Impossible Turn Testing

[IrishAl]

Jim -- was there an appreciable difference in glide ratio when doing dead stick versus idle?

 

Maybe Jim or some of the other forum members would take a CT up to altitude and do two 360 turns at 45 degrees and 60kts - one with the engine off and one with the engine at idle. The difference in altitude loss would give us the answer.

[Tom Baker]

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?

With a stopped propeller you will have very little drag, just the surface area of the blades. When the prop is rotating it creates a flat plate area the size of the prop disk. How much drag the disk area will create is highly dependent on the amount of drag reduction is being created by the pitch of the propeller blades and amount of power driving them. An airplane with a turbine engine that has a propeller that can be placed in beta or flat pitch will create enough drag the they can point the nose almost straight down with no worries of the speed getting to fast.

[Doug G.]

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.

That says something about force, and torque, but not drag or AoA. You ask a question, give an answer, but no reasoning or explanation of why what you say is correct.

What is the physics behind this?

The speed of rotation enters this equation in some manner as does the angle of the blade.

[Ian]

The paper that Tom referenced earlier ( www.peter2000.co.uk/aviation/misc/prop.pdf )is certainly worth a read - unfortunately it only appears to apply to completely windmilling propellors rather than those with "some" power still applied - his conclusion "it depends" is pretty much what this discussion has concluded too, so we are in good academic company.

 

I think the one think that has been missed in the discussion here is the effect of pitch - I recall when doing my twin rating that single engine performance and getting to the blue-line speed depended quite critically on getting the prop feathered on the failed engine!!

 

Here is the final paragraph from the paper:

 

"The main goal of this investigation was simply to determine whether a stationary or a windmilling propeller has more drag.
The answer is complicatedly simple: it depends. It is clear that it depends on the pitch and length of the propeller, and it is probably
independent of the wind velocity. A crossover point was discovered where the drag forces for the windmilling and stationary states
were the same. This crossover point is also dependent on the pitch, the length, and probably independent of the wind velocity."

[Tom Baker]

The paper was from Fast Eddie.

 

I did mention prop pitch in the post 2 above yours. I know from my model airplane days that if you want to slow a airplane down you put a flatter pitch prop on it. Overall speed will be slower, but deceleration will also be faster. When flying aerobatics the correct prop choice will provide some down line braking.

[Ian]

My apologies to Fast Eddie, indeed the link to the paper was on his post, and to Tom - you certainly have mentioned prop pitch.

 

Of course over this side of the pond we fly with a VP prop, and downwind before turning base, putting the prop to full fine has a really noticable slow down effect.

[cdarza]

Sharing some numbers i had for the 180 degree power off turn. (prop not turning)

 

Test altitude 4000ft.  85degrees

0 Flaps  15 degree bank at 63-65kts -  Lost 500 ft

0 Flaps  30 degree bank at 65-67kts -  Lost 500 ft

Descent rate with wings level was at around 500ft / minute. 

 

In the past, I have taken off and at around 300-400 feet always questioned and thought i could easily make the 180 turn....It visually appears doable --    Not anymore.

[Jim Meade]

Any comments on your choice of airspeed? I wouldn't think we'd be seeking best glide in this situation.

[S3flyer]

Ok - now could one of the CT owners take their bird up and do the same turns with an idling prop as a comparison? Be interesting to know the difference.

 

Side note - I've read a couple articles that a 60 degree turn was optimal as far as altitude loss but 45 degrees was close enough and had less risk of stall/spin.

[IrishAl]

Sharing some numbers i had for the 180 degree power off turn. (prop not turning)

 

Test altitude 4000ft.  85degrees

0 Flaps  15 degree bank at 63-65kts -  Lost 500 ft

0 Flaps  30 degree bank at 65-67kts -  Lost 500 ft

Descent rate with wings level was at around 500ft / minute. 

 

In the past, I have taken off and at around 300-400 feet always questioned and thought i could easily make the 180 turn....It visually appears doable --    Not anymore.

It will be most efficient when done at 45 degree bank, remembering to drop the nose enough to maintain airspeed.  

I believe it can be done in a lot less than 500ft in a CT.

Natural inclination in an engine out: be gentle with it;

Required action: aggressive 45 degree turn will get you the best result.

Just watch your airspeed and your coordination in the turn.

The lowest possible airspeed in the turn will also produce the best result.

[FastEddieB]

I just have to reiterate...

 

Combining an "aggressive 45 degree turn" with "the lowest possible airspeed" has killed a lot of pilots.

 

Speed will likely already be low and decreasing when the turn is begun. History shows that often pilots are loathe to lower the nose enough in the turn, leading to the typical stall/spin scenario. Sudden action also does not always lead to best performance, i.e. that coordinated turn.

 

Just be careful, practice if you want to, but be aware of the hidden dangers.

[WmInce]

I just have to reiterate...

Combining an "aggressive 45 degree turn" with "the lowest possible airspeed" has killed a lot of pilots.

Speed will likely already be low and decreasing when the turn is begun. History shows that often pilots are loathe to lower the nose enough in the turn, leading to the typical stall/spin scenario. Sudden action also does not always lead to best performance, i.e. that coordinated turn.

Just be careful, practice if you want to, but be aware of the hidden dangers.

Concur totally with you Eddie.

[IrishAl]

Not at 4000 feet Eddie - that's where it's safe and where you can learn to do it well,

Practice makes perfect!

[FastEddieB]

Not at 4000 feet Eddie - that's where it's safe and where you can learn to do it well,

Practice makes perfect!

Sure.

 

But best to remember that what we can do successfully at altitude while practicing, may bite us when it's "in anger" from less than pattern altitude.

 

And I played with this the other day. But I still would think long and hard about yielding to the siren song of a return to the runway.

 

But for those who feel skilled and practiced enough to pull it off, go for it.

[cdarza]

Any comments on your choice of airspeed? I wouldn't think we'd be seeking best glide in this situation.

Hmmm - i basically thought of using best glide however, Instructor i was with recommended a higher airspeed for the 30 degree bank as a safety issue.  

[FlyingMonkey]

I have practiced the impossible turn as I related elsewhere.  I also know the danger involved, as well as the danger of even just landing straight ahead.  Most of the "straight ahead" areas off the runways at my home airport are trees and subdivisions.  There is one golf course off one runway.  If I did not find an immediate landing area of sufficient size and terrain at the proper distance, I'd probably pull the chute.  That has the greatest chance of survival anyway, statistically better than any off-airport landing.

[FlyingMonkey]

Hmmm - i basically thought of using best glide however, Instructor i was with recommended a higher airspeed for the 30 degree bank as a safety issue.  

 

I used 60kt.  Plenty above stall speed, even in a 45° turn, to make you not sweat a stall.  Just be aware that 60kt and 45° is *quite* nose low with no power.  Push the nose down first before starting the turn, speed starts to bleed very quickly once the power comes out.  Set your speed, then turn.  You probably will have to lower the nose even more to maintain 60kt once the turn starts due to the increased drag.  

 

This was done at 15° flaps in my case, since that is my normal takeoff config and I will not be playing with flaps in an emergency, other than perhaps to put full flaps in on short final.

[Tom Baker]

Sure.

 

But best to remember that what we can do successfully at altitude while practicing, may bite us when it's "in anger" from less than pattern altitude.

 

And I played with this the other day. But I still would think long and hard about yielding to the siren song of a return to the runway.

 

But for those who feel skilled and practiced enough to pull it off, go for it.

 

Eddie, some have said training in the 180° turn and landing back on the runway is taught as common practice for sailplanes, but I don't know this for fact since I don't have a sailplane rating. Some of the people who have suggested it come from a sailplane background. I do know that coming from a GA background with light airplanes that it is not taught as normal procedure. So the question is, at what point in aircraft performance does the turn back become an option that should be considered?

[Ed Cesnalis]

You need to include altitude AGL when asking this question.  

 

How can it make sense to turn back from 200' from a tow rope break yet go strait ahead from 1,000' in your 2-4 passenger plane?

[Ian]

You need to include altitude AGL when asking this question.  

 

How can it make sense to turn back from 200' from a tow rope break yet go strait ahead from 1,000' in your 2-4 passenger plane?

 

Glide ratio in a modern glider vs a powered light aircraft?

[FlyingMonkey]

You need to include altitude AGL when asking this question.  

 

How can it make sense to turn back from 200' from a tow rope break yet go strait ahead from 1,000' in your 2-4 passenger plane?

 

I think the assumption in the question here is that we fail the engine at an altitude where the answer is not obvious.  100' AGL the answer is obvious, assuming no remaining runway.  at 1000' the answer is obvious, if there is sufficient space from the runway threshold to get turned around.  The 200-700' range I think is where this debate lives most strongly for our airplanes.   

[S3flyer]

I think the assumption in the question here is that we fail the engine at an altitude where the answer is not obvious.  100' AGL the answer is obvious, assuming no remaining runway.  at 1000' the answer is obvious, if there is sufficient space from the runway threshold to get turned around.  The 200-700' range I think is where this debate lives most strongly for our airplanes.   

The debate is interesting and I'm curious of real world examples of what is truly possible.  But.....I'm a conservative pilot and the "debate range" has too many variables built in so MY plan is to turn around at 900' AGL, otherwise forward to the best option with a chute pull.  I MAY pull the chute in doing the turnback to the runway depending on touchdown point (might need an airbrake).

 

This works for MY skill and practice level.

[Jim Meade]

At what minimum altitude will you pull the chute? At some stage, pulling the chute and completing a turn will be conflicting options.

[Tom Baker]

You need to include altitude AGL when asking this question.  

 

How can it make sense to turn back from 200' from a tow rope break yet go strait ahead from 1,000' in your 2-4 passenger plane?

 

At what point in aircraft performance do you go from never attempt this, to maybe it should be a consideration?

If I understand Eddie's opinion correctly he think it should never be a consideration.

[S3flyer]

At what minimum altitude will you pull the chute? At some stage, pulling the chute and completing a turn will be conflicting options.

My GRS has a minimum AGL of 65m (~215ft) so my plan for a controllable aircraft is to fly down to 300' to select the best possible location then pull.