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Thread: Calculating Maximum Range without a Fancy Panel Gadget

  1. #1
    SJ's Avatar
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    Calculating Maximum Range without a Fancy Panel Gadget

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    We all know that in most airplanes (maybe all..) by powering back even a little bit, we can often increase our range and certainly increase our duration. I have done this in my short-legged C-180 a number of times "by guess and by golly" with pretty good results. I googled around and found some really long haired equations that caused my eyes to glaze over. I don't have a fancy gadget in the plane to tell me max efficiency (many do now).

    Are there any shortcut, rule of thumbs, techniques for getting to get close to max MPG quickly?

    Here is my BF&I plan for determining this:
    1. Find a day where the wind is pretty much the same for a hundred miles or so.
    2. Head in one direction at normal cruising altitude and power settings with the AP keeping the heading and altitude
    3. Lean to EGT/CHT as normal
    4. Record fuel flow, airspeed, and ground speed

    Repeat until done...
    1. Reduce power by 1" of MP
    2. retrim for AP
    3 re-lean to temps
    4. Record fuel flow, airspeed, and ground speed

    Go home and do the math...

    Now I DO have a GNS430W in this plane connected to my fuel flow meter and it will display effective MPG - I will also record that.

    Now, your thoughts, rebukes, criticisms, etc... Please!

    sj
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    Don't have my 53 owners manual here at home,
    but the 1957 C180 manual I have here has a chart showing "Landplane cruise and range performance".
    At 2500', 5000', 7500', 10,000', 15,000', and 20,000' (whew!).
    Each altitude has a "maximum range" setting--
    for example, at 7500' it shows 16" / 2000 rpm is good for 7 gph, endurance of 7.9 hours, and range of 840 miles.
    I'd say just use that data and lean as appropriate--
    it's probably not exact but just how close do you want to cut your endurance calculations?
    I'd hate to have a forced landing because " I was supposed to be able to get another 5 miles out of that tankful".
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    SJ's Avatar
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    Unfortunately or fortunately I don't have the stock 1955 engine and prop. so I assume that those performance figures were probably not the same now as in my book.

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    hotrod180's Avatar
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    For those that do have the stock 470 in theirs, here's a scan of the chart.

    Click image for larger version. 

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    One reminder this chart gave me - I need to be adjusting the RPM as well as the MP in my testing. Thanks!

    sj
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    I'd have to leave the math to the fuel flow analyzer. Fly on a neutral wind day, set a distant way point in your GPS (connected to your analyzer) and watch your fuel flow analyzers display of fuel remaining at destination, time aloft, or range as you twiddle the knobs. Mine reacts pretty quickly and was very educational in the Cub.
    Remember, These are the Good old Days!
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    My observations while chasing yotes. O-320 with borer 82-42, 31"ABW, and CGI monitor with fuel flow sensor. For me cruise at 87mph, 2500rpm is about 8 gph. On the deck looking for dogs one notch of flaps 65mph 2200rpm 4.6gph. Average altitude 3500-4500 during cold temps. Assuming (not true) 36 gallons usable at 2500rpm max distance 391 miles over 4.5 hours. At 2200rpm max distance 508 miles over 7.8 hours. Speed is expensive and yet another example of why the cub excels at going slow. Best day every one time I actually had to come back and refuel while chasing dogs.
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    aviationinfo's Avatar
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    SJ, I remember an article I read by Barry Schiff a few years back (Iíve googled but canít find it now) discussing this very topic. He related some research done, based on an airspeed just slightly faster than L/D Max. I wish I could remember the specifics now. Maybe someone else here will remember. I think the basic premise is to take your airplaneís best glide speed and add some margin to it. It is unquestionably a slower speed than most of us use.

    In the article he named the airspeed after the person who did the research (canít remember that either, letís call it SJ Speed). Does this ring any bells?

    PS: My personal calculation for fuel efficiency is based on the amount of preflight coffee consumed prior to launching for the Idaho backcountry. If I can fly a 3.5 hour leg nonstop to Johnson Creek without wasting fuel on extra side journeys seeking an outhouse and a leg stretch along the way, I find I can average less than 8gph quite easily...
    Last edited by aviationinfo; 02-26-2019 at 03:04 PM.
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    Quote Originally Posted by hotrod180 View Post
    For those that do have the stock 470 in theirs, here's a scan of the chart.

    Click image for larger version. 

Name:	C180 power & range chart.jpg 
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    And if you happen to be packing two O-470-M, I think they exaggerated a tad on the true air speeds, unless it is MPH and not in Knots, even than 206 mph at 67% power is better than I thought a 57 310 could do, will find out soon. I haven't bought the plane yet, waiting to test fly it, the weather will not cooperate, but hopefully this weekend. I wanted to see the performance numbers for it, searched the web for a free copy and came up empty handed but I found a site where you can download I think any POH, $14. for the 310B model, pdf format and you can download immediately and save it to as many tablets phones you need. https://www.eflightmanuals.com



    Last edited by supercub1999; 02-26-2019 at 03:42 PM.

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    Steve, Steve, Steve.....With all the moola you haul in from this site, you need to just call up one of those advertisers on here and have them send you a fuel flow computer.

    Seriously, after my assigned 185 broke a crankshaft, our aircraft division guys asked me what I needed in the panel of the replacement. My answer, only a little exaggeration, was the only thing I HAVE to have beyond basics is a fuel computer. I used one of those twice in my work plane, went home and ordered one for my 170. Never looked back.

    i noticed you’re getting a little thin in the tonsorial dept. Prolly from worrying too much about gas.....

    MTV
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    If you want to have a close place to start, multiply your power off stall speed by 1.6. That will get you close to L/D max. In a propeller driven airplane maximum range occurs at L/Dmax. You can refine it by slowing your airplane while maintaining altitude. Then as you steadily slow note the speed at which you cross over to the back side of the power curve and you have to start adding power to hold altitude. The area can be a little flat on laminar wings like a Mooney, fairly well defined on a Cub airfoil. The speed you are looking for is on the forward part of the crossover. It should also be pretty close to your best range glide speed. As for the power, use the lowest selectable RPM on a constant speed prop consistent within the MP limitation. Operation over square is ok as long as your POH does not prohibit it. You want those pistons going slow and extracting every last BTU from the fuel. Lean aggressively even operating LOP. You will be below 70% so LOP operation should not be a problem. The reason why you go a "few knots" up from absolute L/Dmax is two fold. One you trade about 1% fuel for a 3% speed increase. Two, if you operate exactly on L/Dmax you are likely to drift slow to being on the backside of the curve. The power required to get back up on L/Dmax is so great you are better off staying a few knots fast and not go there. Even fancy FMC units do this when you select "cost index" to zero
    Charles Lindbergh did the same thing at the beginning of WWII showing air crews how to get the most out of their airplanes range. Know how to find L/D max, then at a couple knots, run your prop as low as it will go and lean aggressively, you'll be right in the sweet spot.
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    Very interesting stuff, folks! BTW, I DO have an EI fuel gadget and it is hooked up to my 430.

    GeeBee, I am certain you mean power off - flaps UP stall speed - which is something many folks don't do. I am going to give it a try at the next opportunity!

    sj
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    For some interesting reading Google "Carson Speed"

    https://www.flyingmag.com/very-best-speed-fly


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    Quote Originally Posted by SJ View Post
    Very interesting stuff, folks! BTW, I DO have an EI fuel gadget and it is hooked up to my 430.

    GeeBee, I am certain you mean power off - flaps UP stall speed - which is something many folks don't do. I am going to give it a try at the next opportunity!

    sj
    Obviously you would want the minimum drag for maximum range for a given airplane so you would use flaps up. However, you could be in a situation with flaps stuck in a given position and need maximum range (although usually those kind of situations require maximum endurance) So you could do the same thing at a given flap position and that would be the maximum range for that specific flap position but in no case would the specific range be as great as that in a flap up condition.
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    SJ's Avatar
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    https://en.m.wikipedia.org/wiki/Lift-to-drag_ratio

    I especially like the LD ratio of the flying squirrel. I can relate...

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    aviationinfo's Avatar
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    Quote Originally Posted by DJ View Post
    For some interesting reading Google "Carson Speed"

    https://www.flyingmag.com/very-best-speed-fly


    Sent from my SAMSUNG-SM-G900A using SuperCub.Org mobile app
    This is the subject matter I couldn’t recall above. Thanks! I thought Schiff wrote about it but I guess it was someone else.
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    I had an aero prof that once gave an entire test centered around one fact. At L/Dmax parasitic and induced drag are equal. If you knew that one fact, you did not have to chew a single number. Some people finished the 20 question test in 3 minutes and others took and hour and exhausted a calculator battery. Guess who failed.
    Last edited by GeeBee; 02-26-2019 at 08:38 PM.
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    Funny how O320's burn 8 or 9 gallon per hour and O360 'a burn 6 or 7.

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    Don't over think this. Since the specific fuel consumption (pounds of fuel per horsepower) of the stock carburetor equipped O-470 and your carburetor equipped O--520 are in all probability very close you would be able to use the Cessna generated range tables as long as you set power based on fuel consumption. To get a place to start, calculate the number of horsepower required for long range cruise in the O470 range table by multiplying the engine rating (230 hp) by the percent of power from the table. Once you have that then find a power setting for the O520 which will produce that horsepower. Maximum range is a function of air frame efficiency, that hasn't changed with the engine change. You just have to use a percent of power setting which will be lower than what you would use for the O470. Your larger three bladed prop has more drag than the original two blade but also has a more efficient blade profile so is probably a wash. Or you could do it the easy way and use the power charts for a IO520 powered 185, the slight reduction in the fuel burn do to the higher efficiency of the fuel injection system will be compensated for by the lighter cruise weight. Just my two cents worth.

    Tim
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    L18C-95's Avatar
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    I find the C-90-8F likes to be flown at 70% plus, it can sip fuel but prefers to be ridden hard - well at 90 ponies thatís all relative


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    Bill Rusk's Avatar
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    Quote Originally Posted by GeeBee View Post
    If you want to have a close place to start, multiply your power off stall speed by 1.6. That will get you close to L/D max. In a propeller driven airplane maximum range occurs at L/Dmax. You can refine it by slowing your airplane while maintaining altitude. Then as you steadily slow note the speed at which you cross over to the back side of the power curve and you have to start adding power to hold altitude. The area can be a little flat on laminar wings like a Mooney, fairly well defined on a Cub airfoil. The speed you are looking for is on the forward part of the crossover. It should also be pretty close to your best range glide speed. As for the power, use the lowest selectable RPM on a constant speed prop consistent within the MP limitation. Operation over square is ok as long as your POH does not prohibit it. You want those pistons going slow and extracting every last BTU from the fuel. Lean aggressively even operating LOP. You will be below 70% so LOP operation should not be a problem. The reason why you go a "few knots" up from absolute L/Dmax is two fold. One you trade about 1% fuel for a 3% speed increase. Two, if you operate exactly on L/Dmax you are likely to drift slow to being on the backside of the curve. The power required to get back up on L/Dmax is so great you are better off staying a few knots fast and not go there. Even fancy FMC units do this when you select "cost index" to zero
    Charles Lindbergh did the same thing at the beginning of WWII showing air crews how to get the most out of their airplanes range. Know how to find L/D max, then at a couple knots, run your prop as low as it will go and lean aggressively, you'll be right in the sweet spot.

    Excellent post.

    Bill
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    SJ's Avatar
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    Quote Originally Posted by L18C-95 View Post
    I find the C-90-8F likes to be flown at 70% plus, it can sip fuel but prefers to be ridden hard - well at 90 ponies that’s all relative


    Sent from my iPhone using SuperCub.Org
    I guess that is the next question, how does this impact effective engine life? Obviously, if you are tooling around at 2000 rpm instead of 2400 you will get 10 or 15% more "tach time life" out of your engine (you will still take off at full rpm), but is it better or worse on the actual engine? Do you shoot for the same CHT's at these lower power settings?

    This has been very educational for me!

    sj
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    I recently read Mike Busch's book "Engines" He subscribes to Lindbergh's operational techniques and it is hard to argue with his results. He has over 4000 hours on a set of TSIO-520's on his C-310. His point is the enemy of engines is heat and contaminants. The more complete the combustion, the less contaminants. So lower RPMs aid that. Thus he recommends as does Lindbergh, running low rpm and LOP operations. Obviously you will have higher BMEPs in your cylinders but at lower power settings that should not represent a problem. I have started running LOP even on my fixed pitch PA-18. I have found if I do I can drop the CHT's by 10 degrees. Yeah it runs a little rougher than ROP but the fuel savings plus the drop in temps is worth the troubles. If you are looking for max range, this is your ticket. Before you run LOP make sure you understand the "red zone" and "the big pull".

    Along the lines of heat, I have been messing with Mooney's lately. The Bravo model came out with a Lyc TIO-540 which had horrendous reputation for eating cylinders. They grooved the outer circumference of the exhaust valve guide and ran 1/4" oil lines to the top of each jug. The concept was to have the oil carry away the heat on the top of the jug. The result is the Bravo now has one of the best reputations for cylinder longevity. In my mind this proves Busch correct, it is all about heat.

    bat443 is correct, specific range is all about horse power, the settings for a given engine will change to achieve that horsepower but for the minor weight change the speed required and thus the horsepower required will not change. Virtually all engines burn .46 lbs/hour per horsepower. So the numbers will change little for a given airframe between the two engines when operating at max range airspeed. In the case of a C-180 series, if you change from a two blade O-470 to a 3 blade O-522/550 that can make a difference, not huge but propeller changes will impact the numbers a few percent.
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  24. #24
    SJ's Avatar
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    One thing that this thread has reminded me of: We have some really smart folks in our group!

    sj
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    hotrod180's Avatar
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    Quote Originally Posted by SJ View Post
    ….BTW, I DO have an EI fuel gadget and it is hooked up to my 430.....
    I'm not familiar with EI's fuel instruments, but it seems that hooking it to your 430 could provide range in either miles or time-
    based on groundspeed, fuel burn, and fuel remaining.
    Then just try different power settings and go with whatever gives you the best result.
    But maybe you're already doing this & I'm just misunderstanding?
    Last edited by hotrod180; 02-27-2019 at 12:22 PM.
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    WhiskeyMike's Avatar
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    Range tables are for new airplanes in perfect condition - and influenced by marketing. Just like "cruise speed." Two identical airplanes are as different as two people. Don't run out of gas.
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    hotrod180's Avatar
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    Quote Originally Posted by WhiskeyMike View Post
    Range tables are for new airplanes in perfect condition - and influenced by marketing. Just like "cruise speed." Two identical airplanes are as different as two people.....
    I can definbitely understand how the actual numbers (speed and therefore range) would be different for real-life airplanes,
    as opposed to hypothetical new & perfect airplanes,
    but although the actual range will be different, shouldn't best range power settings be the same?
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    WhiskeyMike's Avatar
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    Quote Originally Posted by hotrod180 View Post
    I can definbitely understand how the actual numbers (speed and therefore range) would be different for real-life airplanes,
    as opposed to hypothetical new & perfect airplanes,
    but although the actual range will be different, shouldn't best range power settings be the same?

    Maybe, but it would depend upon how your carburetor and intakes system function. Aerodynamic principles don't change but every engine runs a bit differently and every plane flies a bit different from another.

  29. #29
    SJ's Avatar
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    I'm itching to go out and do some testing, until today the weather sucked, and this morning I woke up with a nasty cold.... Maybe tomorrow before the weather closes in again!

    sj
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    On my very first flight home after equipping my new-to-me Commander 114 (IO-540), I was "on the fence" about whether to make another fuel stop before arriving at home. Using the newly installed EI CGR-30P (with fuel flow integrated with the 430W), I could see that there would be 50 minutes of fuel remaining on landing. Since my personal minimums are to have 1 hour remaining, and since I thought it would be a bad idea to bust my personal minimums on my very first flight in the plane, I initially planned to divert to another airport for fuel. But then I started playing around with RPM and MAP, while watching the "fuel remaining at destination" figure, and found out pretty quickly that reducing RPM and running "oversquare" extended the range sufficiently to make it home with 1:15 remaining, with minimal change to the ETA.

    Fuel flow computers are amazing, and I've had one in every airplane I've owned from that point on. Once calibrated, I find they are usually accurate within 1/2 gallon per fill-up.
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    Quote Originally Posted by JimParker256 View Post
    Fuel flow computers are amazing, and I've had one in every airplane I've owned from that point on. Once calibrated, I find they are usually accurate within 1/2 gallon per fill-up.
    ^ This

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    Just remember, garbage in, garbage out. Make sure you initialize with the correct amount of fuel, make sure the fuel you initialize is what is actually in the tanks, (a lot can go wrong there like units conversion etc) and always be aware there could be a leak. If that leak is in front of, or behind the flow meter makes all the difference in the world. Look up Air Transat 236 or Air Canada 143. Had all the fuel flow meters in the world but still ran out of fuel.

  33. #33
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    I used to just figure my known fuel burn from experience and figure the hours of fuel I had on board using my watch bezel.
    13 gallons an hour. 70 gallons (out of 80) that I was willing to burn. At 100 knots eco-cruise on floats. So 5.3 hours, or 530 nautical miles in eco cruise.. And then add another safety margin and I had 475 NM as a max range leg for the C-180 on floats.

    BUT, then I started using a ECI fuel burn totalizer.... religiously. That was when I found out how much I was burning by taxiing around on the lakes, and take-offs, and power climbs over mountains... Once I managed to dial in the fuel flow sensor, it is right-on for accuracy.

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