Determining Best Glide Airspeed for Your Super Cub

After reading the post in which a Doc from Maine lost his life when he experienced an engine out over water, it struck me that given the wide variations in mods to our super cubs (+/- VG's, big tires, etc) it would be good to know what the best glide airspeed is for our aircraft.

This may be a naive question, but how do we determine that airspeed?

Randy

Page 25 in the attached manual states 70 MPH.

http://www.supercubproject.com/downloads/PA-18_Information_Manual.pdf

Thanks, Stewart, but wouldn't this speed be varied considerably by VG's, big tires???

Randy

I don't think the drag of the tires will impact the aerodynamics of the wing. Your glide distance will be reduced but in my thinking the target speed for best glide doesn't change. VGs are a non issue in the discussion.

Randy,

This chart is a good thing to bring on a flight and then just pull power, check each speed against vsi and you'll know for sure.

Jerry

who has a vsi ?

cubflier said:

Randy,

This chart is a good thing to bring on a flight and then just pull power, check each speed against vsi and you'll know for sure.

Jerry

	SPEED IN MILES PER HOUR
DECENT IN FPM	50	55	60	65	70
100	8.3	9.2	10.0	10.8	11.7
150	5.6	6.1	6.7	7.2	7.8
200	4.2	4.6	5.0	5.4	5.8
250	3.3	3.7	4.0	4.3	4.7
300	2.8	3.1	3.3	3.6	3.9
350	2.4	2.6	2.9	3.1	3.3
400	2.1	2.3	2.5	2.7	2.9
450	1.9	2.0	2.2	2.4	2.6
500	1.7	1.8	2.0	2.2	2.3
550	1.5	1.7	1.8	2.0	2.1
600	1.4	1.5	1.7	1.8	1.9
650	1.3	1.4	1.5	1.7	1.8
700	1.2	1.3	1.4	1.5	1.7
750	1.1	1.2	1.3	1.4	1.6
800	1.0	1.1	1.3	1.4	1.5
850	1.0	1.1	1.2	1.3	1.4
900	0.9	1.0	1.1	1.2	1.3
950	0.9	1.0	1.1	1.1	1.2
1000	0.8	0.9	1.0	1.1	1.2
	MILES TRAVELED PER 1000 FEET OF ALTITUDE

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You'll need to balance altitude loss with distance traveled to find the optimum glide. Best glide is about covering the most ground. A VSI will only tell part of the story. Probably need to factor gross weight and average CG, too. Supposedly that's what the factories did when establishing emergency procedures. It must work okay since they don't give a sliding scale for glide speed @ variable operating weights.

I have a VSI. I don't need it but it fills a hole, balances the panel instruments nicely, and weighs almost nothing.

WindOnHisNose said:

After reading the post in which a Doc from Maine lost his life when he experienced an engine out over water, it struck me that given the wide variations in mods to our super cubs (+/- VG's, big tires, etc) it would be good to know what the best glide airspeed is for our aircraft.

This may be a naive question, but how do we determine that airspeed?

Randy

Click to expand...

Randy,

Go grab a glider instructor. There is so much more to this that you need to know.

Tim

I have a copy of a Piper approved flight manual (no.1054) for commercial operations of a stock PA18-150 and it has a lovely graph in the performance section - 'En route Glide NIL WIND (power unit inoperative)' showing distance against height. It shows the glide speed to be 58mph/50.4 Kts and the 'net' distance per thousand feet to be 1.5 NAUTICAL miles.If you are an ace pilot with a slippery aircraft you may reach the 'gross' figure of 1.8 Nauticals per thousand feet.

If anybody is interested in having a copy of said manual (Possibly airline blokes who live and breath 'Scheduled performance numbers') then I will bring mine to New Holstein and you may copy to your heart's content! - just say and I'll do it.

It's a long story involving the UK CAA as to why I have it. I am now authorised, after much hassle, to use Flt. Manual 934. - much simpler.

Frank

coxcub said:

I have a copy of a Piper approved flight manual (no.1054) for commercial operations of a stock PA18-150 and it has a lovely graph in the performance section - 'En route Glide NIL WIND (power unit inoperative)' showing distance against height. It shows the glide speed to be 58mph/50.4 Kts and the 'net' distance per thousand feet to be 1.5 NAUTICAL miles.If you are an ace pilot with a slippery aircraft you may reach the 'gross' figure of 1.8 Nauticals per thousand feet.

If anybody is interested in having a copy of said manual (Possibly airline blokes who live and breath 'Scheduled performance numbers') then I will bring mine to New Holstein and you may copy to your heart's content! - just say and I'll do it.

It's a long story involving the UK CAA as to why I have it. I am now authorised, after much hassle, to use Flt. Manual 934. - much simpler.

Frank

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I would very much enjoy getting a copy of that, Frank! Any chance you can pdf it and post it here? If not, I would be delighted to photograph it at NH...yet another reason to make it to New Holstein. Thank you very much!

Randy

I've always thought that best speed would not change, as stated that is airfoil related but that the decent rate would increase for increase drag from big tires perhaps (hopefully) at the same rate that the cruise speed reduced for big tires (I know we cruse faster than we glide and that would be more drag but this is cowboy math). For me that was 9mph from 8"s to 31's or about 10% speed loss... so I would expect the decent rate to go up 10% for the given speed resulting in a slightly less than 10% reduction in available glide distance if this cowboy math hypotenuse adjustment works out.

My POH says best glide is 65mph.

We need a test dummy, I mean a Volunteer...

Iflylower said:

Randy, I think you can't go wrong with 70mph, but I'm sure that working with your plane to be exact can't hurt either. The chart that cub flier posted will have all the important variable. Best glide (most distance over time) is generally close to L/D Max. If that takes anyone back to some confusing aerodynamic lessons. It's simply the most lift from the wing possible for the drag spent. That drag, in turn, can be increased by speeding up (more parasitic drag), or slowing down (more induced, or lift-created, drag).

The 70 MPH sure is a number for plain factory, gross weight, supercub. Big tires might affect parasitic enough to slow best glide a couple mph. A really lightly loaded plane might also move best glide lower, by reducing lift req(induced drag). Best glide should more be a pitch attitude that speed, but it's probably simple with speed.

If your keeping 70mph all the while running emer proceedures, selecting an evaluating a landing spot, shutting off fuel, mags, popping a door, pressing 911 on your spot, squawking 7700 and talking on 121.5, you're better than most.

Lastly if you think of it, or have time to consider..... Glider people think of Optimal glide and add 1/2 estimated knead wind to Vbg. Or subtract 1/2 estimated tailwind to Vbg. Also consider, if over water, or over an airport, A Vmd or minimum descent (or least sink) might be your best option to give you the greatest amount of time for a restart or getting things together. I hope you don't need any of it. I'm not a gider guy yet, but, I've been thinking about it and researching their material.

Of all the single engine planes I've flown, the cub is my number 1 choice to lose one in, and not for it's great glide.:smile:
Cheers.

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Good post.

I had a glider guy teach me in an aerodynamics class and it was some of the best flying information I ever had. Need to go get my glider rating so I can remember it.

Tim

Hi Randy. If you want the numbers for YOUR aircraft I would suggest. "Performance Flight Testing" By Hubert "SKIP" Smith. TAB Books Inc. Modern Aviation Series. Copyright 1982. ISBN 0-8306-2340-X (pbk). Probably can get an old copy on Amazon. Lots if good tests and it won't be a guess. Most hand calculators will get you through the math. Jerry B.

sierra bravo said:

You'll need to balance altitude loss with distance traveled to find the optimum glide. Best glide is about covering the most ground.

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Can you tell me why my chart does not do that? The bottom line should read MILES TRAVELED PER 1000 FT OF ALTITUDE LOSS.

I just did a check just to see if I still believe in 60 mph and I came up with the following distances traveled per 1000 ft altitude loss.

55 mph = 1.3 miles
60 mph = 1.5 miles
65 mph = 1.4 miles
70 mph = 1.2 miles

All cubs will differ just as do their stall speeds. Since all wings are not bolted to the airframe the same and rigging is all over the map I expect there to be plenty of variation.

Jerry

I was responding to the VSI opponent, not the chart. I wrote what I was thinking without doing much thinking. My mistake. Please continue.

Damn, I can't stand it anymore! Charts, book recommendations, POH's! Anyone else see a simpler answer to the question?

Go up to whatever altitude that makes you feel safe, over whatever terrain you're comfortable over, and shut the frigging engine down and do some honest to god dead stick flying to establish your personal best speed. If you don't have a VSI use a stop watch and the altimeter. Yes I know the question was best glide not best sink rate but you can extrapolate from there. Plus you'll get invaluble real world dead stick experience while you're at it. If you're not comfortable doing that, you will be doing yourself a favor by getting so.

Oh, and if your a constant speed prop person, some cubs are.... Pull the prop controll out for course pitch. The drag is noticeable. Just push it back in to drag it up before you ditch/touchdown.

courierguy said:

Damn, I can't stand it anymore! Charts, book recommendations, POH's! Anyone else see a simpler answer to the question?

Go up to whatever altitude that makes you feel safe, over whatever terrain you're comfortable over, and shut the frigging engine down and do some honest to god dead stick flying to establish your personal best speed. If you don't have a VSI use a stop watch and the altimeter. Yes I know the question was best glide not best sink rate but you can extrapolate from there. Plus you'll get invaluble real world dead stick experience while you're at it. If you're not comfortable doing that, you will be doing yourself a favor by getting so.

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Done it but I was sweating so much with the fan off my notes got soggy

This a simple way to test your airplane, the way it is configured.
1. Fly early am when there is little or no wind.
2. Climb to 3000 feet above the ground, set up your glide speed (stabilize it) with the power off and trim set and follow a set heading.
3. Record two things, the Time it takes to go from 2500 to 2000 feet, and concentrate on your GPS groundspeed (average rate for the time interval). With no wind, and careful airmanship you will be able to tell how fast you are gliding in a horizontal direction).
4. That's it! You only need to record those two parameters as you repeat the test at 50MPH, 55MPH, 60 MPH etc.

Of course you realize it is just a test, with lots of variables that could play into your situation if/when you have to glide for real. I discovered my cub with 31 inch tires, vg's, and the Boher prop glided Farthest at 50 or 55MPH, NOT 70 MPH. Landing in water a 1,000 feet short of the beach would not be good! I found I hah 4,000 feet of horizontal glide at 50MPH or 55MPH for an 8:1 glide ratio. That was for that day at 1550 lbs., 45 degrees etc. If I had to glide into a wind, the problem becomes more difficult and the answer more obscure, but I also discovered I had almost twice as much time to glide at 50MPH than I did at 70MPH (46 seconds vs. 27 seconds per 500 feet of altitude loss). That extra time might give you a chance to either start the engine again, collect your wits, time to communicate over the radio etc.)

To calculate how far I glided in a set amount of time, I multiplied the seconds it took to glide down 500 feet of altitude by how far I would have covered over the ground using the GPS groundspeed which I converted to feet per second (60MPH=88 feet per second). 88/60MPH = 1.466 FPS per 1 MPH. So if a glide took 30 seconds, and your average GPS ground speed was 75MPH you would calculate the horizontal distance traveled as 75 x 1.466 x 30 = 3,299 feet. Do this calculation for several runs, each at a different glide speed using your airspeed indicator, and you can get a feel for best glide speed.

Homerrhoid said:

I discovered my cub with 31 inch tires, vg's, and the Boher prop glided Farthest at 50 or 55MPH, NOT 70 MPH.

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Do you still have enough elevator authority at 50 (no power) for the flair?

Jerry

Old wives tale? Close throttle, full nose up trim. hands off control stick, the aircraft will develop its own best glide.

Best glide is to accomplish longest time airborne before impacting mother earth or furthest distance possible without a power source? (I'm asking) If I have a ten MPH headwind, my PA-12 will develop a 850+ fpm sink rate at 58-60 MPH. It seems as if it is almost a vertical decent. I would imagine (as I do almost 100% power off three point spot landings) that once within 100ft AGL of terra firma that one would hope to have sufficient airspeed to enable for some fancy maneuvering should the need arise. I certainly do not wish to be approaching a landing spot on the verge of a stall without a fan and have a surprise waiting that I did not observe on the way down. I practice on a regular basis. I have found that I can do a turn back from 650 ft AGL with no obstruction. Practiced that as well.

To address cubliers question. I need some speed for elevator authority or I am going to pancake in and wipe out the gear. My 12 does not float like an 18.

Seaworthy said:

Old wives tale? Close throttle, full nose up trim. hands off control stick, the aircraft will develop its own best glide.

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Tom, it has been close enough in every plane I have flown (never tried it in a twin), maybe not 100% accurate, but it gets the job done especially when seconds count.

sj

cubflier, nobody has as much flare as you do!!!

sierra bravo said:

I favor the POH and it's emergency procedure speeds because in the real world an emergency requires situation management where airspeed control is just one part of the pilot's unexpected workload. Add hostile terrain, uncomfortable turbulence, and terrified passengers to the mix. The POH recommended speeds work pretty well for an overloaded and distracted pilot. I took the post topic in the context of emergency procedure. Ideal condition and best case simulations aren't the same thing.

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When the engine quits, I'll use the full nose up trim trick rather than attempting elevator inputs to maintain an IAS. I then have both hands free with the stick locked between my legs. Seconds count. I don't fly that high.

Lots of good suggestions here, and an important topic which is not well understood, but let me offer a few thoughts:

1. Many/most power pilots conflate best glide speed (also known as best Lift-to-Drag, or L/D for short), with minimum sink speed. Except for a very few specialised applications, such as human-powered aircraft, these are almost never the same. Best L/D, to cover the most territory is normally a bit to QUITE a bit faster than minimum sink speed and headwinds/tailwinds can have a huge effect. I'll not drag out the moldy old Aero-E texts nor bore you with a pedantic proof here.

A couple good references for the concept are from the Soaring world: The FAA "Glider Flying Handbook" is a remarkably well done compendium of this and other information; and Bob Wander's "Glider Polars and Speed-to-Fly Made Easy" focuses mainly on this topic. Both available from the Soaring Society of America (www.ssa.org) and from Bob Wander (www.bobwander.com).

2. As far as the airplane is concerned, it's actually angle of attack which matters, although this translates roughly to airspeed. For a given aircraft configuration, the a.o.a. for best L/D (and similarly for minimum sink) is substantially the same regardless of gross weight, but the airspeed for that a.o.a. increases as the gross weight goes up. There are some effects from centre-of-gravity position and density altitude, but these can be largely ignored for Cub type aircraft.

3. Most altimeters have significant hysteresis (lead/lag) at some points of their range as the internal gears rotate. There is also significant lag in indicated altitude, both going up and going down. This varies significantly from ship to ship. Therefore the altimeter and stopwatch technique will give a rough general idea of sink rate for a given indicated airspeed, but should be repeated across several altitude bands to minimise data scatter. GPS altitude "might" be better.

Guess it's time for me to quit droning on and go calibrate glide speeds for the cubscout 'Cub.

Thanks. cubscout

I don't know how to link with this smart phone I'm on tday, but if youtube is searcd for"deadstick ridge soaring" a video of mine comes up. I get several hours in a year deadstick, hopefully it would help in an emergency? It sure is fun and really saves on fuel......

Randy - I can't fathom out how to get a PDF to upload so I'll bring the Scheduled Performance Manual with me to NH - look forward to meeting you - Perhaps the FBO will have a copier. I aim to come on the Tuesday (traveling from UK on Monday) and to go over to OSH on the Thursday.

It seems to me that 60mph is the general concensus which is pretty close to the official Piper scheduled speed of 58mph.

Seaworthy said:

I'll use the full nose up trim trick rather than attempting elevator inputs to maintain an IAS. I then have both hands free with the stick locked between my legs.

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This method will produce the least amount of tail drag because the elevator will not be opposing the stabilizer.