Testing how far can you fly with engine at idle

[dabkb2]

I am getting 1500' per mile, so for a 10 mile stretch with nowhere to land I need 7500'AG to be safe.
Would like to know what kind of numbers other gyro's are getting.

 

[XXavier]

That's an L/D ratio of 3,5.
It seems a low figure, but that's normal with autogyros. I enclose data for the MTO, scanned from 'Flight Performance of Lightweight Gyroplanes' by Holger Duda & Insa Pruter - German Aerospace Center.

 

[WaspAir]

I never expect more than about 4:1 in any rotorcraft (in my Bell 47 with the big bubble, the joke is look forward between your toes, and that's where you'll be landing). Airmass motion makes a huge difference with such steep glides - - tailwinds and rising air help immensely, while headwinds and sink are awful. It's always a big adjustment from my sailplane that can go literally ten times as far from the same altitude.

Cinsider your downwind options when the engine goes quiet to maximize your range (and reverse course before touchdown, of course).

 

[okikuma]

In addition to the very draggy spinning rotor above one needs to consider the additional drag from the propeller at engine idle.

Xavier,

I read through the 'Flight Performance of Lightweight Gyroplanes' paper by Holger Duda & Insa Pruter - German Aerospace Center. None of the computations for gliding include the drag from a windmilling propeller. So, I’m guessing their assumption is that the propeller is “dead” (i.e not spinning) during the glide. For a geared engines such as the Rotax 912 series, the propeller would not windmill. Whereas with a direct drive engine, the propeller would windmill.

This is very general. For testing purposes and all things being unequal. For propellers that normally spin at a maximum of around 2700 rpm. If one sets by throttle the propeller revolutions per minute at roughly 1200. The thrust vs drag ratio of the propeller is roughly equivalent to a non spinning propeller.

Wayne

 

[Jazzenjohn]

How much difference would their be between engine idle, engine stopped with no prop rotation, and engine off with free spinning prop?

 

[Mike G]

The curves that Xavier posted show that the best L/D ratio occurs at the bottom of the Force (or power) curve. The IAS at which this occurs varies with DA and AUW.
This is the point at which the GWS gives the first "behind the curve" warning and this warning is adjusted in real time for DA and actual AUW ( the GWS works that out as your flying).
So if you have a GWS and want the best glide speed for an engine out you have to fly at an IAS just flirting with the behind the curve alarm. You still have to consider Waspair's wind/thermal impact and engine stopped/windmilling effects on ground distance but it gives you a start point.
It's not a function that we'd considered before.
Mike G

 

[okikuma]

How much difference would their be between engine idle, engine stopped with no prop rotation, and engine off with free spinning prop?

A very good question. Oftentimes, the difference of a few feet or meters more is the defining line between life and death.

Many years ago, I performed a test with my Piper Pacer. I wanted to see the difference in gliding distance to a landing in the pattern between the prop being stopped as compared to windmilling with the engine at idle. I had found that indeed the drag was less and I was able to glide farther down the runway with the prop stopped. I then experimented with low power settings until I was able to match the stopped propeller gliding distance. 1200 rpm was the number. Later, some old high time aviators had confirmed that 1200 rpm was the magic number. Many years later, I watched a video of Steve Graves giving instruction in his Marchette Avenger gyroplane. As I recall, he told the student that 1200 rpm on the Lycoming engine on the gyro equaled the drag of a stopped propeller when practicing engine out landings.

Wayne

 

[ArnaudHP]

Practice deadstick landings and you will know

 

[WaspAir]

The curves that Xavier posted show that the best L/D ratio occurs at the bottom of the Force (or power) curve. The IAS at which this occurs varies with DA and AUW.

The indicated speed for best glide goes up with increase in weight (shifting the polar curve down and to the right), but both the best glide ratio you can get and the indicated airspeed at which it is reached are unchanged with air density.

The natural true airspeed increase as you climb for any given indicated airspeed provides density compensation for the optimum speed value.

The L/D ratio is unchanged because both foward airspeed (horizontal) and sink rate (vertical) are affected the same way by density (looked at another way, there's a linear density term in the equation for L and also in the equation for D, so when you take the ratio of the two the density term simply divides away).

 

[dabkb2]

That's an L/D ratio of 3,5.
It seems a low figure, but that's normal with autogyros. I enclose data for the MTO, scanned from 'Flight Performance of Lightweight Gyroplanes' by Holger Duda & Insa Pruter - German Aerospace Center.

View attachment 1162635

The graphs are great, If I'm reading it right it works out pretty close, 50KNTS is where I get the best distance.

 

[dabkb2]

I never expect more than about 4:1 in any rotorcraft (in my Bell 47 with the big bubble, the joke is look forward between your toes, and that's where you'll be landing). Airmass motion makes a huge difference with such steep glides - - tailwinds and rising air help immensely, while headwinds and sink are awful. It's always a big adjustment from my sailplane that can go literally ten times as far from the same altitude.

Cinsider your downwind options when the engine goes quiet to maximize your range (and reverse course before touchdown, of course).

I agree, tailwinds did make a difference. But with the turning to land into the wind there's not a big difference.

 

[dabkb2]

In addition to the very draggy spinning rotor above one needs to consider the additional drag from the propeller at engine idle.

Xavier,

I read through the 'Flight Performance of Lightweight Gyroplanes' paper by Holger Duda & Insa Pruter - German Aerospace Center. None of the computations for gliding include the drag from a windmilling propeller. So, I’m guessing their assumption is that the propeller is “dead” (i.e not spinning) during the glide. For a geared engines such as the Rotax 912 series, the propeller would not windmill. Whereas with a direct drive engine, the propeller would windmill.

This is very general. For testing purposes and all things being unequal. For propellers that normally spin at a maximum of around 2700 rpm. If one sets by throttle the propeller revolutions per minute at roughly 1200. The thrust vs drag ratio of the propeller is roughly equivalent to a non spinning propeller.

Wayne

I will try it. Its going to be a long quiet ride from 4500'.

 

[dabkb2]

The curves that Xavier posted show that the best L/D ratio occurs at the bottom of the Force (or power) curve. The IAS at which this occurs varies with DA and AUW.
This is the point at which the GWS gives the first "behind the curve" warning and this warning is adjusted in real time for DA and actual AUW ( the GWS works that out as your flying).
So if you have a GWS and want the best glide speed for an engine out you have to fly at an IAS just flirting with the behind the curve alarm. You still have to consider Waspair's wind/thermal impact and engine stopped/windmilling effects on ground distance but it gives you a start point.
It's not a function that we'd considered before.
Mike G

I don't think the GWS would give an alarm, I've tried faster and slower but for distance my best speed is 50KNTS

 

[dabkb2]

Practice deadstick landings and you will know

Practice it regularly

 

[WaspAir]

I agree, tailwinds did make a difference. But with the turning to land into the wind there's not a big difference.

For a long glide as you first posted, it can be a very big difference.

For a fifty knot nominal airspeed and 1500 feet per mile loss as you have suggested, a commonly encountered, unremarkable ten knot headwind will cut your groundspeed to 40 at the same sink rate and net you 1875 feet loss per mile, while a ten knot tailwind will increase your groundspeed to 60 with the same sink rate and flatten your glide to only1250 feet per mile. The difference between those two conditions is a huge 625 feet per mile (and it only gets bigger with a stronger wind). That's enough saved altitude to fly a decent pattern after a mile glide is completed.

Turning into the wind for touchdown can be delayed until final moments and need not take much height. Some sort of at least minimal maneuver to cope with field shape, miss wires and obstacles (perhaps not visible from far off) and align with wind is to be expected for any engine-out regardless of which way you head.

The potential touchdown range is not a simple circle centered on your current position. Even light winds move it downwind and distort the zone.

 

[dabkb2]

For a long glide as you first posted, it can be a very big difference.

For a fifty knot nominal airspeed and 1500 feet per mile loss as you have suggested, a commonly encountered, unremarkable ten knot headwind will cut your groundspeed to 40 at the same sink rate and net you 1875 feet loss per mile, while a ten knot tailwind will increase your groundspeed to 60 with the same sink rate and flatten your glide to only1250 feet per mile. The difference between those two conditions is a huge 625 feet per mile (and it only gets bigger with a stronger wind). That's enough saved altitude to fly a decent pattern after a mile glide is completed.

Turning into the wind for touchdown can be delayed until final moments and need not take much height. Some sort of at least minimal maneuver to cope with field shape, miss wires and obstacles (perhaps not visible from far off) and align with wind is to be expected for any engine-out regardless of which way you head.

The potential touchdown range is not a simple circle centered on your current position. Even light winds move it downwind and distort the zone.

It can, but at different altitudes the wind speed can and does change. Ground and airspeed need to be monitored.

 

[WaspAir]

For moderate gyro climbs, the wind typically freshens somewhat with altitude and rotates slightly clockwise in direction owing to surface friction. I'm all in favor of monitoring it. It's important for navigation, anyway.

In any event, your greatest glide range will be downwind and the range difference can be large.

 

[XXavier]

Flying in a quiet day, with little or no wind, and gliding, with the engine at idle, at different airspeeds, say, between 60 and 90 km/h, one can find the airspeed at which the sink speed stabilizes at a minimum during the glide. Then, you're flying, obviously, with the minimum 'gravitational power required' for a glide. Multiplying that airspeed by 1,3 you obtain the minimum drag airspeed, the one that gives you the maximum L/D, and thus, the maximum gliding range...

 

[Mike G]

The indicated speed for best glide goes up with increase in weight (shifting the polar curve down and to the right), but both the best glide ratio you can get and the indicated airspeed at which it is reached are unchanged with air density.

The natural true airspeed increase as you climb for any given indicated airspeed provides density compensation for the optimum speed value.

The L/D ratio is unchanged because both foward airspeed (horizontal) and sink rate (vertical) are affected the same way by density (looked at another way, there's a linear density term in the equation for L and also in the equation for D, so when you take the ratio of the two the density term simply divides away).

Wasp you made me go back to see how the GWS works out at what airspeed the bottom of the drag curve occurs.

Since the GWS can’t measure actual and/or changes in AUW, neither can it measure the drag, it produces some parameters (thanks to Jean Claude) that allow us to know when we are at or just below the bottom of the curve. I knew that one of those parameters used TAS so out of intellectual laziness I assumed that the bottom of the curve airspeed was a function of DA.

Looking at how the GWS does send the “behind the curve” alarm I realised that another of the parameters that enters the equation is also affected by DA and, as in your example, they cancel each other out.

So thanks I learned something today.

Mike

 

[Mike G]

That's an L/D ratio of 3,5.
It seems a low figure, but that's normal with autogyros. I enclose data for the MTO, scanned from 'Flight Performance of Lightweight Gyroplanes' by Holger Duda & Insa Pruter - German Aerospace Center.

View attachment 1162635

Looking at Xaviers graphs I was surprised to see that a German paper presented graphs with metric units except the airspeed (V) which is shown in kts.

One of our test flights when installing a GWS in a new gyro type is to confirm at what airspeed the bottom of the drag curve occurs. In the case of 2 seat tandem gyros this has come out between 50 and 60 kph (kilometres/hour) which is 27 to 32 kts which is a long way from Xavier’s documented 50 kts.

I wonder if there isn’t a simple typo kts replacing kph.

If someone with an MTO could carry out a simple test next flight to see at what airspeed they get for the bottom of the drag curve I’d be grateful.

Mike