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There’s been a bit of discussion (argument) recently regarding the difference between glide ratios at idle power vs with prop stopped. I’ve long known from experience that the glide ratio with prop stopped is less than at idle, but decided to do real measurement tests to settle the argument.

With a Rotax engine there’s no option to keep the prop spinning; it’ll stop and stay stopped very quickly after switching off at glide speed. In an 80kt dive the prop will creep slowly from one compression stroke to the next, but no way spin.
In this test, the idle speed static on the ground was 1670rpm, at 50kts glide speed it was 2350rpm.

All tests done by timing with stopwatch the descent from 3000 to 2000 QNH, early early morning before any thermal activity. Three attempts at idle and three at prop stopped, with the average used for calculations. Just watching a VSI is not nearly accurate enough for this exercise.
For the Savannah, 50kts is the speed for best glide ratio, as determined by earlier testing.
Times recorded for these tests are - at idle 101 seconds, at prop stopped 94 seconds.

At idle -           101sec/1000ft = 594fpm descent rate = 8.5:1 glide ratio @ 50kts.
Prop stopped - 94sec/1000ft = 634fpm descent rate = 7.9:1 glide ratio @ 50kts.

So for each 1000 feet of descent, the aircraft will glide 8,500ft at idle or 7,900ft with prop stopped. So that’s 600ft (182m) less distance at prop stopped than at idle.

That may not seem like much until you’re on a final with prop stopped and find that you’re falling short of you’re aiming point….. So need to aim farther from the intended touchdown point to allow for that difference. Quite a bit farther in practice, and then slip off height to hit the chosen point.

That’s if you’re practiced at idle power approach already, rather than using some power to maintain a glide slope. If you’re using power regularly then you aren’t at all prepared for a deadstick landing…. I always pull power to idle at the start of base leg and don’t change that right to touchdown – good practice. I only need to change that when someone ahead is tying up the circuit with a long powered final….

I’ve done dozens of prop-stopped landings and really value the experience and the confidence that builds. I realize that we’re supposed to only do that with an instructor on board, but I know from early training what cautions are required and follow those lessons carefully. I reckon everyone should get that experience. A convenient way to do that would be when you next do a BFR to ask the instructor to let you do a couple of dead stick landings – you could learn a lot.….. If at first you can’t hit your chosen spot then do it again and again until you know how the picture should look on approach.

But one tip: As soon as the prop fully stops after switch-off, throw the switches back ON to be ready for a re-start. If you don’t, you’re liable to get a heart attack when you need power again and crank the engine and it doesn’t start right away…..

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There’s been a bit of discussion (argument) recently regarding the difference between glide ratios at idle power vs with prop stopped. I’ve long known from experience that the glide ratio with prop st

Interesting clip.   Two observations from the benefit of my arm chair. First I was interested to see a tricycle undercarriage nose over like that  in the rough.   Second, I dunno t

Did you learn anything you could apply to your skills base, re planning glide approaches generally? I'd say not much and if landing right on the keys is your aim you better "unlearn" it. Very few inst

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It's a similar  % effect to a mild headwind. ALL forced landings should plan to land say 300 metres into a field and reduce it when assured of making it if you are able. To not reach your chosen field is a "fail" really. At some place where you are not "knowing" of the exact height of the terrain, (the most likely real situation) you just will not be able to determine your height above the ground and will have to estimate it, so practice at a new place and cover the alt would be to your benefit for realism  As I've previously mentioned the cowl may be damaged  too (Lots more drag). the prop may have left the plane(less drag) Realise what you have going for you when some event happens and adapt to it. Very early in your glide you will have to assess whether you are definitely making it IF NOT assured the quicker you form plan B the better. Avoid hitting something very solid and land into wind (min actual speed) and DON'T lose control. . If an outlanding is required it's better to have the engine running  rather than runout of fuel before finding the perfect paddock. having flown over acceptable ones eventually being forced to land on a really bad one. Nev

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Hello John, This is EXACTLY what i was saying. 

 

If you practice with engine at idle its going to give you BETTER performance than with prop stopped, forget all the drag of a rotating propeller crap, that's just from people who don't understand simple physics.

 

So, when the engine does actually stop you will get WORSE performance than with everything you practiced previously.

 

Thank you for validating what i wrote, which i simply parrot replied here from a really experienced guy at the airfield.

 

 

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I've always said a rotating prop has the POTENTIAL to create a lot more drag than a stopped one and I'll stick to it as I know it's a proven fact. Your example is just ONE plane and quite a slow one. It's not indicative of ALL.  Your prop won't windmill. Geared Rotax's don't. There's others  that are DIFFICULT to STOP turning. even though the engine is delivering NO power whatso ever. .  For someone who is relying SOLELY on your recollection of "a really experienced guy at the Airfield".  you are pretty happy to call others views crap from who don't understand simple Physics.. Some things that seem intuitively right  in aviation, don't always turn out that way in practice. Nev

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Maybe you don't really know as much as you think you do ? 

 

But its not meant as an insult, just experience  with a very small aircraft segment perhaps ?

 

The aircraft is was referencing cruises at 147 knots, stalls at 34 knots. My recollection from the guy at the airport was supported as I said in previous correspondence by a follow-up phone call last week to refreshment I memory after the original post.

 

Anyway, I digress.   This is not a pissing competition it was just an explanation about why very experienced pilots who should  be able to make it back to the airfield don't, they come up short and the difference is that the performance of the aircraft with a propeller stopped is less than the performance of an aircraft with the propeller at idle speed which is what we train for and this as I was highlighting maybe the reason that so many experienced pilots get caught out and why many people think it is a good idea to occasionally go out (at altitude) and turn the engine off, propeller stopped and just get a visual outlook on the descent profile and feet per minute descent.  not every aircraft is a same but at least if you know you can be prepared

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My insurance for any landing is to be high and slip to lose height.
if you lose your engine aim to have the field selected easily within reach and stay high. The great joy of slipping into a landing is that all you do is let go of the forces acting against you on the stick and rudder and immediately you flatten the glide and can increase speed..

The problem with flaps is that they give a great increase in drag and reducing flap may reduce drag, but it also reduces lift and there is a change in attitude required.

I wonder how many here are competent at slipping an aircraft, which is a fun thing to do. No need to be scared as if it all goes bad, just let go of the stick and remove feet from rudder for a moment.

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Yes  Flyboy maybe it's my experience with only a small aircraft segment. I'm not interested in a pissing contest either.  I really only post here to try to make flying safer and encourage the gaining of more UNDERSTANDING of aircraft , not to just pass exams or make arguments .

  Yenn . Re slipping of aircraft, I've found very few do it anywhere near correctly. What is more is that when I do  a "proper" one with the wing well down and a goodly RoD to demonstrate, they are  surprised by what is happening. What does that tell you.?   I suggest they have NEVER been shown  the proper technique is the most likely cause

  I'm a bit more wary than you. I don't advise anyone to push it to low levels till they are very confident and well practiced.. The plane is very  crossed up and also slow. The most effective sideslips are at lower speeds and getting and maintaining the correct pitch is the clue. It also keeps from loading the airframe too much. Full rudder at higher speeds isn't a good idea. If you aren't careful all you do is an uncomfortable SKID,  Nev

   

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YES.

My instructor is great at showing the right way to slip that extra hight off.

I rearlly enjoyed my traing at the Oaks.

spacesailor

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I must emphasize that this only tested glide at idle power vs prop stopped, no other option with a Rotax engine. 

It says nothing about glide with a windmilling prop.

Someone with an aircraft/engine combo that does leave the prop windmilling at best glide speed needs to do the tests to settle that argument that goes on and on.

It's easy to do, and only needs a stopwatch besides the altimeter you already have, and a phone does that.

Early morning before thermal activity, climb to 3200ft, set up a glide at your best glide speed, time the descent from 3000 to 2000ft. Repeat several times at idle and at windmilling. Average the measured times.

If the prop will stop when pulling up to a slower speed and then stay stopped at glide speed then that would make a really good comparison. 

To calculate descent rate - 60,000 / time(sec).  To calculate glide ratio - 101.3 x speed(kts) / descent rate(fpm).

Of course do it with an instructor on board and over a suitable runway in case the engine won't restart.....

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Just a thought, on electric !.

If an electric plane runs out of juice, would the windmilling prop produce a little power back into the battery.

Just enough to get power to flare on landing ?.

spacesailor

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The Jabiru 3300A will not windmill either at least not at 80 knots. It may in a dangerous high speed dive but I am not going to try this unless I have to. Like JG I have always known the performance difference because I have done plenty of purposeful dead stick landings which is why I advocated the switching the engine off on down wind at 1000 feet abeam the threshold and then turning immediately on to base to test it out. Thanks for the test values. These will vary by aircraft and of course any headwind. FH your theory does not pass scrutiny for many high performance aircraft either though it may for some heavy aircraft with large diameter props. My 60 inch prop is 800rpm at idle on the ground and at 60 knots is about 1000 - 1050rpm meaning there is less friction due to the air flowing past the blades at 60 knots but there is still 800rpm producing thrust that wouldn't be there is the prop was free spinning.

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Whether it produces thrust or not depends on the pitch, the RPM and your speed. The higher "idling" RPM is to be expected  but whether it's due to the air pushing the prop or vice versa is not so easy to determine. The prop spinning through the air has some losses. but a spinning prop is aerodynamic and can create thrust or drag. in fairly large amounts.  IF the airflow IS making the engine turn it's on backwards to do that best (More drag)  I've only ever said "has the POTENTIAL" and if its got  adjustable pitch can be  an effective brake. Not all props are easy to stop. The two mentioned do stop so that's fine. Testing them IS illegal  so why not just take some one else's figures and not risk it.  10% would cover it and as I have said the wind effect may be much more. This really is not a big drama or a silver bullet by itself.  It's one factor to consider, analyse and kick around.. I know of 2 props that have departed in flight. The plane in that case would certainly be affected but again which way? The idling prop speed would need less HP than you might think but a too high idle will make A JAB230 float on landing because then it's definitely adding thrust in a lower speed ground effect situation . I know people who shut the motor off at the flair on short strips. That's why you avoid a too fast idle but if you go too far the other way you might have the motor stop in the air at low throttle openings or stall when you open the throttle to go around. Nev

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22 hours ago, facthunter said:

Y

  Yenn . Re slipping of aircraft, I've found very few do it anywhere near correctly. What is more is that when I do  a "proper" one with the wing well down and a goodly RoD to demonstrate, they are  surprised by what is happening. What does that tell you.?   I suggest they have NEVER been shown  the proper technique is the most likely cause

  I'm a bit more wary than you. I don't advise anyone to push it to low levels till they are very confident and well practiced.. The plane is very  crossed up and also slow. The most effective sideslips are at lower speeds and getting and maintaining the correct pitch is the clue. It also keeps from loading the airframe too much. Full rudder at higher speeds isn't a good idea. If you aren't careful all you do is an uncomfortable SKID,  Nev

   

All good advice, I doubt anybody teaches sideslips anymore.

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If you have flaps better to just use them.  The same skills probably make your near limit crosswind landings better. There's a reluctance to land on one wheel same as with 3 pointing tailwheelers.  Not seen very often. .  Needs less runway.  Nev

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  Sideslipping is not permitted in some "flapped" planes but Flaps do the same job .ie, Steepen the approach path AND give you a better view over the nose, AND reduce the stall speed. As I said IF you have flaps better to just use them. All of this is,  I hope, fairly obvious and there should not be any confusion about the basics of it. Nev

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Yes,. They would be the ones left after "some" which were NOT permitted are allowed for . Forgive me, I just had to say that.  In any case Pax normally don't get impressed with what is a fairly uncommon and awkward  manoeuver( for them) anyhow.   When I converted to the supposedly dreadful, scary LOW INERTIA types particularly the quite draggy ones. It was pointed out , and ably demonstrated  that a  steep dive caused only a mild overspeed but produced a quite high descent rate over about 100 ft. A double flare puts it on the ground quite nicely. The first flare arrested the higher than normal, sink rate and the next one did the usual thing. A touch down you hardly felt. This process did what  a slip does in effect, without the drama. OK if you have just crossed a line of Pine trees near the end of the strip and don't want to waste any or don't have the length to spare. Nev

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okay, I have managed to get more information and this is a screenshot from the manual of an aircraft showing the difference between the glide performance when the propeller is completely stopped and the engine/propeller is at idle.   

 

The top row which is blue is the aircraft performance with the propeller and engine at idle.

 

The bottom row is the engine and propeller stationery.

 

As you can see the glide performance is massively better with the engine running until you get to about 70 kn where the running engine will start to deteriorate the performance due to excessive drag.

 

I think this information form a certified aircraft manual really puts the subject to bed.

 

All of the training that we do with the engine at idle practicing forced landings is a good part of our training but the performance is significantly different when the engine actually stops. In this aircraft's case at 52 kn airspeed the difference is more than 20% to graded performance with the engine turned off and a propeller stationary, contrary to Flat Earth people who think an engine at idle is creating more drag than a stationary propeller.

 

In my opinion we should be teaching people precautionary landings with the engine at idle, but they must know that there will be at least 20% difference when the engine is actually stopped.

 

 

SolidCaptureImage3221234.thumb.jpg.4b1fcc5c98e75192cccca6b7d51018c4.jpg

 

 

 

Demonstrated performance reduction between an engine on and at idle, and engine off and propeller stopped.

between V1 = 23 and V2 = 18.8

|V1−V2|[(V1+V2)2]×100=?|V1−V2|[(V1+V2)2]×100=?
=|23−18.8|[(23+18.8)2]×100=|23−18.8|[(23+18.8)2]×100
=|4.2|[41.82]×100=|4.2|[41.82]×100
=4.220.9×100=4.220.9×100
=0.200957×100=0.200957×100
=20.0957% difference

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What sort of aircraft is that with a 22:1 glide ratio?? That's darn near a glider..... Nothing like most of the recreational aircraft we fly, with GR around 10:1.....

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So, it looks like a draggy aircraft performs 10% worse and a really slippery aircraft performs 50% worse with the prop stopped vs idling. That makes sense, and is something for me to remember. Thank you for the info.

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If your thinking of  things like 50% worse, I'd be going on more than assumptions. You need facts relating to individual types and  more specific conditions. Operating speed range.  Turbo props can have drag of double the forward thrust if the prop is still turning. That's why they often REQUIRE auto feathering to be certified on some planes.  Nev

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