Aerodynamics theory question

[Sbrookfield]

Here's the real life scenario: You have a partial engine failure, engine will only put out say 1,600 rpm for whatever reason. The nearest safe landing area is a ways away (out over the ocean or something). Assuming 1,600 rpm is not enough for you maintain altitude, what airspeed would you fly to get the closest to shore as possible before going in? VX or Vglide?

I can remember some tables we looked at in college which explained how Vx was determined, having something to do with the maximum difference in power or (thrust?) required Vs. Power available. We all know V glide is the intersection on the graph of induced and parasite drag (lowest total drag point).

I'm fairly confident that I know the right answer, but I'm hoping someone here remembers or knows the aerodynamics better than I do and can answer this with an explanation defining the difference in Vx and Vglide.


Hope this isn't too much of a common sense question.

Thanks

[8GCBC]

Who publishes Vglide? Never used it on a check ride last 37 years?

[Sbrookfield]

I don't know about too many other POH's but its pretty clearly stated in the 172 POH. Around here if you show up not knowing Vglide, or not using it when the check instructor "fails" your engine, you won't be passing said checkride, in any airplane (SEL). Just from what i've observed.

[8GCBC]

Best glide speed, not Vglide.

[Sbrookfield]

touche... i suppose i've been educated

[8GCBC]

I wasn't trying to find error, I really never saw it before!

[Sbrookfield]

ha there for a second i thought every thing i'd learned since day 1 was a lie

[G44]

You want to fly max L/D if your objective is distance.

[HydroCub]

If not published, best distance glide airspeed is usually very close to Vy as this airspeed is typically at or near V/Dmin. Partial power does throw another factor in. In a glider, the main airspeeds to know are V(min sink) and V(best distance). If you have a headwind, you add airspeed to best distance.... with a tailwind, you can trend slower towards minimum sink airspeed.

With partial power, you're really in need of a drift down airspeed which is not published for single engine aircraft.

In no case would you fly Vx.... it's too slow and would usually result in a higher descent rate with a lesser ground speed, especially with a total power loss.

[Speedo]

I guess you could determine it empirically using a VSI or an altimeter plus a watch. Sounds like a worthwhile experiment; if you do it please post your findings.

[HydroCub]

I guess you could determine it empirically using a VSI or an altimeter plus a watch. Sounds like a worthwhile experiment; if you do it please post your findings.

Remember Eric.... you have to relate descent to distance, not time. It really relates to ground speed, not time. Basically, it's the Vx and Vy thing inverted to a descending condition rather than climb. You are looking for the airspeed that gives the wing the best glide ratio. Too slow, you sink.... too fast, you dive altitude away.

[articfox]

I agree with the VSI approach. Most of the cub stuff is so different in that with additional parasite drag and messing with the wings you will get a different glide from bird to bird. It always seemed to work by using a VSI and determining the min. sink rate. That is if you have one... Trim plays a important role so your gliding trimmed vs gliding fighting. A big mistake at low altitudes is using excess airspeed to gain altitude. Maintaining altitude while slowly increasing AOA to your known Best Glide speed than seems to help. Stopping the prop is usually a good way to screw it all up as you have to slow the airplane past Vg to get the motor stopped. Unless your Buck Rogers, or you can stick your foot out to stop the prop it is best to just descend at that glide speed for your bird (or wing).

[Aviator]

As mentioned above, the best A/S depends on several factors, and difference can be huge. It could even be Vne or more. Any fixed "best range speed" is nonsense. My 2c.

[aktango58]

I agree with Hydro but will add this:

Once you get into ground effect your entire scenario will change. Stories about a gooney bird in ww2 that lost an engine at altitude in the pacific and was going down. They finished the flight for over 500 miles at less than 50', made it to land and put her on the runway.

[HydroCub]

I agree with Hydro but will add this:

Once you get into ground effect your entire scenario will change. Stories about a gooney bird in ww2 that lost an engine at altitude in the pacific and was going down. They finished the flight for over 500 miles at less than 50', made it to land and put her on the runway.

Ever read about Max Conrad's record setting ferry flights..?? He used to fly Twin Comanches so low the ocean spray was hitting his windshield.

[FdxLou]

For a little insight into this discussion you may want to read the article by Barry Schiff in this months AOPA Pilot.....July, 2012, page 18. "Soaring Airplanes"
Food for thought.

Lou

[aktango58]

Will look for that reading.

Thanks

[sierra bravo]

best distance glide airspeed is usually very close to Vy

Not true for the Cessnas I've flown and I'm not sure it applies to my -12, either. In a C-180 the best glide speed is near the middle between Vx and Vy but favors Vx slightly. Vx is 63mph, Vy is 90, best glide is 75. I recall 172s having a best glide speed just a few mph above Vx. Thanks for the topic, though. It made me think about something that I hadn't thought about in a while.

The best demonstration I can recall for the contribution of idle thrust comes when landing on skis on slick ice. To answer the OP question, I'd use best glide in a partial (near idle) thrust emergency descent.

[SJ]

Not true for the Cessnas I've flown and I'm not sure it applies to my -12, either. In a C-180 the best glide speed is near the middle between Vx and Vy but favors Vx slightly. I recall 172s having a best glide speed just a few mph above Vx. Thanks for the topic, though. It made me think about something that I hadn't thought about in a while.

SB, I still think best glide is closer to VY than VX in most planes, including the 172 and 180, I guess googling a little would probably tell us for sure but I seem to remember VY in a 172 around 75 kts, best glide around 70kts, and best angle in the low 60's.

Although it does not matter in this case, what I tell students - especially in cessnas and other similar singles - is don't look for best glide. Go to full nose up trim without flaps and in every cessna single I have flown this gets you really close to the best glide speed - within a knot or two - and gets your attention away from trying to "peg" a number on the ASI and into figuring out the problem and/or concentrating on your landing options. Students (and people on BFRs) will use up to a minute trying to find the best glide never looking outside to see where they are headed. The full nose up trim option eliminates that time waste - and you might have time later to tweak it. Now, if you find out you had unselected fuel and switch tanks, be ready for a lot of forward yoke to avoid an elevator trim stall.

Your mileage may vary!

sj

[SJ]

Ok, so I did some googling after my post. The 182/180 is what I posted above, the 172 is a lower glide number than I remembered.

sj

[sierra bravo]

The V speeds I listed are pretty darn close to what my POH states. I may have missed by one mph but not more. While doing the Google thing I happened across this article. Interesting reading. http://www.swaviator.com/html/issueAM00/basicsAM00.html

I was taught to trim to the desired speed, not to expect a speed from a predetermined trim setting. Loads vary. And so does trim. That said, and consistent with the linked article, consider your operating weight and how that impacts published glide speed, which was established at gross. The lighter the plane, the greater the (theoretical) deviation from Vy.

FYI

V SPEEDS (KIAS)(C-172M)
Vso..........................................41
Vs............................................47
Vr.......................................55-65
Vx (sea level)..........................59
Vx (10,000 ft.).........................61
Vy (sea level)..........................73
Vy (10,000 ft.).........................68
Vfe...........................................85
Vno........................................128
Vne........................................160
Va (2300 lbs)...........................97
Va (1950 lbs)...........................89
Va (1600 lbs)...........................80
Best Glide (flaps UP)..............65
Best Glide (flaps DN)..............60
Max. Demon. X-Wind..............15
Approach (Flaps UP).........60-70
Approach (Flaps DN).........55-65

A Supercub Best Glide at 1750# is identical to Vy according to this

Speeds IAS [mph] @ MTOW (1750lbs)
VR (Rotating speed) --------------- 40 mph
VX (Best angle of climb speed) ---- 55 mph
VY (Best rate of climb speed) ------ 60 mph
VA (Maneuvering speed) ---------- 96 mph
VG (Best glide speed) ------------ 60 mph
VNO(Max.struct.cruisingspeed) -121mph
VNE (Never exceed speed) -------153 mph

[SJ]

I don't disagree at all, and not doubt I am wrong as usual on the numbers. I'm just saying that in a real engine failure situation - not a controlled one on a checkride / bfr - getting within 5% by doing something quick and dealing with other matters could save your life better than fiddling around trimming to a precise speed.

[Cub Builder]

This is one of those been there, done that things... at night. Happened in my Stinson 108-1. Ended up losing all three left cylinders in sequence in about a 30 second time frame. It produced 1500 - 1700 rpm running full throttle on 3 cylinders, which isn't much power... at all. I held my altitude while I trimmed the plane and slowed to best glide while toggling through the various checks (carb heat, full rich, switch tanks, try mags independently). This happened 8 miles from the airport at 8,000' with the nearest airport runway altitude of 6344'. The partial engine extended the glide to 8 miles and made it to the end of the nearest runway with about 150' to spare, so my rate of descent was roughly 1500' for 8 miles distance. I suppose it might have been possible to have pulled a bit more out of the plane, but if so, it's unlikely that I could have with my skills under those circumstances. I thought I had handled it well in the air. Once on the ground I found that I had so much adrenalin in my system that I was incapable of intelligent conversation (like calling a friend to come get me).

-CubBuilder

[8GCBC]

I fly with this in mind....

With power, without power, with a little power, with intermintent power, stuck prop, no prop (it happens) Vy is best (+/- 5% [density altitude, STC's, LBS, hangar rash, rigging vary Vy]).

[aktango58]

SJ,

I always taught students to learn the attitude...

Set the attitude and let the speed come while you are spotting a place to put down, undoing what ever you just did to cause the trouble and run through the check lists...

By the time you are done with this, it is time to reset the attitude...

[8GCBC]

I believe it is possible to achive Vy (within reason) without an Airspeed Gauge (looking outside for attitude reference, attitude gyro, altimeter, VSI, noise). A good exercise for a BFR.

[sierra bravo]

According to the article I linked using Vy rather than best glide speed in a 172 will reduce your glide range by approx 30% while a Cub's best glide is Vy. It seems to me the pilot needs to know the appropriate speeds for the plane he's flying. I'll try SJ's trim nose up idea to see how it works out. I wouldn't trust it until I validate it.

[8GCBC]

Agreed, I don't believe published (or advised) performance number(s) until proven in my aircraft. Even factories have published insufficient performance data. Every airplane and environment is different.

[8GCBC]

Mr. Sierra Bravo:

Question: Best glide speed maybe proven slower than Vy because of propeller thrust used in Vy? But, both are very close to L/D max?

Thank you for the tables.

-- 8GCBC

[sierra bravo]

Standard answer? I just drive 'em. I don't know what makes 'em work.

[8GCBC]

OK with me.

[RCharles]

Take your GPS , and on a calm day , with little wind aloft , and someting to record you results. Make 1000ft or 2000ft decents in opposit directions at different power settings and speeds .Average the results and you can find the best glide for distance or the best time aloft speed just for your aircraft .You can vary the weights or whatever to determine the numbers . I did this with my son when was in 8th grade for a science project . He did the flying and I recorded the results .The test were to determine whether the glide charts for the L21B were accurate .We proved that the plane would not glide as predicted ..The Charts were overly optimistic ...Surprise !

[dougc905]

Excellent project! Has your son come up with another project to match or beat this idea? I just finished marking 50 or so grade 9 projects where the majority of the kids tried the same-old thing. What I like about your idea is that you gathered data over a variety of power settings. What I was reminded of this year is that too many students have no idea how many data points or trials they need. Too many kids think that a graph made with three data points is sufficient.

Take your GPS , and on a calm day , with little wind aloft , and someting to record you results. Make 1000ft or 2000ft decents in opposit directions at different power settings and speeds .Average the results and you can find the best glide for distance or the best time aloft speed just for your aircraft .You can vary the weights or whatever to determine the numbers . I did this with my son when was in 8th grade for a science project . He did the flying and I recorded the results .The test were to determine whether the glide charts for the L21B were accurate .We proved that the plane would not glide as predicted ..The Charts were overly optimistic ...Surprise !

[SJ]

According to the article I linked using Vy rather than best glide speed in a 172 will reduce your glide range by approx 30% while a Cub's best glide is Vy. It seems to me the pilot needs to know the appropriate speeds for the plane he's flying. I'll try SJ's trim nose up idea to see how it works out. I wouldn't trust it until I validate it.

You should never trust anything I say. I run a website, therefore I must have already lost my mind

My point is not that full nose up trim achieves precise best glide, but that the net effect of time lost hunting for best glide vs just using full nose up trim is in favor of the latter. If find it to be pretty close in every airplane I have ever tried it in, including a cub.

It is also very important that the cub you are "VY'ing" in is a stock supercub, no big tires, no VGs, no other lift /drag mods, and no blah blah blah. All of those things could play a role... so don't trust those either!

sj

[SteveE]

Just for the heck of it one day I went to 2000', pulled power back, best glide 60 mph, and went 2 miles before I would have had to land. Best I remember there was no wind. So basically my rule of thumb with no wind I could go 1 mile per 1000'.

[8GCBC]

Some aircraft may do this...(?)

Vy TAS is a little faster than "Best Glide" TAS because propeller efficiency actually increased with airspeed, thus giving more thrust and a small amout of lift too, this helps with AOA.

My theory is Best glide (L/D max)+ excess thrust (if available) = Vy

[SJ]

Oh and to further play advocacy on part of that guy that lives WAY down under (and I don't mean Australia), I have not flown a cub type airplane with an accurate airspeed indicator. Certainly mine is not even close. So a level flight attitude in glide, or slightly nose down from level, will probably be really close to VY / Best Glide in a cub. Give that a try... Oh, dare I say, works in other planes too. Shoot for level flight attitude and see how close you are.

Report back...

sj

[8GCBC]

Using longitudinal outside visual references is a great idea for determining an approximate performance value. Will do a little more practice using longitudinal outside visual references (concerning Best Glide and Vy). Thanks Steve for focusing us on this.

[Frodo]

L/D Max (no wind) regardless of partial power. L/D Max is not an airspeed, it is an AOA! We who have no AOA gauge choose an airspeed that approximates L/D Max for a nominal GW and call it best glide.

Think of it this way, all Cubs have two engines. One is metal and burns gas to create thrust. The other is gravity and burns altitude to create thrust. No gas, you have no thrust from the metal thing up front. No altitude, you have no thrust from that blue ball called Earth. L/D Max requires the minimum thrust. The combination of the two engines have to add up to this amount of thrust. What ever power you get from the metal one allows you to minimize altitude burn rates. If it produces more than required for L/D Max, you now have a chance to fill the tank for your gravity engine.

Other consideration: L/D Max gives you a set glide path regardless of GW. A heavy GW simply means that you will get there faster. What changes the glide path is drag. More drag, steeper glide. So, if you think flaps help with glide path, you are wrong, unless for some weird angle of incidence reason, some flaps reduces your drag (highly unlikely).

[RCharles]

On my son's project , we made power off descents and varied the speeds to achieve the longest glide . I believe our results were very close to what SteveE said above. One thing we realized , is that the glide chart depicts a straight line to touchdown ,where as a real life situation would require some maneuvering to set up for a field or such thereby using up valuable altitude . I am going to try to find that old project and see what our numbers were... RC