Glide Ratio

[GraemeSmith]

I see your autocorrect keeps wanting to make "ratio" into "ration". Mine does too!

I need to go fly this again and get a bigger data set and plot this again. It is distinctly different than when I measured in summer temperatures but with the aircraft at the SAME load condition and Density Altitude AGL.

Because there is Vg and there is VMinSink (which the glider guys use to loiter while looking for lift). They are usually different and that quick dataset from today doesn't show it. On these short stubby wings they may well be almost the same - I know when I did it extensively with my 150 there was only about a 1 knot difference - which was way within the margin of pilot error.

I've shot a note to a mathematician/aerodynamicist to try and glean why I got different answers at the same DA but different temperatures.

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Any Onex owners or glider pilots who know the answer about different Vg for different seasons but same DA?

[Bryan Cotton]

I hope to get this dataset within the next month as well and I'll share. I am going to re-fly my Vx/Vy card (Thanks Jeff and sonex604.com) and add the glide stuff in too.

I am not sure why the same DA but different temps give different numbers. I am a former glider pilot. The one thing I can tell you which may be confusing, is you fly faster into the wind and slower with the wind for best glide. For the non-glider pilots flying at best glide is probably good enough, unless it's really windy and you are trying to go upwind. Then you add about half the winds speed to your glide speed.

[dbdevkc]

I'm a glider pilot also - and you are correct about changing speed when flying into a headwind vs tailwind, and that matters when the rubber meets the road and you're in a situation where you need to glide as far as possible. But you need to start with the correct IAS for the plane. 10 kns off either way will make a huge difference in sink rate and glide ratio. Obviously I'm not flying yet but part of the plan is to get the baseline for my airplane (which if I were a betting man will be different than a factory airplane) by flying some early morning when there is no convection yet and very little wind aloft and track altitude lost over distance at different speeds, repeat in different headings, etc.

[Bryan Cotton]

I'm a glider pilot also

What club? I used to be in Nutmeg Soaring. They were at Candlewick Farms in New Milford, CT until they got kicked out. Then we bought Freehold airport south of Albany/North of the Catskills.

[dbdevkc]

Valley Soaring Club, Randall Airport, Middletown, NY. I think you know Dave Matuska from Sikorsky, he is a member of our club.

[Bryan Cotton]

Valley Soaring Club, Randall Airport, Middletown, NY. I think you know Dave Matuska from Sikorsky, he is a member of our club.

I know him well! Say hi for me.

[GraemeSmith]

So I posed the question about the different numbers I got in the summer and the winter to a friend.

His qualifications include a very experienced glider CFI. He ran the aviation program at Daniel Webster University back in the day and one class he regularly taught was to take their Cessnas out and measure the performance numbers from scratch (in effect Phase 1 testing them) and now he is a senior FAA Inspector. His response was:

It would be hard to find something very direct, but there could multiple factors at issue.

1. Variations in testing procedure, even if not specifically noted, such as rates of deceleration, or acceleration in the test procedure.
2. Published speed are usually a line of best fit for multiple tests conducted in various conditions. One or two tests does not necessarily mean there is a significant difference. In fact I used to teach a class on determining aircraft performance, and we had students do some basic testing in various aircraft. We would get some significant variations from time to time – including discovering that there were some install issues in a particular Cessna 152.
3. Calibration issues – Usually are bigger and have more factors than people will give credit to….
a. Do you have an electronic airspeed indicator? Does it account for temperature difference. Does temperature have an effect on the circuit boards and digital pickups? You would know this better than I, but I would suspect these could have an effect, especially in Experimental equipment that may not meet the TSO requirements.
b. Mechanical Operations – Pitot tube (and Static) material throughout the aircraft, does it expand or shrink based upon temperature? Remember most of it is in an area that is not heated or warm – I.e. it all at 80+ degrees in summer, but likely at or near zero in the winter.
c. Instrument error with Temperature – Mechanical instruments – no matter how precise, also have some error associated with temperature. I am sure diaphragms, gears etc shrink or are slow to react when colder. Remember – even if the cockpit is “Warm” it is likely that instrument was cold soaked and is in a portion of the panel that is not all that warm. It took years, but we finally got temperature corrections on approaches (although still in a messy calculation process). I could easily see some of the same factors applying to airspeed indicator.
4. Density altitude – Be are that moisture plays a bigger role in Density altitude than we give it credit for. You can easily get a 3% difference just with variation in moisture. In addition, recognize that the atmosphere is not “linear”, seeing the difference in temperatures at the surface and then the small variation in MSL altitude tells me that you likely had a temperature inversion you were dealing with in the winter. This could play a factor.
5. Airspeeds – Remember there are variations in change of airspeed (KIAS, KCAS, KEAS, KTAS) etc. In fact more electronic panels, because they can calculate calibration and “compressibility” effects give the Indicated Airspeed displayed to the pilot as KEAS – the Eclipse 500 did this. There are also things that we measure in TAS, but pilots think of in terms of IAS. I do not recall if this is the case with glide speeds, but it does play factor in other speeds, like VA/VNE. In fact I did a webinar on this subject, as more people fly higher (like Gliders in Mtn Wave) or add turbo charging (Hmmm….Sonex engine or Jet) that they can easily exceed VNE without realizing that they actually are.
a. https://www.youtube.com/watch?v=q9TNBU8BryU

So those are just a few thoughts. Not 100% sure, but could see so many reasons.

And my thoughts back:

I suppose my thoughts and questions back include:

I don’t trust the VSI implicitly as mine is electronically derived/computed. I also measure altitude lost over time. – which over 2,000ft is going to be a 3-4min period. That is a decently gross amount of altitude to get repeatable numbers. I also avoid starting measurements until airspeed is stabilized for about 15 secs.

The point about instrument error – especially temperature related I had considered. It’s an interesting issue. Even the MGL avionics try and temperature compensate. But from the bottom end. The units - including the remote mounted critical AHRS module and magnetometer – include heaters to get the chips up to a minimum temperature before they will deliver answers. But in the summer there is no cooling to a regulated temperature – so the room for error exists.

Getting the plane to fly exactly on aTTitude is difficult. Even well trimmed in very smooth air – it is a tough gig. The plane is very short coupled. Another pilot who I am comparing notes with is even trying to differentiate the CG location because that drives the elevator position for trim and changes the drag. His plots demonstrate a very sharp peak optimal “tip” – instead of the more rounded peak you get on a Ce$$na. It’s really had to fly that “tip” precisely.

DA affected by moisture. Yes – but I thought that was compensated for in the DA calculation – taking account of temp and dewpoint?

But I hadn’t fully considered the possibility of inversions. Guess I’m going to have to add temperature measurements at each altitude to ensure that the lapse rate is valid that day.

VNe – Isn’t the airframe stressor here the aerodynamic forces – rather than the TAS? Air gets thinner, IAS drops though the plane still has a higher TAS. But the TAS can’t harm the plane as such. There is less air and aerodynamic effect acting on the airframe. So VNe would be IAS? No? Though I can see that you could do harm pulling G’s as that is relative to the TAS and the earth’s gravitational pull…..

The old “rule of thumb” of putting the chord line parallel to the horizon – definitely gets you in the ballpark – and if you were flying it in anger, and looking around for a field and and and…….. it’s probably a good way of getting close.

[GordonTurner]

Vne is mostly a flutter problem. Flutter is excited by velocity, not density, so the actual Vne is really a TAS. This is inconvenient to display on a steam gauge airspeed indicator (turbine aircraft have a moving redline), so typically a conservative IAS is marked based on the highest altitude the plane is expected to be flown at. Gliders on the other hand, typically have sea level IAS marked but with a chart showing the reduction in IAS for Vne as altitude increases.

Gordon

[Bryan Cotton]

Here is my data from today. I am really suspicious of the 60kt point as I my idle is set too low and I was trying to set a consistent RPM by hand. I think once I get the idle stop set better I can get better data. Also it really appears I need to go higher than 84kts to get a full data set. Takeoff was at 1083 lbs. This brings up another point - best glide speed is a function of weight, because it is dictated by angle of attack. So at some point I need to do some light weight testing and some heavy weight testing to get a better polar and correction.

Waiex 191YX glide data 11-22-22.png (22.69 KiB) Viewed 1234 times

[GraemeSmith]

Here is my data from today. I am really suspicious of the 60kt point as I my idle is set too low and I was trying to set a consistent RPM by hand. I think once I get the idle stop set better I can get better data. Also it really appears I need to go higher than 84kts to get a full data set. Takeoff was at 1083 lbs. This brings up another point - best glide speed is a function of weight, because it is dictated by angle of attack. So at some point I need to do some light weight testing and some heavy weight testing to get a better polar and correction.

Waiex 191YX glide data 11-22-22.png

Yeah - that "60" looks a little suspect.

One thing you can do to "imitate" a stopped prop is to set an idle in the 800-1,000 range - find the rpm where it "just" contributes thrust. That "very slightly" thrusting prop will imitate a stopped prop as the tiny bit of thrust is better that the drag of a windmilling non powered prop. If you see what I mean.

And yes - need to try different load factors.