Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 FD set the props very coarse for a long time, this worked (limited speed) until they were adjusted. A fixed pitch prop would be needed to keep it that way. We know that coarse works to limit speed but flat to limit speed would not permit you to use more than 1/2 throttle to prevent over speeding the engine. Link to comment Share on other sites More sharing options...
FastEddieB Posted December 9, 2015 Report Share Posted December 9, 2015 Isn't that a climb setting, as I suggested?? Yes. I read "cruise". Blame Mature Onset Dyslexia! Link to comment Share on other sites More sharing options...
FlyingMonkey Posted December 9, 2015 Report Share Posted December 9, 2015 FD set the props very coarse for a long time, this worked (limited speed) until they were adjusted. A fixed pitch prop would be needed to keep it that way. We know that coarse works to limit speed but flat to limit speed would not permit you to use more than 1/2 throttle to prevent over speeding the engine. I would think either way would work to keep you slow using a Rotax 912. Way too coarse would not let you develop full power and thus limit speed, and way too flat would RPM limit the speed. Neither is efficient or really good for the engine. The other way to slow things down is to add drag to the airframe with steps, less efficient fairings, longer wing tips, etc, and then let you set the prop pitch correctly -- probably the better way. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted December 9, 2015 Report Share Posted December 9, 2015 Yes. I read "cruise". Blame Mature Onset Dyslexia! Aerodyslexia = read climb as cruise and ascent as descent! Link to comment Share on other sites More sharing options...
Tom Baker Posted December 9, 2015 Report Share Posted December 9, 2015 How could a climb prop slow it down? Climb prop = race prop. We all pitch for climb to go faster, not slower. the compromise is speed vs economy (climb vs ecomomy cruise) not climb vs speed. In my experience with both model and full sized aircraft if prop pitch is reduced the aircraft has a slower speed. In theory when you decrease pitch by 2" for each revolution of the propeller you will travel 2" less forward than before. At 2500 RPM you will travel 5,000 less inches per minute. In this example if you reduced pitch from 52" to 50' you would need an increase in RPM of 100 to maintain the same speed. All of this assumes that you are operating in a perfect world. Flight Design seems to be an exception to the rule. In the case with the CT I think it was way over pitched, and when the pitch is reduce it allows the engine to increase power and turn enough faster to overcome the decrease in forward motion caused by the reduction in pitch. Link to comment Share on other sites More sharing options...
Anticept Posted December 9, 2015 Report Share Posted December 9, 2015 It's because prop lift is not linear. They have efficiency curves. If the prop is below the optimum angle of attack, then we don't reach the full potential. However, above optimum, we begin to see significant drag increases with littlw lift gain, eventually stalling the blade. Also, the engine's power curve is a factor too. So, if the prob is already significantly overpitched, we will see an increase in speed as we bring the prop and engine combination into their efficient configurations. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 Long ago I had a friend that flew his home built in the formula 1 class at Reno. Composite planes with 100hp Conti O200 engines. To prepare for the race he would install his climb prop, rest of the year he had a cruise prop. Max speed in this class is from a flat prop and weight and power wise it isn't far from LSA. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted December 9, 2015 Report Share Posted December 9, 2015 Long ago I had a friend that flew his home built in the formula 1 class at Reno. Composite planes with 100hp Conti O200 engines. To prepare for the race he would install his climb prop, rest of the year he had a cruise prop. Max speed in this class is from a flat prop and weight and power wise it isn't far from LSA. The best speed around a course probably does not translate into highest cruise speed. I'm guessing the climb prop gave him better acceleration/deceleration with power changes that enabled him to make better overall times around the course even though his top end might have been lower. A higher top speed in a race doesn't help if you can never get there because it takes too long to accelerate to that speed. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 In aircraft like ours as well as the formula 1 planes a flat/climb prop give both best speed and best acceleration. Its very similar to my sports car and my motorcycle, my top speed is in 4th gear not 5th (Sportster) or top speed is in 5th not 6th. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted December 9, 2015 Report Share Posted December 9, 2015 In aircraft like ours as well as the formula 1 planes a flat/climb prop give both best speed and best acceleration. Its very similar to my sports car and my motorcycle, my top speed is in 4th gear not 5th (Sportster) or top speed is in 5th not 6th. A stock 100hp O-200 and a Rotax 912 are very different...RPM ranges, rev rates, and power curves are not the same. Link to comment Share on other sites More sharing options...
Tom Baker Posted December 9, 2015 Report Share Posted December 9, 2015 Long ago I had a friend that flew his home built in the formula 1 class at Reno. Composite planes with 100hp Conti O200 engines. To prepare for the race he would install his climb prop, rest of the year he had a cruise prop. Max speed in this class is from a flat prop and weight and power wise it isn't far from LSA. Once again, the increase in RPM has to outweigh the decrease in pitch. That is not always the case. On my dads Taylorcraft we removed a 44" pitch prop and installed a 42" pitch prop. The result was a higher static RPM, a Higher max RPM in level flight, and a reduction in top speed by 5 MPH. I have seen this time and again with other aircraft too. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 A stock 100hp O-200 and a Rotax 912 are very different...RPM ranges, rev rates, and power curves are not the same. true but the result is the same, flatter = faster in O200 racing no redline, in Rotax 5,500 is the limit any other approach limits available power. You can't get close to max power with a cruise setting. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 Once again, the increase in RPM has to outweigh the decrease in pitch. That is not always the case. On my dads Taylorcraft we removed a 44" pitch prop and installed a 42" pitch prop. The result was a higher static RPM, a Higher max RPM in level flight, and a reduction in top speed by 5 MPH. I have seen this time and again with other aircraft too. Granted Link to comment Share on other sites More sharing options...
FlyingMonkey Posted December 9, 2015 Report Share Posted December 9, 2015 true but the result is the same, flatter = faster in O200 racing no redline, in Rotax 5,500 is the limit any other approach limits available power. You can't get close to max power with a cruise setting. Based on what you are saying, isn't it more correct to say you need the pitch that gets you max continuous RPM (or a little more if you have a 5min setting like the 912) at WOT at your race altitude? Just trying to make sure I'm seeing your gist here. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 Based on what you are saying, isn't it more correct to say you need the pitch that gets you max continuous RPM (or a little more if you have a 5min setting like the 912) at WOT at your race altitude? Just trying to make sure I'm seeing your gist here. Yes its the same thing. Remember you suggested a 'big climb prop' to limit speed in the FK14. A climb prop in such an aircraft would generally make max power available where a cruise prop that could save a lot of gas if you throttle back would not generally make max power available. In our world, like the form 1, generally speaking a climb setting is a setting where max power is available or close to it and it results in meaningfully faster speeds then the cruise settings that many of our CTs where delivered with. There was a time when most believed that the coarse pitch was there to limit speed and the question of flattening the pitch for additional speed/power was open to dispute. There are some LSA that spec a cruise setting. Today in a FD the prevalent thinking is to pitch in a range that minimizes engine wear and it permits us to be a bit on the fast side. Link to comment Share on other sites More sharing options...
gbigs Posted December 9, 2015 Report Share Posted December 9, 2015 In the CTLSi Sport my prop is set for cruise. I still climb at 5000 agl at over 800fpm in less than 500 feet of runway. Why sweat the little stuff? Link to comment Share on other sites More sharing options...
FlyingMonkey Posted December 9, 2015 Report Share Posted December 9, 2015 In the CTLSi Sport my prop is set for cruise. I still climb at 5000 agl at over 800fpm in less than 500 feet of runway. Why sweat the little stuff? What is your max RPM in straight and level flight at wide open throttle? Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 What is your max RPM in straight and level flight at wide open throttle? Much like a constant speed prop should not be operated at cruise prop settings and high power settings what is the limit with a CT? At some point you would be too coarse to use full throttle, no? Perhaps when limited to less than 93hp it becomes a non-issue? What kind of wear / damage occurs when a constant speed uses a high power setting and a cruise pitch? Can those with CTs with adjustable props operate the entire flight at the coarsest setting? If not what happens? Link to comment Share on other sites More sharing options...
Anticept Posted December 9, 2015 Report Share Posted December 9, 2015 When a constant speed prop is set "over square", mwaning manifold is higher than RPM, then you are lugging the engine. Increased bearing wear is a major issuse, and in extreme cases, the high pressure in the head will bend a rod or blow a jug. Some engines are OK with an inch or two over at higher rpm. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 When a constant speed prop is set "over square", mwaning manifold is higher than RPM, then you are lugging the engine. Increased bearing wear is a major issuse, and in extreme cases, the high pressure in the head will bend a rod or blow a jug. Some engines are OK with an inch or two over at higher rpm. Sounds like a flat pitch is the conservative choice for engine wear and a coarse pitch and less than full throttle is the conservative choice for fuel burn. With no manifold pressure gauge plus and need for speed I like flat. Link to comment Share on other sites More sharing options...
gbigs Posted December 9, 2015 Report Share Posted December 9, 2015 For your engine, what is the static takeoff RPM? Once established in the climb, say 1000 AGL and Vy . . . at full throttle, what RPM are you indicating? On takeoff at 5k msl WOT in the climb at Vy (about 65kt to 70kt IAS) the RPM is around 4800 to 5100. I reach 800 agl pattern altitude by mid runway or lower the nose, gain speed, climb slower, about 500fpm and get to 5800 msl on the turnout. I cruise at 80 to 90% throttle at 8k msl at 5350 rpm. I never use WOT on cruise. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted December 9, 2015 Report Share Posted December 9, 2015 think: AGL(above ground level) and MSL(mean sea level). as in 800' agl is often pattern altitude. Link to comment Share on other sites More sharing options...
Anticept Posted December 9, 2015 Report Share Posted December 9, 2015 Sounds like a flat pitch is the conservative choice for engine wear and a coarse pitch and less than full throttle is the conservative choice for fuel burn. With no manifold pressure gauge plus and need for speed I like flat. There are engines that have to be throttle managed as they don't have a waste gate designed with an upper limit; they will happily go 36+ inches and help you separate your cylinders from your engine case. These aren't seen much anymore with proper waste gates being standard on turbo installs, but that's what they had to do. The problem with pitching excessively coarse or flat, is you suffer takeoff power. That is what the real benefit is to constant speed props; they let you access your full take off power, and then pitch coarse as you pick up speed. They take the efficiency band and expand it greatly. Link to comment Share on other sites More sharing options...
gbigs Posted December 9, 2015 Report Share Posted December 9, 2015 think: AGL(above ground level) and MSL(mean sea level). as in 800' agl is often pattern altitude. fixed it....was typing in a hurry. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted December 10, 2015 Report Share Posted December 10, 2015 Burger, I know you don't cruise at WOT, but it would be helpful to know what rpm you are turning at that setting. Just climb to your normal cruise altitude and go wide open, long enough for the rpm to stabilize, maybe 30 seconds. That will tell a lot about how close to optimal your prop pitch is. Link to comment Share on other sites More sharing options...
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