The International Cessna^® 170 Association

Hello Everyone

This would be my first post on this site. I am a relatively new 170 owner (1956 170B) and have to say that the plane is a blast to fly. It is a fairly stock 170 with original O-300 engine. I am still getting a feel for the plane and have a few questions that I would like to pose to all the group.

My question is regarding V speeds. What is the manuevering speed and the best glide speed for 170B?

Thanks again and I look forward to learning quite a bit from you all!

Kevin

Manuevering speed (Va) is 115 mph. (found in the limitations section of the Owners Manual.)
Best glide is 76 mph. (although not published, this speed is equivalent to best rate of climb (Vy), found in the performance section.
Both these speeds are dependent upon aircraft wt, and should apply to 2200 lbs. Lighter weights would result in lesser speeds in both categories.

George is right (as usual) but before I would put a number on best glide (since every aircraft is different, after 50 years of people tweaking them) take your aircraft up and level the wings -pull the power- and watch the airspeed indicator....this will give you best glide, at the weight at the time....this just happens to correspond inversely to VY (best climb over time @ full power)....normally....there's always a reason for math not to work under certain circumstances.....

bradbrady wrote:George is right (as usual) but before I would put a number on best glide (since every aircraft is different, after 50 years of people tweaking them) take your aircraft up and level the wings -pull the power- and watch the airspeed indicator....this will give you best glide, at the weight at the time....this just happens to correspond inversely to VY (best climb over time @ full power)

I don't get this. How is the airspeed indicator by itself going to tell me anything about best glide?

If I have an airspeed indicator and a VSI and I fly a series of tests at different airspeeds and note the associated Vertical Speed for each given airspeed, I can calculate the glide ratio at various airspeeds and see where I get the best glide. Is that what you meant or is there another trick

wings level + No power = best glide, VSI should show least sink.....

bradbrady wrote:wings level + No power = best glide, VSI should show least sink.....

Nope...wings level, + no power, VSI will show least sink when flying at ... min sink speed, not best glide speed. Best glide in still air will be achieved at a somewhat higher speed. Refer "Stick & Rudder" section on "Working Speeds" of airplanes for more detail.

For gliders, we have charts, known as "polars" that plot sink rate against airspeed as airspeed changes and the shape of that curve allows us to determine min sink and best glide speeds readily.

Flying into a headwind, you need to fly even faster to achieve best glide over ground (rule of thumb... add 1/2 the wind to your still air best glide speed) and in a tailwind, you fly slower, but never slower than min sink, to let the wind carry you further downwind.

The same principles apply to an 170 or a 747 in a glide.

That said, I would still like to determine my best glide speed and glide angle, both of which change significantly if you get the prop stopped.

Pete,
there is no argument here....but your splitting hairs.....I understand what your saying.....but if you can fly an aircraft +1or -1 knot your a much better pilot than I am.....I'm talking general...wings level+no power= best glide I'll stand by that!!! I'm talking about aircraft that has been tweaked, you never know what the real glide rate is...until you try what I 'm saying, (in general) depending on how the aircraft has been rigged.......the glide can very 5 to 10 knots. from book....using best climb from the book.....so if you try what I'm saying you should be within 1 knot.(in theory)

For gliders, we have charts, known as "polars" that plot sink rate against airspeed as airspeed changes and the shape of that curve allows us to determine min sink and best glide speeds readily.

Flying into a headwind, you need to fly even faster to achieve best glide over ground (rule of thumb... add 1/2 the wind to your still air best glide speed) and in a tailwind, you fly slower, but never slower than min sink, to let the wind carry you further downwind.

The same principles apply to an 170 or a 747 in a glide.

So Pete did last weeks British Airlines glider pilot look up the speed he used in the Triple Seven Polar Chart or just make a penitration and hope for the best? What ever he did it worked for him.

Pete's description of best glide and how it is achieved is something the average private pilot does not understand. I am surprised during BFR's by pilots, during an emergency landing exercise, who peg some book figure and do not adjust for headwind or tailwind as necessary to manage their glide. Very few instructors provide their basic students with the opportunity to practice managing a glide. As a result, with the exception of glider pilots, managing a glide is a lost technique. I believe obtaining a glider rating does as much for a pilot's safety and proficiency level as the instrument rating.

"Best glide speed" is ordinarily defined presumeing still-air, and is intended to provide maximum time aloft for the available altitude, for the purpose of trouble-shooting a failed engine, and/or selecting a suitable landing site. I'm sure that's what Kevin was seeking.

"Best glide speed for distance/range" considers wind, and is used to reach the selected site.

And extreme example would be an airplane with a "best glide" speed of 100,... with a headwind of 100. ANY increase of indicated airspeed will increase the gliding range of the aircraft despite the loss of time aloft.
However with a 100 mph tailwind it might be prudent to fly the Best Glide speed in order to maximize the time aloft, if maximum distance is also desired. This is why I always taught students/clients to look first upwind, then downwind when searching for a site. (Off-shore situations might also make good use of knowlege of best glide time vs distance.)

johneeb wrote:

So Pete did last weeks British Airlines glider pilot look up the speed he used in the Triple Seven Polar Chart or just make a penitration and hope for the best? What ever he did it worked for him.

John:

I don't know but I can tell you that I own a glider in partners with a NWA 747 pilot who knows the numbers and tells me that best glide i/e the speed that gives you the best L/D for a 747 is somewhere upwards of 200 kts and achieves a glide ratio of about 15-20/1. That upper end is better than the early training gliders I learned in. However, I would rather be touching down off field at around 40 in a Schweizer 2-22 than a 747 doing about 200!

BTW, the guy flying the Gimli Glider , an Air Canada 767 that ran out of gas in mid flight, deadsticked into an old RCAF base turned into a drag strip. He was/is an experienced glider pilot and did use best glide to make the field and reportedly slipped to a landing when he was a bit high.

Also, let me repeat my plug for my favorite flying book, Stick & Rudder, by Wolfgang Langewiesche. Its a classic and contains information that lots of new pilots were never taught and that old pilots have forgotten. In particular, his chapter on the Working Speeds of an Airplane covers Vx, VY, min sink, best glide, etc in an easy to understand manner that has practical application still today for all of us.

gahorn wrote:"Best glide speed" is ordinarily defined presumeing still-air, and is intended to provide maximum time aloft for the available altitude, for the purpose of trouble-shooting a failed engine, and/or selecting a suitable landing site. I'm sure that's what Kevin was seeking.

You are correct Gahorn. Now first off, great discussion guys. Being a fairly new pilot (within the last 2 years), a lot of this information (at least in my experience) wasn't discussed in flight training. The idea of increasing/decreasing best glide speed based on winds was not discussed in my training. I was only told to target your Best Glide speed right from moment of engine out to landing. I am a firm believer however in the PPL as a "License to Learn". I believe it it is my responsibility to continue to learn as I gain hours.

Now back to your original response "Gahorn". I am a bit confused as to the speeds that you quoted. For the Va of 115mph. The only place in my POH that I see this quoted is in the "Operating Limitations" section regarding speeds for Steep turns, Lazy Eights etc. This speed is quoted under the Utility category of 1900 lbs. Wouldn't the Va be >115mph (as it is usually quoted for gross wt)?

You also mentioned that Best Glide is equivalent to Vy (or in this case 76mph). I have been a bit confused as to the actual Vy for my plane? In the Operating Checklist section it states 88mph as Best Rate, in the Operating Details section it mentions 78-85 mph (based on alt) and then in the Performance Section 79-89 (based on alt). Which of these numbers should I be using?

Again, thanks guys for all the responses!

Kevin

gahorn wrote:"Best glide speed" is ordinarily defined presumeing still-air, and is intended to provide maximum time aloft for the available altitude, for the purpose of trouble-shooting a failed engine, and/or selecting a suitable landing site.

Usually "best glide" gives you the most distance for altitude lost (calm wind) and "minimum sink" speed gives you the most time in the air regardless of wind.

Well, I should not post after working all day and all night. My synapses sometimes take shortcuts under those conditions. (I posted that msg at a ridiculous hour and apologize.)

Best glide speed, does indeed, provide best distance for loss of altitude in still air.

Kevin, those are good questions.

Firstly, it's important to remember that the Owner's Manual not a Pilot's Operating Handbook. It is not an "approved" document, nor is all the performance-related information contained therein, (although some approved-data contained in the Approved Flight Manual (AFM) is repeated there.)

I believe the take-off-climb recommendation of 88 mph makes the assumption of a standard day, sea-level takeoff.... which would agree with the data contained in the performance section as well as in the AFM.

The recommended entry-speeds in the limitations section are combined with the weight limitations of the utility category, and Va (normally computed at gross wt.) would in most cases be a greater, not lesser, speed, (At reduced weights, aircraft experience increased limitations on manuevering speed, therefore speed must be reduced. This is because Va is the speed at or below which aircraft will stall and relieve flight-load-factors before full/abrupt control movement will cause permanent structural damage. Since aircraft stall at lower speeds when lighter, their manueving speed must also be reduced accordingly.)

The figure of 115 mph for Va was derived from the FAA Type Certificate Data Sheet, which (curiously, for all Cessna 170's, ragwings, A and B models) is stated to be 115 mph or 100 kts. I find that curious due to the obvious differences between the 170 wing structure and the structure of the later, more similar, A and B models.

I believe that Cessna is merely making the recommendation of 115 mph as an entry speed for the approved aerobatic manuevers listed, and that they are not claiming that 115 mph is an equivalent "Va" at those weights.

Similarly, the seaplane speeds for those manuevers is computed at a wt 75 lbs heavier and might lead one to feel one could develop a guide or graph, but I believe that any such method might not be accurate considering that the seaplane has additional structural considerations lacking in the landplane. The AFM tells us that the seaplane Va is 105... regardless of Cessna's differing recommendations for those maneuvers... notice that Cessna ignores seaplane Va in both directions, recommending 100 mph for steep turns, but 110 for lazy 8's/chandelles.

As for "best glide" speed.... while Vy is not always equivalent to best glide in all aircraft, it is a good approximation in this one. Likewise "minimum sink" speed might be comparable to best-angle (Vx) which Cessna indicates to be 76 mph in the takeoff-distance charts (which is approved data, also found in the AFM). Therefore, at flaps zero, gross wt, standard conditions, I believe 76 mph is equivalent to Vx, and 88 mph is equivalent to Vy. At flaps 20 (B model) 67 mph is best angle.

I've written this after a nights rest and hopefully it is more reasonable.

Did someone mention the Gimli glider? Ironically Air Canada just retired her last week. Fin 604 did a final fly-by leaving Montreal for MHV on January 27th. They used the call sign "the Air Canada glider".