Glide Ratios

[carusoam]

Seth,

What would restore the prop to spinning again, if you decide to push in the blue knob and start the engine rotation. Does it work?

The oil pressure going to zero, feathers the prop, as apposed to most other Mooneys, the prop goes to most flat pitch.

Would it require turning the key to rotate the prop, build some oil pressure and re-flatten the pitch?

Or did I way over think your ship's inverse relationship?

I like the missile's glide ratio.

Best regards,

-a-

[xftrplt]

That is correct.  My Missile has a fully feathering prop, though I have never shut the engine down in flight to test it.  The glide ration according to Rocket Engineering is 16:1 with the prop feathered.  Personally, I'd plan 3 miles for every 1000 feet, but I'd want to test that first on my own to see if my plane was indeed capable of the STCed 16:1 ratio.

 

Have any Rocket or Missile owners on this board ever feathered their props in flight?

 

-Seth

3 NM/1000ft = 18/1

You sure you want to plan for that?

[RJBrown]

I worked with Rocket back in the 90s and talked to the test pilot about this issue. According to him the data is real. I never took the opportunity to verify it personally though. Sometimes in the Rocket I would feather the prop on shutdown just to see if I got a reaction from someone on the ground. I often got that "somethings different but I don't know what" look from the linemen.

[aaronk25]

Perhaps so.  I have shut the fuel off several times inflight in our plane, anywhere near glide speed the prop stabilizes at ~1700 RPM and the blue knob does nothing. It also wont do anything below ~1900 on the ground during a runup. It is a stock ('78 and up) McCauley C214 that has been in the shop 130 hours ago and checked out fine, and this is the second engine and second prop governor that is plugged into it.

 

I have also fooled around with super low speed cruise and the prop will only govern down to 1950 RPM, then the knob hits the rear stop.

Thats got me thinking...the plane I was flying was a 82' J "round tip prop" and at 85Kts when I pulled the blue knob all the way out to the stop with engine at idle it was idling 700rpm.   The idle on this plane is set a at 700 on the ground too, a bit low.  

 

Prop was just flipping by......wonder why yours doesn't go any lower?  I'll have my 77j with the round tip prop back end of the week and I'll try it in that and see what it does. 

 

 

Aaron

[Seth]

3 NM/1000ft = 18/1

You sure you want to plan for that?

Thank you.  Good point, I was thinking statue miles but on all our charts it's NM.  That right there is breaking a link in the accident chain.

[aviatoreb]

Thank you.  Good point, I was thinking statue miles but on all our charts it's NM.  That right there is braking a link in the accident chain.

 

Ummmm....I was roughing out 3 mi/1000ft as 16:1 also - but quite right as that is using 5280ft/mi but that is statute miles indeed.  So Seth - don't feel bad - I am supposed to be a math prof and I goofed too.

[aviatoreb]

That is correct.  My Missile has a fully feathering prop, though I have never shut the engine down in flight to test it.  The glide ration according to Rocket Engineering is 16:1 with the prop feathered.  Personally, I'd plan 3 miles for every 1000 feet, but I'd want to test that first on my own to see if my plane was indeed capable of the STCed 16:1 ratio.

 

Have any Rocket or Missile owners on this board ever feathered their props in flight?

 

-Seth

 

I have - during my 10 hours of check out with my CFI, about 10 mi out but lined up for a runway we agreed to try and feather, while the engine was in idle but with forward blue knob.  Pulling the blue knob all the way back, Two things happened.  1 the plane practically felt like it leaped upward as the glide ratio increased dramatically.  It was amazing.  It felt like we were suddenly rocketing upward.  It really didn't feel as if it was decending at all.  It really can glide.  2 - shortly there after the engine in idle was having a hard time turning the now full feathered prop and began to sputter and shut down.  Ooops - just as it shut off pushed in the black and push in the blue and she coughed back to life.  It was a good experience to know what happens but I wouldn't do it again unless lined up for a good landing site.  I would recommend it though Seth to do once so you know.

[jetdriven]

Part of that is the momentary boost of thrust as the prop goes into feather. What was the steady-state?

[aviatoreb]

Part of that is the momentary boost of thrust as the prop goes into feather. What was the steady-state?

 

What boost effect is this Byron?  I never heard of it.  Please explain.

 

I took it just to be a sudden change of decent angle feeling as if it were a climb.  I think we were in the higher glide ratio for more than an instant - enough time to get a feel for an impressive glide but I do not have instrumentation to tell you directly the glide ratio and I was not carefully enough recording the VSI and indicated speed and so forth to compute it exactly.

[jetdriven]

I have only done it in twins, but if the prop is turning 1000 RPM or so, when it starts to feather, it turns that rotational energy into a momentary boost of thrust as the prop increases pitch to 45-50-75 degrees. When the prop stops, the drag is less than a windmilling prop, but the transient effect is a momentary boost.

[aviatoreb]

Makes sense. Well that boost effect is really striking in a small airplane with a big prop in front.

[Seth]

i'll have to give it a shot sometime.  However, I'll make sure to have a Mooney CFI with me the first time as well as the procedures for starting a dead engine out while a few thousand feet over an airport with a long runway (preferably multiple runways).  I'll look in the POH, but I can't remember if I have to crank to get it turning in order to change the pitch in the prop.  I'll look over that the next time I'm at the plane (and I'll remind myself to make a copy of the POH for my files).  My guess is I have to crank the starter if my prop was stopped.

 

-Seth

[aviatoreb]

i'll have to give it a shot sometime.  However, I'll make sure to have a Mooney CFI with me the first time as well as the procedures for starting a dead engine out while a few thousand feet over an airport with a long runway (preferably multiple runways).  I'll look in the POH, but I can't remember if I have to crank to get it turning in order to change the pitch in the prop.  I'll look over that the next time I'm at the plane (and I'll remind myself to make a copy of the POH for my files).  My guess is I have to crank the starter if my prop was stopped.

 

-Seth

 

Hi Seth, Please report what you learn regarding restart if the prop is in full feather.

[WardHolbrook]

Guys, I don't think that there is a sudden, short-term increase in thrust when a prop goes into feather. A windmilling propeller has the same amount of drag as a flat plate of the same diameter. I think what you're experiencing is the sensation of acceleration as all of that drag from the windmilling prop is removed. For you Missile owners that might be tempted to try it in flight, I'd recommend that you check the feathering on the ground first - just to make sure it still works as advertised. I've flown more than one twin that one or both propellers wouldn't feather because something was mis-rigged. Did some of the Missile conversions retain the optional unfeathering accumulators from the original installations? If so, that would make restarting inflight a bit simpler. 

[Seth]

Because I've never thought about it before, until now, and some have mentioned it in this thread, can it hurt your engine to feather it on the ground?

 

I though you were not supposed to turn off your engine except for with the prop knob forward.

 

-Seth

[Awful_Charlie]

I don't think that there is a sudden, short-term increase in thrust when a prop goes into feather. A windmilling propeller has the same amount of drag as a flat plate of the same diameter.

 

Sorry, I disagree with that a bit.  A windmilling prop absorbs a lot of power into the engine, in addition to the drag - if you don't believe it, on the ground pull the prop through at 500+RPM and see how much of a sweat you break out in (best turn the mags off first though eh!)  The slower you can get the prop to turn, the less power it is absorbing (pull the prop through at 100 RPM - much easier!)

[bd32322]

Sorry, I disagree with that a bit.  A windmilling prop absorbs a lot of power into the engine, in addition to the drag - if you don't believe it, on the ground pull the prop through at 500+RPM and see how much of a sweat you break out in (best turn the mags off first though eh!)  The slower you can get the prop to turn, the less power it is absorbing (pull the prop through at 100 RPM - much easier!)

I think what you are calling as ansorption of power, wardholbrook is calling drag. Drag 1 for the airstream to actually turn the propeller and drag 2 is the effect of a windmilling prop to act like a flat plate of some size.

I think thats what he meant

Good point however

[WardHolbrook]

A windmilling propeller is not absorbing energy (power), it's creating drag. If it was absorbing there would be heat involved. Brakes absorb energy (convert to heat).

[carusoam]

I would consider it like downshifting into a corner. Energy is adsorbed by compressing air and expelling it as heated exhaust.

Or similar. It works for my F-body anyway...

Down shift prior to turning base, ready to accelerate on the go-around.

Best regards,

-a-

[N601RX]

After reading some of the earlier post I decided to see how my plane responded while flying yesterday  I went to idle and pulled the prop all the way back.  My RPM went to 650 immediately. I have a Hartzell prop and governor.

[RJBrown]

Drag does absorb energy.

 

A windmilling propeller is not absorbing energy (power), it's creating drag. If it was absorbing there would be heat involved. Brakes absorb energy (convert to heat).

Drag is friction between the plane and the air. It does create heat. It does convert kinetic energy to heat through friction just like brakes. That heat is then dissipated into the surrounding air in both cases.  The term absorb is rather inaccurate in either situation but does loosely describe what is happening. Brakes convert kinetic energy into heat through friction. The heat generated is then dissipated into the surrounding air. Brakes do their job by dissipating energy not absorbing it.

[bd32322]

Drag does absorb energy.

 

Drag is friction between the plane and the air. It does create heat. It does convert kinetic energy to heat through friction just like brakes. That heat is then dissipated into the surrounding air in both cases.  The term absorb is rather inaccurate in either situation but does loosely describe what is happening. Brakes convert kinetic energy into heat through friction. The heat generated is then dissipated into the surrounding air. Brakes do their job by dissipating energy not absorbing it.

And the air then transfers the heat to water molecules, forms large cumulonimbus clouds, which in turn transfer energy back to an unlucky aircraft - circle of life complete.

Sorry - in a 3rd rate philosophical mood today

[aaronk25]

So tried the glide on my 77j after doing it last week on a 82j the both ha e round tip props, my 77 would only go down to 1320rpms with the prop full aft. The 82 when down to 650rpms in the glide. The difference is my 77 required 700fpm decent where the 82 required only 550fpm same weight.

In a real engine out I would pull the nose up close to stall and stop prop if I needed the glide distance.

[Super Dave]

I've always wondered if opening the throttle would make any significant differance in getting a prop stopped on a gliding airplane. I know that my '72 Beetle used to be harder to turn over with the gas pedal to the floor.

I've also wondered at what speed the stopped prop would start spinning again.

Anyone ever tested either of these out?

[bumper]

I stopped my Husky prop (O-360, 180 horse) at 14.5K climbing in mountain wave. Pulled up almost to the stall to get the prop stopped. Quickly decided I shouldn't leave it stopped long as it was sub-zero, so  I pushed over nose down but prop wouldn't windmil. Didn't want to get too close to indicated Vne at that altitude (I was over 16K by then), so reverted to starter. On a previous 7AC Champ w/ Sensenich wood prop, it took almost Vne (129 mph) in a dive to get it to re-light - - no starter and good compression on that one.

 

bumper