FastEddieB Posted January 23, 2015 Report Share Posted January 23, 2015 As long as you don't delete your post because I disagreed with you. deleted Link to comment Share on other sites More sharing options...
WmInce Posted January 23, 2015 Report Share Posted January 23, 2015 . . ."in Europe are altitudes assigned in meters? It seems like that would be very confusing for pilots of aircraft with altimeters calibrated in feet." . . .Western Europe, no, altitude expressed in feet. Further east, i.e. Russia, yes, altitude is expressed in meters.Newer generation aircraft have instrumentation to accomodate both measures. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 23, 2015 Author Report Share Posted January 23, 2015 Even if you succeed making an impossible turn in a practice there is no assurance you can do it when you DO NOT EXPECT the engine to quit. Even the POH describes the engine out emergency in more dire terms. If at or near pattern altitude and calm wind and not at MTOW there may be enough room for error and delay in reaction to make the turn, otherwise it's a chute pull and/or a straight ahead set down. Is there any assurance that if you practice landing straight ahead, that you will not kill yourself doing it if it happens unexpectedly? Practice and proficiency increase the odds in your favor, but assure nothing. If you want assurance of a great outcome due to practice, give up aviation and take up knitting. Link to comment Share on other sites More sharing options...
FastEddieB Posted January 23, 2015 Report Share Posted January 23, 2015 Knitting??? But you'll put your eye out! Link to comment Share on other sites More sharing options...
FlyingMonkey Posted January 23, 2015 Author Report Share Posted January 23, 2015 Knitting??? But you'll put your eye out! Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 the airplane will glide farther with the propeller stopped than with it turning at 1200 rpm. This really made me think. My first response was to say you were wrong. But now that I've thought it through, I understand it a lot better. I have always taken Eddie's view on this - that an idling prop will produce some thrust in a glide, and so will extend the glide. The argument is (see above) that if you stand behind an idling airplane you can feel how much thrust it generates and this will extend a power off glide over an engine out glide. But if the plane is gliding faster than the prop is spinning (considering pitch and rpm in relation to airspeed) then the prop will have a windmilling (braking) effect rather than a thrust effect. It's like an idling car rolling at 1mph - the idling engine wants to push it up to 5mph. But if the car is already coasting at 50mph, the idling engine will act the opposite way - as a brake. What this means is that I may actually have a better glide ratio at 60kts with a stopped engine than with an idling engine, because a windmilling prop creates far more drag than a static one. I have, up till now, always assumed it to be the other way round. Link to comment Share on other sites More sharing options...
FastEddieB Posted January 23, 2015 Report Share Posted January 23, 2015 It probably depends on the idle speed and prop diameter and a bunch of other stuff. But I was taught a stopped prop was definitely less drag than one rotating with a dead engine. So much so that getting the prop stopped by slowing down enough was one option to extend a glide. I think the ROTAX takes care of that by stopping the prop if the engine loses power, due to the effect of the gearing. Found an apparently erudite paper here: http://www.peter2000.co.uk/aviation/misc/prop.pdf Let's see what it says. Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 It would be useful to know at what glide speed an idling prop hits its 'balance point' - were it's neither producing thrust or drag. Anyone know the pitch in inches that a typical CT is set up for? Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 Jim -- was there an appreciable difference in glide ratio when doing dead stick versus idle? That's a good question and one I'm going to have to think about a little. It might help answer this question. Link to comment Share on other sites More sharing options...
FastEddieB Posted January 23, 2015 Report Share Posted January 23, 2015 An interesting chart here as well: http://www.casa.gov.au/SCRIPTS/NC.DLL?WCMS:OLDASSET::svPath=/fsa/2005/jun/,svFileName=32-33.pdf Link to comment Share on other sites More sharing options...
gbigs Posted January 23, 2015 Report Share Posted January 23, 2015 I'm wondering why you think you have something to bring to this discussion. Did you take the time to practice it? You called me a liar a few posts back... I don't remember an apology or a correction. Frankly, and I'm trying real hard not to make a personal attack, I think you should just keep your thoughts to yourself if you don't know what you are talking about. Remember, there are lurkers who may not know that guys trying to do impossible turns are playing with fire (and lives). What you do in your plane is up to you but suggesting that you are beating the odds on such turns is irresponsible... Frankly, if you don't like my commentary, use the ignore feature. I do, it works well... Link to comment Share on other sites More sharing options...
Jacques Posted January 23, 2015 Report Share Posted January 23, 2015 Anyone know the pitch in inches that a typical CT is set up for? 57'' according to this http://www.indoorduration.com/PropCalc.asp 20° at 25'' from center Link to comment Share on other sites More sharing options...
Doug G. Posted January 23, 2015 Report Share Posted January 23, 2015 Frankly, if you don't like my commentary, use the ignore feature. I do, it works well... This is an idea I can agree with - everyone should ignore him. Link to comment Share on other sites More sharing options...
Tom Baker Posted January 23, 2015 Report Share Posted January 23, 2015 It probably depends on the idle speed and prop diameter and a bunch of other stuff. But I was taught a stopped prop was definitely less drag than one rotating with a dead engine. So much so that getting the prop stopped by slowing down enough was one option to extend a glide. I think the ROTAX takes care of that by stopping the prop if the engine loses power, due to the effect of the gearing. Found an apparently erudite paper here: http://www.peter2000.co.uk/aviation/misc/prop.pdf Let's see what it says. To me the tell tale sign that the idling prop is producing drag is the fact that the engine RPM decreases to the no load idle speed as you slow down. Link to comment Share on other sites More sharing options...
FastEddieB Posted January 23, 2015 Report Share Posted January 23, 2015 To me the tell tale sign that the idling prop is producing drag is the fact that the engine RPM decreases to the no load idle speed as you slow down. We may be going in circles, but my point is still a fully failed but turning engine produces more drag than an idling one. Which should be taken into account when viewing "return to airport" maneuvers with an idling engine. Agreed? Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 Thanks Jacques. Taking 1700 for an average idle speed, @2.43 reduction = 700rpm for the prop. 700 x 57" divided by 12 = 3325 FPM, which is 33kts (online calculator). Therefore, an idling 100hp engine is acting like a brake throughout all phases of flight above 33kts. As the glide speed increases, the braking effect also increases. I think that's correct - ? Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 We may be going in circles, Touche! Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 but my point is still a fully failed but turning engine produces more drag than an idling one. Agreed? Agreed. And so does an idling one, it seems. Link to comment Share on other sites More sharing options...
FastEddieB Posted January 23, 2015 Report Share Posted January 23, 2015 See my addendum to my post above. Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 Doing engine out simulations at idle are going to be a lot more useful if one knows how differently the plane will behave with a stopped prop. At 33kts an idling prop is 'neutral' - it produces no thrust and no drag. At 40 kts, it will produce a little drag, and a little more at 45. The question is, at what speed does that drag match the drag of a stopped prop. If someone can answer that, then we can determine the airspeed where a static prop behaves like an idling prop, in terms of the glide ratio it permits. Link to comment Share on other sites More sharing options...
Doug G. Posted January 23, 2015 Report Share Posted January 23, 2015 Can this be thought about in terms of AoA of the relative wind striking the prop? If that is the case wouldn't a rotating prop (even without power) have a lower AoA than a static prop? What am I missing here? Link to comment Share on other sites More sharing options...
IrishAl Posted January 23, 2015 Report Share Posted January 23, 2015 Doug, We're talking about gliding with a windmilling prop and gliding with a stopped prop. So, it's easier to understand it if you imagine a windmill instead of a plane. Imagine a 3 blade windmill on a pole with a 72 inch diameter. When it spins it makes a 6ft wide 'disc' of spinning blades. When the wind blows against the windmill with the blades stopped, the static blades will catch the wind on their surface area and exert a force on the top of the pole. When the blades are spinning a different amount of force is exerted on the top of the pole. Question: will there be more 'push' against the top of the pole if the blades are stopped, or if they are turning. A. when they are turning. The surprising thing here is just HOW much more. The windmilling blades will create more resistance to the oncoming wind than a single, solid metal disc 6ft wide mounted on top of the pole would create. That seems astonishing, but it's true. Thus spinning blades driven by an oncoming wind will generate MANY TIMES more drag than a set of static blades. When a plane is gliding with the propeller stopped, the only drag is created by the face of the propeller into the wind. But if the prop is spinning and the wind is either causing it to spin or is assisting it to spin, then far more drag is created than with the stopped prop. Effectively, you could imagine yourself flying with a solid metal disc on the front - that's how big an airbrake a windmilling prop is. That's my understanding of it - please let more experienced heads comment or correct as necessary! Hope that helps. Link to comment Share on other sites More sharing options...
S3flyer Posted January 23, 2015 Report Share Posted January 23, 2015 I understand that stopped prop will be 100% drag and an idling prop will have some drag but it also provides thrust. My question is what is the net drag (drag - thrust) of an idling prop at best glide speed? Link to comment Share on other sites More sharing options...
Tom Baker Posted January 23, 2015 Report Share Posted January 23, 2015 We may be going in circles, but my point is still a fully failed but turning engine produces more drag than an idling one. Which should be taken into account when viewing "return to airport" maneuvers with an idling engine. Agreed? No circles! Link to comment Share on other sites More sharing options...
IrishAl Posted January 24, 2015 Report Share Posted January 24, 2015 I understand that stopped prop will be 100% drag and an idling prop will have some drag but it also provides thrust. My question is what is the net drag (drag - thrust) of an idling prop at best glide speed? That's also my question. So far, I've established that at 33kts glide, there's neither drag nor thrust from an idling prop - the prop is 'screwing' through the air at the same speed as it's being pushed through the air by the airplane. If you increase that glide to 43 kts, then drag is the equivalent of a windmilling prop in a 10kt wind. At 60kts glide, drag is the equivalent of a windmilling prop at 27kts. That drag has to be compared to the drag generated by a static prop at 60kts. If the two are the same, then an engine off glide at 60kts will be identical to an engine idle glide at 60kts. I'm fairly certain that the static prop would generate considerably less drag at 60kts than a windmilling prop at 27kts. If so, then the average LSA will have a better glide ratio at 60kts with no engine than with an idling engine. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.