Help please
Thread Starter
Join Date: May 2000
Location: Phuket
Posts: 297
Likes: 0
Received 0 Likes
on
0 Posts
Help please
I am sure this has already been talked about waayy too much however here goes. I already read the discing thread and got no definitive answer. Here is the scenario. Forget the dark and stormy night bit. Multi engine aircraft with a variable pitch propellers. Engine fails with no restart as an option. The failure is in such a way it will not feather either and it is windmilling. You are unable to maintain altitude and there is a ridge line in front of you. Turning around or landing there in the valley is not an option either. You must cross that ridge. After setting up best glide and cleaning up all you can you start figuring glide angles/path you will be just short of the crest if everything else stays constant.
Here is the question....shall you slow down the aircraft (by trading altitude for airspeed) until the prop stops then regain the airspeed or keep going as is and hope for the best? The big question is will a stopped prop produce (NOT feathered) less drag then a windmilling one?
I have heard a lot of conjecture and no supporting trials. I think a stopped one will produce less drag. What say you? (With references)
Jerry
Here is the question....shall you slow down the aircraft (by trading altitude for airspeed) until the prop stops then regain the airspeed or keep going as is and hope for the best? The big question is will a stopped prop produce (NOT feathered) less drag then a windmilling one?
I have heard a lot of conjecture and no supporting trials. I think a stopped one will produce less drag. What say you? (With references)
Jerry
Join Date: Jun 2000
Location: UK
Posts: 683
Likes: 0
Received 0 Likes
on
0 Posts
I tend to believe that a stopped propeller will usually produce less drag. Certainly, my single-engine experience would seem to bear this out in that glide angle certainly seems to improve with the prop stopped compared with it windmilling under no power (i.e. fuel off).
Against this is the fact that you will usually need to reduce speed significantly below VMD (Min-Drag speed) before the prop will cease windmilling due to forward speed, with the result that your net glide angle is increased during that time.
However, you may wish to read this thesis, which may help you to decide for your particular set of circumstances ...
Hope this helps ...
JD
Against this is the fact that you will usually need to reduce speed significantly below VMD (Min-Drag speed) before the prop will cease windmilling due to forward speed, with the result that your net glide angle is increased during that time.
However, you may wish to read this thesis, which may help you to decide for your particular set of circumstances ...
Hope this helps ...
JD
Prop drag
Hello BLCL and Jumbo
My feeling has always been that a stationary prop will generally produce less drag than a windmilling one, and that the finer the pitch the greater the difference will be. This is based on my own, not very scientific, experiments on an Aztec.
With the prop in full coarse pitch (min rpm) there was not much difference between windmilling and stopped prop, so if you can't feather the prop then the next best thing is to set it at the coarsest pitch available.
MJ
My feeling has always been that a stationary prop will generally produce less drag than a windmilling one, and that the finer the pitch the greater the difference will be. This is based on my own, not very scientific, experiments on an Aztec.
With the prop in full coarse pitch (min rpm) there was not much difference between windmilling and stopped prop, so if you can't feather the prop then the next best thing is to set it at the coarsest pitch available.
MJ
Thread Starter
Join Date: May 2000
Location: Phuket
Posts: 297
Likes: 0
Received 0 Likes
on
0 Posts
Driver, I saw that also and I was disappointed with the "depends" answer. Mach dude is right about the pitch of the prop when it stopped and that is why the answer is "depends" I suppose.
Thanks guys
Thanks guys
Join Date: Oct 2004
Location: UK
Posts: 762
Likes: 0
Received 0 Likes
on
0 Posts
My opinion
Principles of Flight/Aircraft Gen was a headache when I did my ATPL's and it still is now.
I wonder if anyone has been in this type of position. It would be great to hear the story.
All i know is what everyone else said; feather and Vmd good.
I wonder if anyone has been in this type of position. It would be great to hear the story.
All i know is what everyone else said; feather and Vmd good.
Join Date: Mar 2006
Location: London
Age: 48
Posts: 53
Likes: 0
Received 0 Likes
on
0 Posts
Wow...this is a tough one.
Just some of my ideas......
1. Burn of fuel by circling within the valley to reduce weight. It'll buy you some thinking time and maybe allow you to feather the prop correctly or stop the prop. If it still won't feather then just fly it as safely and accurately as possible. No more than 10 degrees AOB. Declare the emergency 121.5 They might be able to help with a plan or a divert alternate that you don't know about. Get all the help you can.
2. Did you plan a good safe altitude above the ridge? What is your planned MSA? Did you work out your single engine climb performance before you went flying at full weight at the altitude you planned to fly? The POH should be able to tell you if you can maintain an altitude on one engine at a specific weight. If you can't then you should not have even been flying in that region in the first place and only have youself to blame.
Personally, I wouldn't just aim for the ridge and hope for the best.
Lots to think about though
Just some of my ideas......
1. Burn of fuel by circling within the valley to reduce weight. It'll buy you some thinking time and maybe allow you to feather the prop correctly or stop the prop. If it still won't feather then just fly it as safely and accurately as possible. No more than 10 degrees AOB. Declare the emergency 121.5 They might be able to help with a plan or a divert alternate that you don't know about. Get all the help you can.
2. Did you plan a good safe altitude above the ridge? What is your planned MSA? Did you work out your single engine climb performance before you went flying at full weight at the altitude you planned to fly? The POH should be able to tell you if you can maintain an altitude on one engine at a specific weight. If you can't then you should not have even been flying in that region in the first place and only have youself to blame.
Personally, I wouldn't just aim for the ridge and hope for the best.
Lots to think about though
Thread Starter
Join Date: May 2000
Location: Phuket
Posts: 297
Likes: 0
Received 0 Likes
on
0 Posts
Thanks Bob and you are right on all the above however that was a political answer.(Just kidding) "The big question is will a stopped prop produce (NOT feathered) less drag then a windmilling one?"
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
A windmilling propeller will always absorb more energy and cause more drag than a stopped propeller, feathered or otherwise. If the slipstream is performing work on the propeller and making it turn, overcoming the engine's internal resistance and it's flow being altered by the propeller disc, then you've got a higher drag operation than the same propeller sitting still.
The "depends" part comes into play when you consider the benifits of stopping vs. allowing it to windmill as you perform your drift-down. It depends on how much energy and altitude you'll lose trying to get the prop stopped. If you consider that you're starting from your best-glide condition, then altering it to a higher or lower airspeed in will increase descent and increase drag. You may be able to quickly stop the prop, or it may take some time. How much altitude is lost, how much descent takes place beneath your best glide flight path, will determine if you've lost or gained anything.
Generally speaking, you don't want to deviate from your glide to try to stop that prop if you don't have a means of feathering it, or if it doesn't stop on it's own.
I've experienced engine failures involving props that didn't feather, and wouldn't feather, several times. I've never attempted to slow the airplane down to get the engine to stop turning. Most of the failures have occured at low altitude and I didn't have the option of playing with it. In most cases other engines have enabled the airplane to return to land; in one noteable case I ended up on the ground not far from where the engine quit. In that particular case there was little time from the loss of the engine until contact with the surface, and there was little to be done from such a low altitude.
Most of those cases took place in very mountainous terrain. In most cases I wouldn't have attempted to slow the airplane to stop the propeller, including cases where it happened in single engine airplanes.
Night, narrow canyon descending with questionable terrain clearance, attempting to slow the airplane and fly while concentrating on stopping the propeller is recipe for trouble. A better choice is to backtrack a little in the scenario, and make better choices that won't put one in that position to begin with.
The "depends" part comes into play when you consider the benifits of stopping vs. allowing it to windmill as you perform your drift-down. It depends on how much energy and altitude you'll lose trying to get the prop stopped. If you consider that you're starting from your best-glide condition, then altering it to a higher or lower airspeed in will increase descent and increase drag. You may be able to quickly stop the prop, or it may take some time. How much altitude is lost, how much descent takes place beneath your best glide flight path, will determine if you've lost or gained anything.
Generally speaking, you don't want to deviate from your glide to try to stop that prop if you don't have a means of feathering it, or if it doesn't stop on it's own.
I've experienced engine failures involving props that didn't feather, and wouldn't feather, several times. I've never attempted to slow the airplane down to get the engine to stop turning. Most of the failures have occured at low altitude and I didn't have the option of playing with it. In most cases other engines have enabled the airplane to return to land; in one noteable case I ended up on the ground not far from where the engine quit. In that particular case there was little time from the loss of the engine until contact with the surface, and there was little to be done from such a low altitude.
Most of those cases took place in very mountainous terrain. In most cases I wouldn't have attempted to slow the airplane to stop the propeller, including cases where it happened in single engine airplanes.
Night, narrow canyon descending with questionable terrain clearance, attempting to slow the airplane and fly while concentrating on stopping the propeller is recipe for trouble. A better choice is to backtrack a little in the scenario, and make better choices that won't put one in that position to begin with.
Join Date: Jul 2007
Location: France
Posts: 610
Likes: 0
Received 0 Likes
on
0 Posts
Simple way to know you will clear the ridge
Before landing check list:
Here is a simple way to know when you are high enough to clear the ridge. When you can see some terrain on the other side of the ridge. It works day and night and especially in places like Papua New Guinea.
Tmb
Here is a simple way to know when you are high enough to clear the ridge. When you can see some terrain on the other side of the ridge. It works day and night and especially in places like Papua New Guinea.
Tmb
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
If you can see lights or terrain on the other side of the ridge, you'll clear the ridge given your present altitude and approach to the ridge. However, if you experience a change such as a change in rate of descent, you can easily descend below the ridgeline.
If lights begin to disappear, you're going below the ridge and you're not going to make it. Conversely, if lights or the amount of terrain visible increases, you're clearing the ridge.
If you're approaching the ridge on the leeward side (downwind), then you may experience descending air as you approach the ridge. If you're in an engine-out minimal performance situation, then approaching the ridge with descending air may mean that the approach to the ridgeline looks good, but will unexpectedly change for the worst.
The usual mountain techniques apply, including crossing the ridge at an angle. Making your approach from the side of the valley instead of down the middle gives you turning options. Planning before hand so that one isn't in a minimum performance situation in a box canyon or tight valley is the key; preventing the situation from happening is far better than getting there and then trying to salvage the potentially impossible. If you must turn, and you're not going to be able to climb, reducing speed reduces the radius of the turn, and a climbing or descending turn will take a little less radius than a level one.
Application of flaps in the turn can reduced the radius in an emergency, and well as improve your stall margin. However, in the scenario in discussion you're talking about an engine-out, and we have assymetrical thrust issues. You can see how attempting to slow to stop the prop may compromise your options with crossing the ridge. If you need to turn to get back, and can't cross the ridge, then retarding power on both engines and making a tight turn back may be in order.
If lights begin to disappear, you're going below the ridge and you're not going to make it. Conversely, if lights or the amount of terrain visible increases, you're clearing the ridge.
If you're approaching the ridge on the leeward side (downwind), then you may experience descending air as you approach the ridge. If you're in an engine-out minimal performance situation, then approaching the ridge with descending air may mean that the approach to the ridgeline looks good, but will unexpectedly change for the worst.
The usual mountain techniques apply, including crossing the ridge at an angle. Making your approach from the side of the valley instead of down the middle gives you turning options. Planning before hand so that one isn't in a minimum performance situation in a box canyon or tight valley is the key; preventing the situation from happening is far better than getting there and then trying to salvage the potentially impossible. If you must turn, and you're not going to be able to climb, reducing speed reduces the radius of the turn, and a climbing or descending turn will take a little less radius than a level one.
Application of flaps in the turn can reduced the radius in an emergency, and well as improve your stall margin. However, in the scenario in discussion you're talking about an engine-out, and we have assymetrical thrust issues. You can see how attempting to slow to stop the prop may compromise your options with crossing the ridge. If you need to turn to get back, and can't cross the ridge, then retarding power on both engines and making a tight turn back may be in order.
Thread Starter
Join Date: May 2000
Location: Phuket
Posts: 297
Likes: 0
Received 0 Likes
on
0 Posts
Yes to all of the above and I already knew that but does anyone have any references about the propeller question? I do have an airplane ATPL with CFI/II so airplanes are not new to me however I do not have a physics background. I need to fess up though. This whole thing began with a sailing (yes sailing) question. When your auxillary is shut off and when you are under sail is less drag (through the water) produced with the propeller free wheeling or stopped when the aux is in gear? Since air and water are both fluids and reffering to my aviation experience (and yours) I thought I knew the answer.
Jerry
Jerry
Join Date: Jul 2000
Location: My views - Not my employer!
Posts: 1,031
Likes: 0
Received 0 Likes
on
0 Posts
A mate of mine used to slope soar a c172 (for a laugh). He said they used to reduce airspeed to stop the engine primarily to reduce engine wear but it also improved performance.....
So yes it would improve things but you would have to balance the loss of performance (from deviating away from best se climb speed and recovery) with the distance to go to the ridge and the amount of improvement in climb rate...It may not give a better result if you are close.
I would go for chucking your passengers out!
So yes it would improve things but you would have to balance the loss of performance (from deviating away from best se climb speed and recovery) with the distance to go to the ridge and the amount of improvement in climb rate...It may not give a better result if you are close.
I would go for chucking your passengers out!
Join Date: Aug 2000
Location: Near Stuttgart, Germany
Posts: 1,096
Likes: 0
Received 1 Like
on
1 Post
Hello!
Even with a physics background (or aerospace engineering in my case) your question is nearly impossible to answer in a general manner. There are too many unknowns there to start making calculations. Who has ever measured the coefflicient of drag of a standing, unfeathered propeller blade? For all possible angles and airspeeds? For your specific propeller! You could write down some formulas, but without these basic input numbers you will not get reliable figures from them...
Next, the force required to turn over your engine by the airflow is also variable and depends on things like airspeed, RPM (that again will also depend on the angle the blades got stuck at!) and mostly engine temperature: In the beginning, when the engine is still hot and the oil thin, it will generate _much_ less drag than after a few minutes when it has cooled to ambient temperature. Especially if these are below freezing.
I think, the only way to find out is to perfom the experiment in the real aeroplane. In your specific aeroplane, at your specific cruising altitude... Far away from the mountains of course
Greetings, Max
NB: My "gut feeling" tells me that the standing propeller generates less drag - but the difference is probably not big!
I do not have a physics background
Next, the force required to turn over your engine by the airflow is also variable and depends on things like airspeed, RPM (that again will also depend on the angle the blades got stuck at!) and mostly engine temperature: In the beginning, when the engine is still hot and the oil thin, it will generate _much_ less drag than after a few minutes when it has cooled to ambient temperature. Especially if these are below freezing.
I think, the only way to find out is to perfom the experiment in the real aeroplane. In your specific aeroplane, at your specific cruising altitude... Far away from the mountains of course
Greetings, Max
NB: My "gut feeling" tells me that the standing propeller generates less drag - but the difference is probably not big!
Join Date: Jan 2007
Location: ...FL510
Age: 43
Posts: 65
Likes: 0
Received 0 Likes
on
0 Posts
Already tried
Consider... as the prop has stopped you have to accelerate to the V of best glide. The propeller may start to windmilling again... it happened to me in a single engine (for training purpose ). If that happen now you have two problems instead of one: you loose altitude and still a propeller in windmilling.
Just a tough: what about to help the prop with the starter ???
). If that happen now you have two problems instead of one: you loose altitude and still a propeller in windmilling. Just a tough: what about to help the prop with the starter ???
