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brian(at)lloyd.com
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 Posted: Wed Jun 29, 2011 7:27 am Post subject: turn-back, props, drag, glide, feathering |
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(This needed a new thread.)
On Wed, Jun 29, 2011 at 6:48 AM, Bill Geipel <czech6(at)mesanetworks.net (czech6(at)mesanetworks.net)> wrote:
| Quote: | And u shut the mags off on your yak? I would advise against it. But u r a grown up.
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It is far easier than moving the mixture to idle cut-off to ensure that the engine is not making power. 
I m? Darn. 
I suspect that you are right that most have not turned off the mags in flight. I certainly don't make a habit of it but it is useful to see what the behavior of the airframe is when the engine is truly producing zero thrust. This is one of those things like practicing the turn-back maneuver. You don't really know what is going to happen until you try it. There are other ways to ensure the successful completion of the flight should the engine fail to restart. I do real engine-out practice directly over a very large runway (McClelan is 300'x12,000') so that if the engine doesn't restart, a dead-stick landing is a non-event.
(I have made multiple dead-stick landings both intentionally AND unintentionally in my flying career. There is no teacher like experience.)
| Quote: | It depends on the airplane to be sure. It was never mentioned. I am sure that some twins won't windmill at lift off speed.Got the T shirt.
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I am sure it is possible. There are [almost] always exceptions. The ones I have flown don't want to stop turning at any rational speed, i.e. some reasonable value above stall and/or Vmc. If they did, a feathering prop wouldn't really be needed.
| Quote: | I am also sure that when the engine seizes it won't windmill,
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That is true and at that point the propeller airfoil is deeply stalled and producing very little drag so the point is moot. Now the drag is from the frontal area of the blades and not from the disk area.
| Quote: | and I am equally sure that at that speed and altitude, u may not have a chance to feather it.
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Yes, that is why I mentioned the "dead zone".
| Quote: | Sure, once u reach 400' u have options.
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It varies with the aircraft and it varies with energy. In the case of my Aztec the magic "go" numbers are 100 and 100. If I have 100' AGL and 100mph on the clock, I have sufficient energy to deal with a failed engine, i.e. to feather and clean up the airframe for continued flight on one engine. If I am faster I don't need as much altitude. If I have more altitude I can fudge on the airspeed. One of my take-off call-outs is, "100 and 100, we are good to go one one."
| Quote: | But the engine may not have read the same game plan as u. Many light twins need to be windmilling to feather.
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That is true but that was not the issue. If the engine isn't turning, feathering isn't really needed. The deeply stalled prop does not produce nearly the drag of a windmilling prop. This started out as a turn-back discussion. Aircraft with constant-speed props are at a distinct disadvantage. In fact, I have never found an aircraft with a constant-speed prop that I felt I could safely execute the turn-back maneuver from any altitude while on upwind on climb-out from take-off. I need to be either on cross-wind or, worst-case, after turn to downwind. When teaching transition to the CJ6A I point out that the earliest that the pilot can make it back to the runway after engine failure is after the turn to downwind. From that point it is possible to make it to a downwind landing on the runway. I am currently flying a YAK-52 with the 400hp engine and large 3-bladed MT prop. I have yet to figure out from where in the pattern I CAN make the field if the engine quits making power. Pulling the prop back to corse pitch is the only possibility for reducing drag enough to make the field.
Now this does bring up another possibility and you more-or-less allude to it. If the prop is stopped, drag decreases markedly. I would like to suggest trying to see what happens to the glide ratio of a Yak or CJ if one could get the prop to stop turning, but at that point it could be difficult to get a restart thus making a dead-stick landing a necessity. I know that, if I had an engine that completely packed it in but was still turning, i.e. windmilling prop, if I didn't have a clear landing spot already made I would probably stall the aircraft in an attempt to get the prop to stop turning in order to increase my glide ratio. This would be a desperation maneuver on my part because I have no experience flying either Yak-52 or CJ6A with a stopped prop. Still, If the choice is the trees or a field beyond, I would definitely try it. The outcome can't be any worse as I would just end up in the trees anyway.
For those of you who live in SoCal, there is El Mirage dry lake. As a kid I used to go out and land on the lake and camp out. (I was 17. I had an airplane. What would you do?) While there I used to try things that I would never try anywhere else. I practiced dead-stick landings as I could land in any direction. If I screwed up, I would just roll wings-level and land in whatever direction I was pointed at the time. No sweat. It also allowed me to learn how to judge power-off spot landings better. I would suggest going out and trying to land a Yak and/or CJ there with the prop both stopped and windmilling to see what the effect on glide ratio is and whether it would be possible to stop the prop and execute a turn-back maneuver. (I would try a chandelle in an attempt to trade airspeed, i.e. kinetic energy, for altitude, i.e. potential energy, and also end up at minimum airspeed where the prop might stop. Definitely an interesting experiment.)
But I am not you. I find that not many people are comfortable doing some of the things that I do with airplanes even though I know they are perfectly safe, as these operations go against conventional wisdom. (This was fodder for many discussions on this list 10+ years ago back when this was often the Brian Lloyd/Mike McCoy show.) Examples include practicing shutting off the engine and making intentional dead-stick landings, practicing the turn-back maneuver to a landing (for real - but with throttle at idle), 450nm flight legs in a CJ6A with stock fuel, etc. (The Huosai engine can be operated at an average 11gph giving 130kts TAS at 11,500', resulting in a no-wind range with 30min reserve of 430nm. It takes a very small tailwind component to give a 450nm range.)
--
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
brian(at)lloyd.com (brian(at)lloyd.com)
+1.767.617.1365 (Dominica)
+1.916.877.5067 (USA)
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brian(at)lloyd.com
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| Posted: Wed Jun 29, 2011 7:52 am Post subject: turn-back, props, drag, glide, feathering |
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On Wed, Jun 29, 2011 at 7:19 AM, myersf(at)comcast.net (myersf(at)comcast.net) <myersf(at)comcast.net (myersf(at)comcast.net)> wrote:
| Quote: | I want chime in here a bit although not too much. As a lurker I hate to bring this up but...
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No problem. Too few people actually think about what they were taught originally, especially with airplanes. The problem is, what we were taught may be wrong but the only way to find out is to do something that would be considered dangerous and/or fatal. Welcome to flight testing.
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It has always been my understanding that constant speed props are different whether on singles vs twins. I was taught that and I taught it that way.
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Ouch! I understand the being taught that way. I have determined that several things I was taught were wrong. That is why it behooves you as CFI to be absolutely sure before passing on "knowledge."
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When oil pressure goes to zero, singles go to flat, twins go to course. Heck, I had to create a lesson plan for it on my CFI-ME or MEI.
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It depends on the airplane and the prop. Both types of prop exist in both worlds. Case in point, both the CJ6A and Yak-52 have props that go to course pitch when oil-p goes to zero. And some twins require oil pressure to force the prop to feather, implying that they will go to fine pitch (high RPM) upon loss of oil-p. This is why you really need to know the characteristics of the specific propeller and governor, even on the same aircraft. And you might not be able to discern the information from the POH. You might actually have to call the prop manufacturer or prop overhaul shop to get the straight poop. (Welcome to transition training. This is why we spend 5 house in ground school before flying the new airplane.)
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Turboprops are different again. Autocoursen, auto feather, garret type engine vs Pratt...
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True. The point is that systems differ between aircraft and even within the same make and model of aircraft. It pays to know the airplane.
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Now I have been corrected by other pilots about OWT I have perpetuated but I am pretty confident on this issue.
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It is best to find out on a per-aircraft basis. Making assumptions is not a safe thing to do even if you are attempting to, "err on the safe side."
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I don't know Mr Sapp but I think he had an experimental 4 blade prop on his plane. If the prop went to flat pitch when the engine failed, drag probably really increased even more than if he had the standard fixed found on a super cub.
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That is very likely true. And if it were a constant-speed prop, it would be worse still.
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On the other hand I wouldn't at all be surprised to hear that this prop was created to go course on failure or that Mr Sapp pulled the prop to full course to get more glide. Either way I think it was a great demonstration of airmanship.
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No question about it. Clearly he kept his head and flew the plane as far into the crash as possible, thus saving both he and his wife. Good on ya, Doug!
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That plane must have had a lot of power to need a 4 blade prop....
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Could be. More blades tend to produce more static thrust at the expense of greater drag in cruise. Greater static thrust means better STOL performance but at the greater drag results in either higher fuel burn or lower TAS in cruise. For a bush plane this seems like a reasonable trade-off to me.
--
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
brian(at)lloyd.com (brian(at)lloyd.com)
+1.767.617.1365 (Dominica)
+1.916.877.5067 (USA)
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