Lift Strut Data

[GAHorn]

I'm always skeptical of such claims. Some of you engineering types help me out here. If drag increases as the square of velocity,...then in order to achieve this remarkable increase in speed, what percentage of drag would have to be reduced by adding this additional equipment?

[Dave Clark]

Yeah George, I'm also skeptical, to the point of not doing any of the speed mods. I think you'd have to do about all of the things possible to see any real benefit. By that time you could have afforded a 180 or some other plane that can go wheels up. I still cling to my 6:00x6 tires though.

[cessna170bdriver]

I'm always skeptical of such claims. Some of you engineering types help me out here. If drag increases as the square of velocity,...then in order to achieve this remarkable increase in speed, what percentage of drag would have to be reduced by adding this additional equipment?

At a given horsepower, drag multiplied by speed is a constant. If you do the math, a 5 percent increase in speed only requires a 4.76% reduction in drag. Because drag is proportional to flat plate area times the square of the velocity, to accomplish that drag reduction, the equivalent flat plate area only has to be reduced by 2.4%. For the same speed increase by adding horsepower alone, the increase would have to be 15.76%.

Rev 1: OOPS!! Looks like I mixed apples and oranges and slipped a cog in my algebra the first time through. Actually a 5% increase in speed will require a 13.6% reduction in flat plate area.

I guess that's what happens when electrical engineers start messing around in the real world


Miles

[Bruce Fenstermacher]

Yep Miles I couldn't have said that any better.

Why just the other day I saw a friend dragging around some flat plates and I told him it was obvious he could use more horsepower.

[N170CT]

Miles,

Dumb question, I know...but have you ever done any calculations or taken any measurements to determine an Equivalent Flat Plate area for the Cessna 170B??? I would be keen to know that value. I realize that there are a whole bunch of parameters that would influence such a number, but one could make assumptions. For instance, the number of external antennas, venturis, type of spinner, tire size, etc .
chuck

[cessna170bdriver]

Yep Miles I couldn't have said that any better.

Why just the other day I saw a friend dragging around some flat plates and I told him it was obvious he could use more horsepower.

That's right Bruce, the larger your flat plate, the more horsepower it takes to drag it around at a given speed. EQUIVALENT flat plate area just gives us a common means of comparing the drag of different shapes, the same as horsepower is a means of comparing the performance of different engines. We're no more dragging around actual flat plates than actual horses are turning our propeller.

Miles,

Dumb question, I know...but have you ever done any calculations or taken any measurements to determine an Equivalent Flat Plate area for the Cessna 170B??? I would be keen to know that value. I realize that there are a whole bunch of parameters that would influence such a number, but one could make assumptions. For instance, the number of external antennas, venturis, type of spinner, tire size, etc .
chuck

Calculating actual drag from the shape of the airplane and the shape of things attached to it is an exercise in computational fluid dynamics. That's what Kent is doing to analyze the drag reduction of possible new fairing shapes.

However, if we know how much horsepower it takes to pull our planes through the air at a given speed and air density, we can calculate the drag (in pounds force), and equivalent flat plate area(in square feet):

Assumptions:

1. Book performance (1956 Owner's manual) of 126mph at 77% power at sea level.

2.Propeller efficiency:80% (Essentially the percentage of power input into a propeller that can be converted to useful thrust) This is just a ballpark number for discussion sake.

3.Standard sea level conditions (Air density of 0.076 pounds mass per cubic foot)


The math:
1. Horsepower = drag(pounds force) * speed(feet per second) / 550 (In other words, if you know the horsepower required to generate a particular speed, you can calculate the drag.)

2. Bernoulli's theorem(English units): drag(pounds force) = Flat plate area(square feet) times air density(pounds mass per cubic foot) times velocity squared (feet per second, squared) divided by 32.2 (Relates area, fluid density, and velocity to drag. Assumes a constant density (incompressible) fluid - not a bad assumption at the speeds we operate.)


Running the numbers:

77% of 145 HP is 112HP into the propeller. An 80% efficient prop will convert this to just over 89 thrust horsepower. At 126 mph, this converts to about 266 pounds of drag. Using Bernoulli's theorem (remember ground school 101 ) we find that to generate 266 pounds of drag at 126 mph would require a flat plate of roughly 6.6 square feet.

I would assume that the book numbers, are for new, aerodynamically clean airplane, along with a smattering of marketing optimism. A more common occurrence might be 115mph at 75 percent power, yielding a flat plate area of almost 8.5 square feet.

I welcome any of you aero types to check these numbers. I'm just an Electrical Engineer that knows how to look up the formulas and turn the crank.

If you have read through all this without your eyes rolling up in your head then you’re too much like me…. GET A LIFE!

Miles

[Bruce Fenstermacher]

I guess I should have been more specific Miles

He WAS dragging around some flat plates and I said you need a John Deere for that. He was using an old Farmall.

[N170CT]

Miles,

You da man...What a delightful exercise in mathematics.

Now all I gotta do is understand what you writ.

Thank you very much for taking the time.

chuck

[Dave Clark]

Ok Miles good work. So to ballpark a 5.575 mph increase in airspeed (5% of the 115mph) one would need to decrease the flat plate area by 5% of the 8.5 square feet or .425 sqare feet. That's 61.2 square inches if my math is correct. That's a 6x10 plate! Hmm, maybe I could remove one of my main wheels...

Seriously I look at my dirty plane but just can't see how I can accumulate enough areas to make a significant difference in flat plate area. Takes me back to my point in my prior post. It's tough to get significant speed increase without doing everything possible, efficient wheel fairings (including tailwheel), flap and aileron gap seals, hidden antennas, wingtip lights, strobes, gear leg fairings, strut cuffs etc etc.

Now if we were starting at a cruise speed of 180mph or so it might be a different story. I'll keep my antique just the way it is as I've got better ways to spend the money.

[cessna170bdriver]

Ok Miles good work. So to ballpark a 5.575 mph increase in airspeed (5% of the 115mph) one would need to decrease the flat plate area by 5% of the 8.5 square feet or .425 sqare feet. That's 61.2 square inches if my math is correct. That's a 6x10 plate! Hmm, maybe I could remove one of my main wheels...

Seriously I look at my dirty plane but just can't see how I can accumulate enough areas to make a significant difference in flat plate area. Takes me back to my point in my prior post. It's tough to get significant speed increase without doing everything possible, efficient wheel fairings (including tailwheel), flap and aileron gap seals, hidden antennas, wingtip lights, strobes, gear leg fairings, strut cuffs etc etc.

Now if we were starting at a cruise speed of 180mph or so it might be a different story. I'll keep my antique just the way it is as I've got better ways to spend the money.

It's worse than that, as I screwed up the math the first time through. I've posted a correction to what I THINK is the right answer. It looks like it would take about a 13.6% reduction in EQUIVALENT flat plate area to get the 5% increase in speed at the same horsepower. Remember though that you don't necessarily have to reduce the FRONTAL area by that much; just streamlining the area that is there will help. Having said that, 13.5% is a VERY tall order, and like you, I'm just not sure it's there.

It's looking more like more power IS the way to get modest speed gains. A 180HP running at 75% power should give you a 7.5% increase in speed over the 145 at 75% power, 123mph, everything else being equal, at the expense of 25% more fuel. If you have the will power, could just use the full 180 for takeoff and "when you really need it", and exercise the option of running it at a low enough power setting in cruise for economy. Two 180HP 170's stayed back with me from Idaho to Alaska a few years ago, and all our fuel burns all ran fairly close.

Miles

[Dave Clark]

"It's looking more like more power IS the way to get modest speed gains. A 180HP running at 75% power should give you a 7.5% increase in speed over the 145 at 75% power, 123mph, everything else being equal, at the expense of 25% more fuel. If you have the will power, could just use the full 180 for takeoff and "when you really need it", and exercise the option of running it at a low enough power setting in cruise for economy. Two 180HP 170's stayed back with me from Idaho to Alaska a few years ago, and all our fuel burns all ran fairly close"

EXACTLY! And, I have the will power My cross country ideal altitude is 7500-8500 and a setting of 20" at 2300 rpm netting 115 knots at 10gph consistantly. Without my power charts at hand I can't tell you what % that is, 65? That would be 117hp and the plane rides and feel far better than it did with the C-145 at those altitudes but that's another story. Plus I can go higher easier than before.

At any rate I stick to my guns that speed kitting a 170 is not going to be of great benefit for the money and time involved.

[cessna170bdriver]

One of my first acquaintenances in the 170 Association was the owner of a mid 1970's Cessna 172 tail dragger. He lived and breathed his airplane, and mentioned to me one time that he had made 22 "speed mods" to his airplane, including such things as strut and brake fairings, a streamline behind the tail wheel, and covering the wing root air inlets, for a net of 21 knots increase. The tailwheel conversion was among the first, and got him 9 knots. That leaves 21 modifications to gain the other 13 knots. He enjoyed the challenge, and never regretted any of the modifications, but did admit that the tailwheel conversion was about the only one that was really worthwhile.

Miles

[jrenwick]

Some of the Swift folks will tell you that the best way to increase cruise speed (and also to improve engine cooling) is to work inside the cowling: stop air leaks, and otherwise make sure the engine baffles are in good condition and tight. The Swift is a cleaner airframe than the 170, so cooling drag may be more of a factor for them than for the 170 -- but maybe this is worth looking at for 170s as well.

Best Regards,

John

[cessna170bdriver]

Some of the Swift folks will tell you that the best way to increase cruise speed (and also to improve engine cooling) is to work inside the cowling: stop air leaks, and otherwise make sure the engine baffles are in good condition and tight. The Swift is a cleaner airframe than the 170, so cooling drag may be more of a factor for them than for the 170 -- but maybe this is worth looking at for 170s as well.

Best Regards,

John

I've heard one long-time 170 driver/aeronautical engineer say that he thought that cooling drag presented the greatest opportunity for improvement in the 170, although I don't recall him saying just how much improvement there might be.

I am warming up to the idea of more ponies under the hood; I just need to warm up the idea of more $$$ out of the bank account.

Miles

[johneeb]

Miles etal,

Do you have an explanation of cooling drag and any ideas on how to eliminate it?