineers to get out their flight test equipment and go on the road and do a prop survey is expensive. To the credit of these companies they have invested a lot in the RV market. For example, and no offense to MT, but there props are not optimized for our planes. There 3-bladed prop is awesome on a Pitts, but on a RV it's about 6-12 MPH slower depending who you ask. I think 9 MPH is typical, although some say their MT prop is only 2 mph slower. It is not a bad prop with the lighter weight and better vibration dampening (pilot perceived smoothness). Despite the higher cost they are popular. However I think they are missing the boat with the 74" diameter and 3-blades. I think Mickey said he is using a 69" dia 3-bladed prop, which makes sense.
As far as vibration and metal props, you are right, they must be surveyed on every and each engine (and airframe). This is the advantage of going with a certified prop. Of course the engine and airframe may be experimental, but if they where certified tomorrow Hartzell could certify their props on our RV's, with limitations, already published. (Note no restrictions of any kind with the blended airfoil on stock O360XP.) Even on the same engine, changing one thing, like ignintion can change the vibration characteristics. This is where Sensenich and Hartzell have stepped up to the plate, and it's a big advantage of going with a Lycoming or clone engine using a certified prop extensively testing. I am not sure MT has not tested there prop on modified Lycomings or reduction drives bolted to those Mazda rotary and Subaru engines RV'ers are flying. Metal bladed prop design from a vibration stand point are more critical, but properly designed and tested is very reliable and safe. Metal blades are also lower maintenance overall and easier to repair, in my opinion.
George
