Rene Posted December 31, 2018 Report Posted December 31, 2018 I have been wondering what others experience when doing a "simulated engine out" exercise in their Mooney M20Js. My information manual has a glide distance chart which I have attached. At midweight of 2740 lbs. max glide is 90 knots with the prop windmilling. Using the chart, hypothetically my glide distance from 10,000 feet would be 19 nautical miles. When I do the math, this computes to a vertical speed of -787 ft/min at max glide. However, when I try this exercise in my aircraft my actual vertical speed at 90 knots is just about 1000 ft/min. Now the only thing that is different about my Mooney is that I have a 3 blade McCauley prop instead of the standard 2 blade. Would the extra blade on my prop be responsible for the difference in vertical speed? Does the extra prop blade mean that my best glide speed should be different from what is stated in the information manual? Quote
carusoam Posted December 31, 2018 Report Posted December 31, 2018 Welcome aboard Rene... Find the thread where people actually measured their planes performance... It is a great discussion. clean. prop pulled back or stopped. speed vs. weight... and balance Improvements can be found beyond the POH, but not much. One MSer even wrote a book about it... find that guy too. You know, the usual... Best regards, -a- Quote
DonMuncy Posted December 31, 2018 Report Posted December 31, 2018 In my experience, the third blade does increase the descent rate. It is a price you pay for increased climb rate. Or looking at it another way, another benefit, as you can descend more rapidly. Quote
Rene Posted December 31, 2018 Author Report Posted December 31, 2018 I tried to search for "performance measurements" and "glide performance" with no luck. Can you give me an idea of what to search for? Quote
jaylw314 Posted January 1, 2019 Report Posted January 1, 2019 43 minutes ago, Rene said: I have been wondering what others experience when doing a "simulated engine out" exercise in their Mooney M20Js. My information manual has a glide distance chart which I have attached. At midweight of 2740 lbs. max glide is 90 knots with the prop windmilling. Using the chart, hypothetically my glide distance from 10,000 feet would be 19 nautical miles. When I do the math, this computes to a vertical speed of -787 ft/min at max glide. However, when I try this exercise in my aircraft my actual vertical speed at 90 knots is just about 1000 ft/min. Now the only thing that is different about my Mooney is that I have a 3 blade McCauley prop instead of the standard 2 blade. Would the extra blade on my prop be responsible for the difference in vertical speed? Does the extra prop blade mean that my best glide speed should be different from what is stated in the information manual? What altitude are you doing this at? Remember, your actual descent rate will vary from indicated according to altitude the same way your TAS varies from IAS with altitude. Your descent angle (not rate) is what remains constant at best glide speed. Since at 10,000' MSL, your TAS is about 20% faster than IAS, it figures your descent rate would be about 20% faster at 10,000' MSL than at sea level. 2 Quote
Hank Posted January 1, 2019 Report Posted January 1, 2019 (edited) One other thing to consider is the position of your prop knob. Pull it all the way back, it makes a big difference, more for those of us with the extra blade. Edited January 1, 2019 by Hank 4 Quote
exM20K Posted January 1, 2019 Report Posted January 1, 2019 I have a couple hundred hours in an hk36 motorglider, and let me tell you: stopped prop vs windmilling is a huge difference. Admittedly, the HK would feather, but it’s not the blade angle so much as the work a spinning prop will do that consumes energy. Contemplate how difficult it would be to pull a prop through at windmilling speed. You are working against all the compression strokes each turn. ill have to look for the AFM or my notes to see if it’s measured for the motorglider. one caveat: stopping the prop requires flying very slowly, and a stall on top of an engine out would be sporty. i use the published chart as an “easily achievable” distance. -dan 1 Quote
Rene Posted January 1, 2019 Author Report Posted January 1, 2019 As I understand it, altitude should not matter when doing these experiments to determine glide ratio. So for example if we are at 10,000 feet, gliding at 90Kt IAS, of course our true airspeed is say 20% higher, but then our true rate of descent is also 20% higher than indicated on our VSI. These two factors cancel each other out, so our glide distance is the same whether we are 5000 feet over sea level terrain, or at 10,000 ft MSL over terrain that is at 5000 MSL. Do I have that right? The other interesting thing is that glide distance does not depend on weight, as shown on the glide distance chart. Heavier weights require higher glide speeds, but end up at the same distance. I always thought that this was a fascinating and very counter intuitive fact. Next chance I get, I am going to try the glide with the prop control all the way out, to see how much better I can do with my 3 blade prop. Quote
carusoam Posted January 1, 2019 Report Posted January 1, 2019 Rene, I think you are missing a detail... Weight and balance have direct effects... The induced drag generated by the horizontal stabilizer is pretty strong. Weight induced AOA generates more induced drag... When at MGTOW, the horizontal stabilizer is going to be pulling down pretty hard to maintain a higher AOA that is appropriate for lifting the higher gross... When people are trying to maximize their cruise speed, they will discuss moving the cg towards the back of the envelope... and staying coordinated with the rudder as well... and removing all the excess weight possible. They may not have mentioned very clearly, weight does matter... often, if they have skipped this detail, they have done everything assuming MGTOW, because this is ‘worst case’, everything else will be slightly better.... Glide distances in modern POHs include weight when selecting best glide speed. They don’t go into too much detail beyond maximizing the distance you can get with the altitude you have. The focus is more in line with not stalling the plane... Pp thoughts only, not a CFI... Best regards, -a- Quote
M016576 Posted January 1, 2019 Report Posted January 1, 2019 (edited) 50 minutes ago, Rene said: The other interesting thing is that glide distance does not depend on weight, as shown on the glide distance chart. Heavier weights require higher glide speeds, but end up at the same distance. I always thought that this was a fascinating and very counter intuitive fact. This is due to the fact that your “best glide” is actually an angle of attack for the airfoil- not really a speed. The best glide angle of attack is estimated in your POH by using a speed and weight combo which should yield near to the best glide AoA in stabilized flight. For those of us with an AoA instrument- you should be able to use your gauge for best glide (also referred to as max range)- which to me is not only more accurate, but simpler to fly. best glide can also be referred to as L/D max. Edited January 1, 2019 by M016576 2 Quote
carusoam Posted January 1, 2019 Report Posted January 1, 2019 Using an AOAi allows you to glide the farthest without doing the math or knowing what your current weight is... Note the similarity to effects of weight on stall speed. You get to understand a higher level of AOA, and where the limits are... the critical AOA... Our wing has a certain range of AOA it can use... some of that range gets used up by Weight, bank angle, and speed... PP thoughts, not a CFI... Best regards, -a- 1 Quote
DonMuncy Posted January 1, 2019 Report Posted January 1, 2019 40 minutes ago, carusoam said: Using an AOAi allows you to glide the farthest without doing the math or knowing what your current weight is... Note the similarity to effects of weight on stall speed. You get to understand a higher level of AOA, and where the limits are... the critical AOA... Our wing has a certain range of AOA it can use... some of that range gets used up by Weight, bank angle, and speed... PP thoughts, not a CFI... Best regards, -a- Carusoam, Any advice as to how to determine that AOA. Other than using POH information or doing actual testing. Quote
carusoam Posted January 1, 2019 Report Posted January 1, 2019 30 minutes ago, DonMuncy said: Carusoam, Any advice as to how to determine that AOA. Other than using POH information or doing actual testing. Don, The POH is where the data collection is, based on in flight data from the tests Mooney ran in certification... The factory then slimmed things down for us, to be determined by the instruments that are/were in our planes (at the time). Things That are interesting to us... Vx, Vy, Vz, stall warning, and stall speeds are all directly associated to AOA... So when they print charts and graphs that have huge amounts of memorizable data... weight, bank, speed... using an AOAi can be very helpful to alleviate the giant memory challenge... Watching Mikey @201er land smoothly time after time, on a short surface target... he has demonstrated great speed control. He uses an AOAi to control his speed better, by knowing what the right speed of the day is... He is still the PIC of his ship and does a great job of the flying part too... Aside from that, no, I don’t believe we have the actual data available in a table that we could look up... If we did, we would still be calibrating and proving we have the AOAi set up properly... We do run into some additional memory load using the AOAi though... when the configuration changes, adding flaps, the AOA and target numbers do shift... So some displays are better than others to handle the change in configuration... flaps up vs. flaps down... PP thoughts only, I don’t have an AOAi yet, not a CFI or mechanic... I’m looking forward to learning more about the new devices without air sensors(?) Best regards, -a- 1 Quote
DonMuncy Posted January 1, 2019 Report Posted January 1, 2019 I have a three blade prop which I think probably changes my best glide speed some (unknown to me) amount. I have an AOA, and calibrated it with POH numbers. I know exactly where on the gauge the stall buffet begins. I believe I can interpolate from the POH numbers to come up with a best glide AOA indication, but I was wondering if there was some other way. Quote
gsxrpilot Posted January 1, 2019 Report Posted January 1, 2019 5 minutes ago, DonMuncy said: I have a three blade prop which I think probably changes my best glide speed some (unknown to me) amount. I have an AOA, and calibrated it with POH numbers. I know exactly where on the gauge the stall buffet begins. I believe I can interpolate from the POH numbers to come up with a best glide AOA indication, but I was wondering if there was some other way. I was hoping you'd just figure it out and then share it with me :-) Which AOA do you have? I have the Aspen and really need to get it calibrated properly. Quote
DonMuncy Posted January 1, 2019 Report Posted January 1, 2019 1 minute ago, gsxrpilot said: I was hoping you'd just figure it out and then share it with me :-) Which AOA do you have? I have the Aspen and really need to get it calibrated properly. I have the Alpha, with the partial circle of lights. We calculated the 1.3 x stall speed number based on the existing weight via the POH. Took the avionics guy up and let him adjust it until the "blue" light came on at that speed. Then slowed down to the buffet and noted it happened on the second light below the blue one. Quote
M016576 Posted January 1, 2019 Report Posted January 1, 2019 12 minutes ago, gsxrpilot said: I was hoping you'd just figure it out and then share it with me :-) Which AOA do you have? I have the Aspen and really need to get it calibrated properly. I have the aspen AoA as well. I wish they provided a little more detail about “what’s under the hood” in the manual- but I do like the simple approach to flying the unit for landing. Set the flaps down carry on the line between yellow and green and hold that AoA to the flare. all these AoA gauges share a common attribute- none of them are divided up into degrees true markings- they are all “calibrated” into equal “units” in relation to the wings stall margin, and then some form of algorithm is applied to show where the “on speed” marking is. Depending on that algorithm, a pilots inputs may change the display of the AoA in either a linear, or some sort of non-linear way. I have a 3 bladed prop, too- so I don’t think my best glide numbers are in line with the poh. I think the best way to find best glide as it pertains to your AoA indicator is to literally go out and test it, using a known weight, and a known IAS, and then see what that gets you. One could back up the test data with something like cloud ahoy... and then be able to see just how your plane did based on the number of test trials conducted, and the set input parameters. I’m not sure if there would be a “better way” unless your AoA gauge has some sort of L/D max input or calibration feature- as these units are all meant to be airframe “agnostic”- focusing mainly on stall margin. BUT... that’s not to say that they can’t be used to aid in finding best glide- because they can be. 2 Quote
aviatoreb Posted January 1, 2019 Report Posted January 1, 2019 1 hour ago, M016576 said: all these AoA gauges share a common attribute- none of them are divided up into degrees true markings- they are all “calibrated” into equal “units” in relation to the wings stall margin, and then some form of algorithm is applied to show where the “on speed” marking is. Depending on that algorithm, a pilots inputs may change the display of the AoA in either a linear, or some sort of non-linear way. hah! You beat me to it. Well said - You most surely must have been exposed to an excellent mathematics education, wherever it was you went to school. You said the most insightful thing in your previous post. Best glide is essentially the same as Vx angle of attack. So knowing where Vx marks on our own individual AoA, no matter what might be the actual speed, we might be able to repeat that in the glide state. But for accuracy, we are still in the same boat because knowing the true Vx for a given weight, we must either read that out of our POH, or somehow do our own engineering test flights. In any case, if we trust we are flying Vx then we can just observe whatever our AoA is reading in its own funking "fractions of" deviation from stall AoA units. 1 Quote
PT20J Posted January 2, 2019 Report Posted January 2, 2019 4 hours ago, M016576 said: I have the aspen AoA as well. I wish they provided a little more detail about “what’s under the hood” in the manual The Aspen patent has more details... US20160298985A1.pdf 2 Quote
DonMuncy Posted January 2, 2019 Report Posted January 2, 2019 In my case, I know where the stall buffet took place. I did not take it to a full stall. But the 1.3 x stall appeared to be fairly close. I am satisfied that my AOA is doing the job. I only asked in the first place as an academic exercise. Quote
PT20J Posted January 2, 2019 Report Posted January 2, 2019 A few points to keep in mind: Best glide occurs at the angle of attack (AOA) for L/Dmax which occurs at the point where total drag is a minimum and induced and parasite drag each account for half the total drag. It is not dependent on weight. The speed for best glide does depend on weight, of course. Perhaps the easiest way think about this is to remember that in unaccelerated flight, Lift = Weight and Lift is proportional to TAS2 and AOA. If you keep AOA constant for best glide, then the best glide airspeed varies as the square root of the weight. At higher altitudes, the required TAS increases due to the decrease in air density, however this is compensated by using the same indicated airspeed as at sea level. The descent angel will be constant at best glide, but the vertical speed will vary as the TAS. A stopped prop will always produce the best glide performance regardless of the number of blades. If you keep the prop turning, the best thing is to pull it back to minimum rpm as others have mentioned. A windmilling three bladed prop will theoretically have more drag than a two bladed prop and should reduce the best glide speed a little. If the STC doesn't mention it, the effect was likely pretty small. Vy is the speed where there is maximum excess power available above the power required. Vx is the speed where there is a maximum of excess thrust available above the thrust required (drag). In many high performance aircraft it occurs very near the stall speed in which case a higher (safer) speed will be listed in the POH. Since Vx and Vy depend on the characteristics of the propulsion system as well as aerodynamics, they are not strictly determined by AOA. Skip 1 Quote
jaylw314 Posted January 2, 2019 Report Posted January 2, 2019 8 hours ago, Rene said: As I understand it, altitude should not matter when doing these experiments to determine glide ratio. So for example if we are at 10,000 feet, gliding at 90Kt IAS, of course our true airspeed is say 20% higher, but then our true rate of descent is also 20% higher than indicated on our VSI. These two factors cancel each other out, so our glide distance is the same whether we are 5000 feet over sea level terrain, or at 10,000 ft MSL over terrain that is at 5000 MSL. Do I have that right? The other interesting thing is that glide distance does not depend on weight, as shown on the glide distance chart. Heavier weights require higher glide speeds, but end up at the same distance. I always thought that this was a fascinating and very counter intuitive fact. Next chance I get, I am going to try the glide with the prop control all the way out, to see how much better I can do with my 3 blade prop. You are correct, at best glide speed, the glide ratio does not change with altitude, but the descent rate does change with altitude. 1 Quote
Shadrach Posted January 2, 2019 Report Posted January 2, 2019 Rene, unless you’re actually shutting down the engine in the air, the prop is providing very little drag and is in fact likely providing some thrust. It is certainly not robbing energy from the glide. If your are not making book numbers at idle, you’d likely do worse with the prop stopped. 1 Quote
M016576 Posted January 2, 2019 Report Posted January 2, 2019 5 hours ago, aviatoreb said: hah! You beat me to it. Well said - You most surely must have been exposed to an excellent mathematics education, wherever it was you went to school. Indeed- I studied from the best!!! 1 Quote
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