RPM = HP (by a fairly fixed schedule)
B. Hoover said:
1. Sorry guys, I?m not the Hoover you?re thinking of? Maybe I need to post like the ?other? Doug Reeves
2. I was just wondering if anyone had an idea at what point sheer horsepower with a fixed pitch prop would overcome the available power of a smaller engine with CS prop during take-off and climb out.
Maybe a better way of phrasing it would be ?what is the difference in static horsepower between a 160 hp engine with a cs vs. a fixed pitch prop?.
I am under the impression that during run-up and acceleration, an engine cannot turn a fixed pitch cruise prop at full RPMs, therefore preventing the engine from making full power.
Both constant speed prop setups and higher hp engines cost more money than a base level horsepower/fixed propeller system. I know high hp/fixed pitch cruise propellers would be faster than a lower hp/CS setup. I am just trying to get an idea how much horsepower over the ?base? engine one would need to make up the difference of not having a constant speed prop.
3. I was not aware of reasons not to install fixed pitch props on 200+ engines. Please explain more!
4. Thanks for sharing some of the costs on the zero time engines. I was under the impression that 180 hp injected engines would be more expensive than was indicated.
The "other" B. Hoover
#1 - Darn
#2 - For every 100 RPM you lose about 5-7 HP. So if you can only turn your 210HP engine (rated at 2700 rpm) to 2400 rpm on take-off/climb, worst case, is going to make 190 HP. Than the fixed prop is not as efficient at that RPM, so knock another 5% off or about 10 hp for a total "thrust" hp of 180HP. Where a 160 HP (or 180HP) at the full RPM will be at rated hp (assume sea level).
I see where you are going, the 210 HP would not be any dog on takeoff but you will never see full power on t/o and climb. Why have the big engine if you are not going to use it for t/o and climb? Also on the same note you will likely turn the engine faster in cruise than ideal, because the prop is a compromise cruise prop, to assure that takeoff/climb is reasonable. This cost gas. That is the middle name of a constant speed prop, compromise. A fixed prop is only at it's optimal in one flight condition (speed, altitude, power applied). A constant speed prop has a wider range of conditions where its at peak or near peak efficiency over a wide range of conditions, simply because it can change its angle of attack.
There are side benefits of C/S like more drag in the pattern, which allows you to slow faster and maintain speed on final with some power to keep the engine warm (like down shifting going down hill in a manual transmission car). With a fast RV and short field a c/s prop can help. A proficient RV pilot can get the job done with the fixed prop however, but fixed props have more residual idle thrust.
Also with a C/S prop when doing Aerobatics, a loop in particular, does don't need you to play with the throttle to keep the RPM in the green, set RPM and forget it and loop away. I'm a C/S prop fan, but there is nothing wrong with fixed, especially on the budget. I would at least consider if going fix, having the option to upgrade later (eg hollow crank engine).
#3 - You said you are new, so you may not know about the search function in the tool bar? Type a key word or several key words to tighten the search down. I did an example search on: constant speed fixed. Here's a select few of the hotter thread hits of many that turned up:
http://www.vansairforce.com/community/showthread.php?t=12425&highlight=constant+speed+fixed
http://www.vansairforce.com/community/showthread.php?t=9843&highlight=constant+speed+fixed
http://www.vansairforce.com/community/showthread.php?t=4601&highlight=constant+speed+fixed
http://www.vansairforce.com/community/showthread.php?t=7206&highlight=constant+speed+fixed
http://www.vansairforce.com/community/showthread.php?t=4060&highlight=constant+speed+fixed
There is NO reason not to use a fixed on a +200hp engine, just many trade offs like cost, weight and performance. My main problem is the IO390 does not have a Sensenich available. The Sensenich performs better than any fixed prop on the market because of the metal blades, which are thinner than wood. It is also made (optimized) just for the RV. The Sensenich in my opinion is the best value and performing fixed prop.
My first choice is the Hartzell BA prop. It is the best performing, best supported and proven prop. Also because its from a +30 year design improvements, its very mature and should last a lifetime. Also you can get the Hartzell prop for the 160, 180 and 200 hp (and think the 210HP?) engines, which like the Sensenich is optimized for RV's.
The idea of going BIG engine to me is getting MAX performance. This is one of those hotly debated subjects. There is some beautiful about a super light RV with a wood prop and 150 or 160 hp engine. It may not have the same top speed or Rate-O-climb but its light, simple, cheaper and handles like a dream. Light weigh RV's are magic. Later RV models (-7, -8) are better able CG wise to take the weight on the nose, but keeping it light is usually a good thing.
Metal props weigh more than wood/composite. Metal props last longer and are more resistant to rain and generally more resilient from damage. Repair on a metal prop (file out a nick) is easier. Composite requires it to be removed. A prop strike with a wood prop is less likely to hurt the engine crank, compared to a metal prop. A wood prop needs constant re-torquing due to wood swelling/shrinking with weather. Metal props, you install and forget till overhaul. Even the complex constant speed needs just an annual grease job ,which takes 10 minutes.
#4 - What has happened in the last years is we have three companies making complete 180HP 360's (ECI, Superior, Lycoming), the most popular size/HP, so price goes down from supply/demand/competition.
Unfortunately like everything prices go up. Lyc clones engines and Hartzell props have all taken a $500 or more bump just recently.
