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  • Engine mount question

    There is a kit available to convert the mounting system on the O-200 I'm using from Lord type mounts to conical mounts. Conical mounts would reduce engine movement somewhat, and move the engine about an inch closer to the firewall. Has anyone used this? Anything built using a C-85 case would have this type mount anyway. Bob

  • #2
    I had to do this,

    I mocked everything up with a C-90 with the "cone" style isolators but then actually used an 0-200. If I used the "Lord Style" isolators for the 0-200 it was going to move the engine forward too much and I would have had interfence with my nose bowl and cowl..

    I turned up a set of adapters on my lathe. All seemed to work out fine.

    So if you don't have a lathe to make your own, I expect the ones on Ebay would be fine.

    Comment


    • #3
      I am building a conical mount for my O-320/Patrol. I've been looking around at various mounts trying to decide how long the "bushings" at the end of the mount's legs and at the rubber isolators have to be. Bob specs 2" length bushings on front corners of fuselage. Some of the mounts I've looked at from smaller engines have very short bushings. I am considering going with 1.5" pieces of tubing, because that is what I get if I cut my 12" piece of 9/16 4130 into 8 pieces. Anyone have a factory mount measurement?

      Comment


      • #4
        I am committing to cutting/fitting the engine mount tubing to length for my Patrol's O-320. When doing calculations using engine-placement recommendations on the plans, there appears to be only about 8.5 inches between the accessory-housing and the firewall. That doesnt leave a lot of space for my Bendix magnetos. It appears that the plug-wires would have a big kink to clear the firewall. Just wondering what the necessary clearance for Bendix mags might be.

        Comment


        • #5
          Still looking for answers here ....

          At OSH I asked Bob about Patrol engine position. The 2005 Bearhawk
          Newsletter gives a figure of 56" to 58" as the recommended
          datum/prop-flange distance. Bob said to use the 58" figure if I intended haul heavy rear loads.

          The Newsletter specs that CG should be in the range of 12.8" to 20" behind datum.
          The newsletter sample weight/balance sheet for N289R (which I assume is for the O-360) is with metal prop and no electrical system.

          I have an O-320 and hope to license it at 1320# so will probably use a wooden prop.

          So if I save about 13# in prop-wt and lets say that the bare 0-320 is 20# lighter than the O-360, I should be about 33# lighter. Lets say
          the 360 engine with prop is 300# and my engine with prop is 267#.

          Let us also say that the actual aircraft CG we are shooting for is 16" behind datum and that the 56" datum/prop-flange figure was used on N289R. Let us also say that the actual center of gravity for the engine/prop combination (ECG) is somewhere in the middle of the engine; say 10" behind the prop-flange. This would give the actual moment contribution of the O-360 engine/prop combination as:

          303# x (CG to ECG distance) = 303 x (56 + 16 -10) = 303 x 62 = 18786 inch-pounds

          Now if I use a lighter engine and a lighter prop, but want the same weight/balance contribution for my aircraft, I want my O-320 positioned such that it balances the rest of the aircraft with an equivalent 18786 inch-pound moment.

          Since my engine-prop combo weighs only 267#, I can now calculate its preferred position

          My CG to ECG distance for my O-320 should be 18786/267 = 70"

          Again assume that my engine-prop combo has ECG 10" behind the prop-flange and that desired aircraft CG is 16" behind datum. My O-320 position can be calculated as follows.

          Location of O-320 prop-flange = (70 -16 + 10) = 66" in front of datum

          Now this 66" figure seems really excessive, a full 8" in front the 58" max recommended in the Newsletter.

          I must be doing something wrong, but I don't know what.



          Comment


          • #6
            A few inputs:

            1. Look up the type certificates(search Google for TCDS for 0-320 and a second search for TCDS 0-360) for the two engines in question.I've attached copies I have but there may be newer.They will have the actual dimensions and CG locations listed. I expect they may be different between the two.

            2. To simplify things, you don't need to work to the desired empty weight CG envelope. You have one known lever arm(56 to 58") and mass combination(0-360 and metal prop) that results in the correct moment. You just need to work out a new lever arm that works with the new mass combination to give the same moment.

            3. If you have some computer skills, set up a simple spreadsheet that calculates the engine CG position relative to the known prop flange location(basically 57" adjusted for the CG location given in the TCDS) and the prop itself(57" plus half the prop thickness, say 3"). Then you can play with the numbers independently.

            Attached Files

            Comment


            • #7
              Thanks for the pdf files. After hours of searching I had not found that CG and weight info. I realized I could work simply from changes in the the engine component of moment, but I when I got the numbers I did, I thought I would go back to the actual CG envelope of the aircraft to make sure I was not making bad assumptions.

              Comment


              • BTAZ
                BTAZ commented
                Editing a comment
                It's good to have a "Sanity estimate" in mind before diving into the calculations.

                The math is simply moment = mass x distance. You are reducing the mass by about 10% so the distance will need to go up about 10%.

                Of course, it is a bit more complicated than that(you are changing the mass of two items, both of which have CGs offset from your baseline moment arm) but if you start seeing answers too far away from a 6" delta either something isn't right or you need to thoroughly understand and justify why..

            • #8
              Your 10% rule works, but I think it is always most accurate if one uses the actual moment-arm as measured from the aircraft's actual CG on a given day (which is 13 to 20 inches behind the arbitrary datum). So if the actual moment-arm for the engine/prop on prototype N289R is 56", then using a new configuration with lighter engine/prop that is 10% lighter should require that the mount move the engine ahead approximately 5.6", In our current example it is a coincidence that this also happens to be ~ 10% of the distance from the prop-flange to the datum. But since the datum is an arbitrary point that could just as well have been initially defined as anywhere on (or even off) the aircraft, using the 10% increase in datum-to-engine/prop-cg distance could get a guy into trouble.

              Comment


              • #9
                The 10% just gives you an idea of the range of the correct answer(which is good to have in mind before formally working the problem). IT IS DEFINITELY NOT THE FINAL ANSWER!

                The actual desired new flange location will depend on the change in the moment between the two engine's differing CG and mass as well as the change in the moment between the two propellers cg/mass.

                Actual real world will be a bit different again as the mass/cg location numbers on the TCDS will be a bit different then actual installed weight given variances in mass of the "accessories"(lightweight vs. standard starters/alternators, exhaust system variance, mags vs. electronic ignition, carb vs factory vs experimental fuel injection, standard vs. lightweight starter ring gear, carbon fiber vs. fiberglass nose bowl/spinner at the different locations,etc.).

                The accuracy of a calculated result is only as good as the accuracy of the data going in. If one doesn't have actual "as installed/as weighed" numbers for the 0-360 engine/prop combination and the same for the 0-320, I expect one would be fooling themselves to think the calculated result will be within 5% of the actual result.

                Also, keep the big picture in mind. Given the moment arm to the tail is probably three times as long as the moment arm to the engine, a 5 lb. delta in the tail between the two Patrols in question(different covering systems, coats of paint, tail wheel, etc.) would be the same as a 15 lb delta in the engine weight.......

                Comment


                • #10
                  Originally posted by bergy View Post
                  Still looking for answers here ....

                  At OSH I asked Bob about Patrol engine position. The 2005 Bearhawk
                  Newsletter gives a figure of 56" to 58" as the recommended
                  datum/prop-flange distance. Bob said to use the 58" figure if I intended haul heavy rear loads.

                  The Newsletter specs that CG should be in the range of 12.8" to 20" behind datum.
                  The newsletter sample weight/balance sheet for N289R (which I assume is for the O-360) is with metal prop and no electrical system.

                  I have an O-320 and hope to license it at 1320# so will probably use a wooden prop.

                  So if I save about 13# in prop-wt and lets say that the bare 0-320 is 20# lighter than the O-360, I should be about 33# lighter. Lets say
                  the 360 engine with prop is 300# and my engine with prop is 267#.

                  Let us also say that the actual aircraft CG we are shooting for is 16" behind datum and that the 56" datum/prop-flange figure was used on N289R. Let us also say that the actual center of gravity for the engine/prop combination (ECG) is somewhere in the middle of the engine; say 10" behind the prop-flange. This would give the actual moment contribution of the O-360 engine/prop combination as:

                  303# x (CG to ECG distance) = 303 x (56 + 16 -10) = 303 x 62 = 18786 inch-pounds

                  Now if I use a lighter engine and a lighter prop, but want the same weight/balance contribution for my aircraft, I want my O-320 positioned such that it balances the rest of the aircraft with an equivalent 18786 inch-pound moment.

                  Since my engine-prop combo weighs only 267#, I can now calculate its preferred position

                  My CG to ECG distance for my O-320 should be 18786/267 = 70"

                  Again assume that my engine-prop combo has ECG 10" behind the prop-flange and that desired aircraft CG is 16" behind datum. My O-320 position can be calculated as follows.

                  Location of O-320 prop-flange = (70 -16 + 10) = 66" in front of datum

                  Now this 66" figure seems really excessive, a full 8" in front the 58" max recommended in the Newsletter.

                  I must be doing something wrong, but I don't know what.


                  Bergy, I think your numbers are about right. I ran the numbers and thought I would show my work and expose it in the public domain for critical feedback.

                  I think 1) your target is a CG at 16 inches aft of the datum and 2) you are trying to understand how your O-320 with 33 pounds less weight will effect the empty weight CG. 3) you want to know how far you have to adjust your engine mount to have the same CG with a light engine/prop.

                  If you have a 1000 pound airplane and move its 300 pound engine forward 2 inches it would shift the CG .6 inches. (Formula for change in CG caused by shifting weight .....Change in CG = (Weight Shifted x Distance weight is shifted) / Total Weight. So, (300 pounds x 2 inches) / 1000 pounds = 600 inch pounds / 1000 pounds = .6 inches

                  If you removed 33 pounds from a station that is 56 inches ahead of the Datum (Leading edge) of a 1000 pound airplane with an empty weight CG of 10.0 inches aft of datum, it will shift the CG 2.23" aft. (Formula for adding weight....Change in CG = (Weight added x distance between Old CG and the Weight) / New Total Weight. So, we are removing 33 pounds that is 66 inches in front of the empty weight CG. The math looks like (-33 pounds x -66 inches) / (1000 pounds - 33 pounds) = 2178 in lbs / 963 lbs. = 2.26 inches.

                  If you want to remove a 300 pound engine @ -56 " aft of datum from a 1000 pound airplane with an empty weight CG of 10.0 aft of datum, then you will have a 700 pound airframe with a CG of 38.3" aft of datum.

                  Weight x Arm = Moment
                  1000 lb x 10 in = 10,000 in lb
                  -300 lb x -56 in = 16,800 in lb
                  +700 lb x __ in = 26,800 in lb

                  The new empty weight CG is 26800 in lbs / 700 lbs = 38.3 inch empty weight CG.

                  If I add a 263 pound engine to the 700 pound airframe, and want the CG to remain at 10.0 inches aft of Datum that I had before, I need to put that engine then it looks something like this.

                  Weight x Arm = Moment
                  700 lb x 38.3 in = 26,800 in lbs Empty weight of airframe w/o engine
                  263 lb x ___ in = _____ in lbs
                  963 lb x 10.0 in = 9,630 in lbs Emtpy weight/cg of airframe that I desire

                  So, it looks like we need a moment for the engine to be 26800 - 9630 = -17,170 in lbs.
                  -17,170 in lbs / 263 lb engine is 65.28 inches in front of the datum

                  700 lb x 38.30 in = 26,800 empty weight
                  263 lb x -65.28 in = -17,170 in lb
                  963 lb x 10.00 in = 09,630 in lb

                  So. the old engine was sitting at 56 inches, and new one is at 65 1/4". You came up with 66
                  Last edited by Bcone1381; 09-06-2017, 01:42 PM.
                  Brooks Cone
                  Southeast Michigan
                  Patrol #303, Kit build

                  Comment


                  • BTAZ
                    BTAZ commented
                    Editing a comment
                    Remember, the actual cg of the engine is not at the prop flange where the 56" is coming from.

                    It is some inches behind it as defined on the TCDS.

                  • Bcone1381
                    Bcone1381 commented
                    Editing a comment
                    That is correct. My numbers are not actual measurements from the TCDS.

                • #11
                  Also,

                  I am concerned that you won't like an aft datum of 16 inches. The CG limits for a fully loaded Patrol are 10.5" to 21.5" Every single piece of weight that is put into a finished Patrol will move the CG aft. The fuel moves it aft. When you hop in, it will shift aft. When your grand child hops into the rear seat and you put the pic nic basket into the cargo area the CG shifts aft.

                  I have some time today and figured a weight and balance of a Patrol with a 900 pound empty weight and a 16" CG. These are the number I used

                  900 pound Patrol
                  172 pound Pilot
                  100 pound Passenger
                  78 pounds of fuel
                  70 pound pic nic basket

                  This has you at 1320 lbs. I picked these numbers and worked a sample weight and balance because I thought a 16" empty CG seemed a bit aft to me. I thought about conditions that I might want to throw into the Patrol, and thought you might get some benefit out of them.

                  Weight X Arm = Moment
                  900 lbs x 16 in = 14,400 in lbs. Empty Weight & CG. This is the airplane with a 16" empty weight CG that I think you are shooting for.
                  078 lbs x 24 in = 01,872 in lbs. This is 13 gallons of fuel in one tank, the other tank is empty.
                  172 lbs x 30 in = 05,160 in lbs. This is the pilot sitting in the front seat
                  100 lbs x 56 in = 05,600 in lbs. A passenger in the rear seat
                  070 lbs x 79 in = 05,530 in lbs. A large Picnic Basket
                  1320 lbs...............32,562

                  32,562 in lbs/ 1320 lbs = 24.67 inches

                  So, the CG is about 3 inches aft of the limit. So lets cancel the picnic leave the basket at home and just go flying. The new weight and balance results in a 21.62" CG. Still aft of the 21.5 limitation. So, I was wondering what would happen if a 172 pound pilot climbed in with 41 gallons of gas, no passenger, no cargo. The CG was 19.3"

                  So, It looks to me like a Patrol with a 16" empty weight CG will only be able to carry a light weight pilot and fuel.

                  SIGNIFICANT NOTE#1....I don't have access to actual ARM measurements. But I would bet dollars to donuts that the numbers I spewed out are close enough to back up my point, that a 16" empty weight CG is pretty far aft. I would rather have scientific actual numbers though.

                  I don't have the Airfoil page of the plans with me, so I don't have a scientific placement of the Datum. I determined all measurement by looking on the plans on my computer, and making my best effort to get some accurate numbers. The cargo ARM is figured at the center of the aft cargo area.

                  -The Datum used was the Wing leading edge. I estimated its location first and figured it was 12 inches in front of the front spar using the plans on my computer. I am curious if that is close.
                  -The Fuel Tank CG was figured 12 inches aft of the front spar. It looked like the fuel tank bay was 24 inches and its center was 12" aft of the spar, so 24 inches aft of the L.E.
                  -The seat locations were estimated, and I guess easily with in 1.5 inches of accurate.
                  -The Cargo area is accurate and centered in that area aft of the rear seat if the Datum is accurate.

                  SIGNIFICANT NOTE #2 This is thread creep...we should be putting this information out under Patrol - Weight and balance thread that was started there. The moderators might consider moving it.


                  Last edited by Bcone1381; 09-06-2017, 01:48 PM.
                  Brooks Cone
                  Southeast Michigan
                  Patrol #303, Kit build

                  Comment


                  • #12
                    Since I was really building the mount, yesterday, i don't feet too far off-thread.
                    I will rarely carry a rear passenger and much fuel, except for check-rides and such. My wife weighs 106#, and if she ever dares ride with me I can assure you it will not be a picnic.

                    However I do want to build an aircraft that will sell when I get bored or too old, so that is why I am playing with the numbers.
                    I might make a mock-up mount using my best guess for CG, put the plane together except for the cowl and weigh it before making the actual mount. Then when I am happy with the W/B I can make my cowl. I have dreams of designing a lightweight alternate ignition systems (Bendix mags are 12# alone) and some other weigh-saving features.

                    Comment


                    • Bcone1381
                      Bcone1381 commented
                      Editing a comment
                      I may have edited my message after you posted #12.

                      What did you decide? Did you make the mount longer?
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