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Glide Ratio

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An idling engine adds more thrust than you would expect.

 

You would have to do this with engine completely shut down.

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3 hours ago, Anticept said:

An idling engine adds more thrust than you would expect.

 

You would have to do this with engine completely shut down.

In my experience while an idling engine does produce thrust, it does not help glide ratio. The velocity of the air moved by the propeller at idle is often less than the forward velocity of the airplane in flight. Because of this the rotating propeller disk produces drag. If you stop the propeller from rotating the glide is improved.

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Specifically in a CT or in some other type?

It's true that a windmilling prop creates more drag when the engine is shut down (the energy from moving the propeller movement is converted to heat due to friction inside the engine), but if it's operating, the engine has to be dumping that surplus energy somewhere. If the air mass is moving faster through the propeller than what the propeller is imparting on it, then this is the same as unloading the prop. So it's going to speed up until the drag comes from something, be it accelerating the air or creating so much trailing edge turbulence... but that trailing edge turbulence doesn't really play a significant factor until at a much higher RPM.

Anyways. It would be an interesting experiment to compare.

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9 hours ago, Anticept said:

Specifically in a CT or in some other type?

It's true that a windmilling prop creates more drag when the engine is shut down (the energy from moving the propeller movement is converted to heat due to friction inside the engine), but if it's operating, the engine has to be dumping that surplus energy somewhere. If the air mass is moving faster through the propeller than what the propeller is imparting on it, then this is the same as unloading the prop. So it's going to speed up until the drag comes from something, be it accelerating the air or creating so much trailing edge turbulence... but that trailing edge turbulence doesn't really play a significant factor until at a much higher RPM.

Anyways. It would be an interesting experiment to compare.

I have not shut the engine down in a CT, but I have in other aircraft. When the CT is at idle the RPM slows down as you slow the airplane, that indicates to me that the forward motion is greater than the airmass being pushed by the propeller. That equals drag.

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Makes sense...sort of like how autorotation in a helicopter slows down a descent 

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I would think it would depend on what your idle set is. When I first got my CT the idle was about 1950 and I think it definitely increased  the L/D. 

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I've landed many times full dead stick. I haven't noticed any real difference in a glide. It may be there, but too small to be noticeable. There are more things that affect the glide far more than this. Things like you have to land in a specific spot so wing, OAT, aircraft weight, pilot skill and speeds, ect... glide is there, but don't depend on any specific number. In an emergency all bets are off and you deal with what' you're dealt.

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I think the primary thing that affects your glide in an emergency, is your ability to de clutter your mind and calmly make immediate and decisive decisions and - Oh Yes - fly the plane.

This can be practiced to the point that if it happens, your confidence/experience level makes the glide and landing a routine experience.  Like US Air landing in the Hudson River. We used to practice that scenario annually at TWA and American.

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On 6/9/2018 at 10:19 AM, Tom Baker said:

I have not shut the engine down in a CT, but I have in other aircraft. When the CT is at idle the RPM slows down as you slow the airplane, that indicates to me that the forward motion is greater than the airmass being pushed by the propeller. That equals drag.

It's slowing down because the prop is loading up. If you think about the air it moves while idling on the ground, what limits the RPM is how much energy you are putting into the prop, which in turn spends it on accelerating the airmass.

However, if the airmas entering the propeller disk is already moving, then it limits how much energy the propeller can impart on the airmass. This works the same if the air is moving or the airplane is. The airmass already has energy in it, reducing the amount of energy that can be accepted (laws of thermodynamics). Either the propeller needs to adjust the pitch to compensate (constant speed propellers), or it needs to do something else. In a fixed pitch, this surplus of energy will accelerate the propeller until it comes back into balance with the energy expenditure.

The airmass will push the propeller only once there is an energy deficit. That means when the engine is shut down, or there is a large enough energy loss elsewhere, such as higher prop RPM and the subsequent losses from turbulence drag caused by the high speed movement.

Now, when I say idle has a bigger affect than one thinks, I am referring to the fact that one might assume there is *no* effect from idle. I'm not going to state in all cases that there is a surplus or deficit of thrust, but often there is a surplus that might give a few hundred feet to a few thousand feet horizontally to a vertical mile. I'd be very interested in seeing more tests, because there's a lot of factors in play, possibpy the most significant is prop pitch and idle settings.

 

Ben29k: autorotation slows the decent because you are trading vertical velocity for rotor disk rotational energy. In turn, as it rotates faster and faster, and due to the absolute sheer size of the disk, you're going to be dumping some of that energy into friction forces in the helicopter and in the turbulent airstream being generated.

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I have shut down the engine in flight and found better glide with the prop stopped with  -12 flap at best speed for weight say about 75 kt.

                                                                                                                  almost no difference with 0 flap at best glide for weight say about 63 kt.

 Also notice a longer float in the flare with the engine running compared to landing dead. This suggests that at idle thrust is more than drag at low speed and the reverse is also true.

Makes sense to me, open to other ideas though.

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When the emergency arrives and you have to deal with a different glide ratio than you got when you practiced there is a built in fix.  The fix is the sight picture.

 

Personally I am not going to head to a field that is visually rising in my view as I approach.

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On 10.6.2018 at 3:13 AM, Anticept said:

Now, when I say idle has a bigger affect than one thinks, I am referring to the fact that one might assume there is *no* effect from idle. I'm not going to state in all cases that there is a surplus or deficit of thrust, but often there is a surplus that might give a few hundred feet to a few thousand feet horizontally to a vertical mile. I'd be very interested in seeing more tests, because there's a lot of factors in play, possibpy the most significant is prop pitch and idle settings.

Corey, I will do the same test with engine stopped and prop without windmilling. I expect a small amount of a better glide if the engine is stopped. My expectation comes from the experience, that a windmilling engine at idle can lead to a surplus of thrust, if the idle RPM at ground is high. The CT I flew for the test has a idle RPM at ground of about 1600. I am convinced, that this produces drag and not thrust. We will see.

I need to wait for better weather and will test again.

Markus

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You have a really low set idle. I can't keep the engine running for long at 1600.

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Wind milling prop has a major drag issue. During a training flight in my 310 the instructor pulled my right engine to idle. I was downwind about one mile from the runway and was told to land without feathering the prop. At full power on the left engine I could not make the runway. He said never forget this lesson. I would never have believed it if I hadn't done it

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310's are quite a bit different from a CT, and it's why I had asked Tom if he was referring to a CT vs another aircraft. I wouldn't put it past a lycoming or continental being draggy, especially with the size of a 310 propeller since their idle is extremely low by comparison.

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40 minutes ago, Anticept said:

310's are quite a bit different from a CT, and it's why I had asked Tom if he was referring to a CT vs another aircraft. I wouldn't put it past a lycoming or continental being draggy, especially with the size of a 310 propeller since their idle is extremely low by comparison.

Cory, you can't think in terms of idle speed, you need to think in terms of propeller speed. The propeller speed is about the same, if not slower for the Rotax. Propeller diameter will be about the same for comparable HP direct drive engines.

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I already did the math. With the idle speeds I typically set (1800), the prop is 740 rpm. That's 15-20% higher over typical settings for other engines. I generally accept an idle RPM of 1800 to 1850.

As said, when I saw 1600 rpm, I was surprised. I have a real difficult time keeping an engine running at that RPM for a ULS. They feel like they are going to shake off their mounts and after 20 seconds or so, the RPM drops and quits. I have this happen on several aircraft.

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1 hour ago, Anticept said:

They feel like they are going to shake off their mounts and after 20 seconds or so, the RPM drops and quits.

yes, I know this behaviour. However at 1400 and below. Hmmm, how accurate is our tachometer? At 1600 all ULS in my hangar are smooth and stable.

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1 hour ago, Anticept said:

As said, when I saw 1600 rpm, I was surprised. I have a real difficult time keeping an engine running at that RPM for a ULS. They feel like they are going to shake off their mounts and after 20 seconds or so, the RPM drops and quits. I have this happen on several aircraft.

When I was still learning my CT's behavior, I preferred an idle of around 1600-1650.  I never had any problem with keeping it running, just with gearbox noise.  On the ground I'd give it a little throttle to keep the RPM above the yellow, but I liked the lower residual thrust at idle when landing as it helped me manage the energy better.  Now that I'm more skilled at approach and landing in the CT and managing speed and energy, I set the idle around 1800rpm, which I think is pretty typical for CTs.

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10 minutes ago, NC Bill said:

I CLAIM THE RECORD!

For the longest living forum subject to date at 7 YEARS, 2 MONTHS and 17 DAYS :clap-3332:since March 27, 2011.

 

Congratulations, Bill.

I had no idea you had been on here that long.

Now . . I feel like a guy. . . . :D

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18 hours ago, NC Bill said:

I CLAIM THE RECORD!

For the longest living forum subject to date at 7 YEARS, 2 MONTHS and 17 DAYS :clap-3332:since March 27, 2011.

 

How about the most viewed?

 

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