"Micromaster"-- Centerline twin using small industrial engines

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Vigilant1, Nov 13, 2018.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Jan 1, 2019 #501

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,483
    Likes Received:
    2,524
    Location:
    Port Townsend WA
    The Rutan Voyager on world flight takeoff was probably the most underpowered aircraft of all time. :gig:
     
    BJC likes this.
  2. Jan 1, 2019 #502

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,330
    Likes Received:
    588
    Location:
    capital district NY
    It is like a Sky-Pup in the front and a Firefly in the back.
     
  3. Jan 1, 2019 #503

    Wayne

    Wayne

    Wayne

    Well-Known Member

    Joined:
    Jan 15, 2013
    Messages:
    412
    Likes Received:
    516
    Location:
    Chicago, IL
    Blane, I think you mean "Party in the front, business in the back!" :roll:
     
  4. Jan 1, 2019 #504

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,330
    Likes Received:
    588
    Location:
    capital district NY
    I like the whole concept of the Micro-Master. I have a cup with Bushmaster on it, it is the definitive cup for the dominant male. Seriously though I just wish the thoughts about making it a three engine that were around briefly would have prevailed. The whole idea that it must be centerline thrust is kind of screwy because people seem to be willing to cheat the thrust-line above and below the CG but a little side to side is taboo. ? . But three 22hp is 66hp and 2/3 of 66hp is 44hp so were is the downside of a little off center thrust? I mean 44hp minus 28hp (two 28hp on the twin) is 16hp and I doubt you are going to get the full net 16hp used controlling adverse yaw. And remember the adverse yaw is between two 22hp engines the center engine doesn't really contribute significantly to yaw. But two of these little engines is far wiser than one any day of the week and so I like it.
     
  5. Jan 1, 2019 #505

    Sockmonkey

    Sockmonkey

    Sockmonkey

    Well-Known Member

    Joined:
    Apr 24, 2014
    Messages:
    1,711
    Likes Received:
    439
    Location:
    Flint, Mi, USA
    I have no problem with three or more props, but going over two means it becomes simpler to hybrid the thing.
     
  6. Jan 1, 2019 #506

    mcrae0104

    mcrae0104

    mcrae0104

    Armchair Mafia Conspirator HBA Supporter

    Joined:
    Oct 27, 2009
    Messages:
    3,075
    Likes Received:
    2,095
    Location:
    BDU, BJC
    A few inches above or below is no big deal in terms of static equilibrium, and the elevator would be sized to manage it. A few feet to one side or the other is a lot bigger deal for static equilibrium (regardless of engine output). It's the moment arm (feet, not inches) that matters, not the direction of the offset (vertical vs. horizontal).

    Asking the HS & elevator (which are usually much larger than the VS & rudder because they have to balance out all other moments in a wide variety of regimes) to do a little extra work is not nearly as hard as asking the rudder to do a lot of extra work (compared to a rudder that isn't required to do this extra job and ordinarily doesn't have to generate much lift). This is why VSs on non-centerline twins are disproportionately huge.

    Also, note that a trimotor losing either wing's engine suffers from the same yaw equilibrium problem as a non-centerline twin.
     
    Vigilant1 likes this.
  7. Jan 1, 2019 #507

    rotax618

    rotax618

    rotax618

    Well-Known Member

    Joined:
    Nov 1, 2005
    Messages:
    735
    Likes Received:
    287
    Location:
    Evans Head Australia
    My Boorabees had zero power on/off pitch change, stick another engine on an extension of the root tube, moove the pilot back to balance it and you have your centreline twin.
     

    Attached Files:

    Tiger Tim likes this.
  8. Jan 1, 2019 #508

    cluttonfred

    cluttonfred

    cluttonfred

    Well-Known Member HBA Supporter

    Joined:
    Feb 13, 2010
    Messages:
    6,635
    Likes Received:
    2,375
    Location:
    World traveler
    Normally, high-mounted engines will cause nose-down pitch with increased power unless a strong horizontal stab is mounted in the propeller slipstream to compensate. Why do you think your Boorabees behaved differently?
     
  9. Jan 1, 2019 #509

    Vigilant1

    Vigilant1

    Vigilant1

    Well-Known Member Lifetime Supporter

    Joined:
    Jan 24, 2011
    Messages:
    4,240
    Likes Received:
    1,973
    Location:
    US
    +1 to mcrae0104's points above. One minor quibble about the part I bolded: The direction of the offset/asymmetrical thrust does matter somewhat because of the nature of the control we use to counter it. To counter an up/down thrust offset, we use the elevator which is a "pure" control. When we apply it, all we influence is the aircraft pitch. If we have a left/right offset (engine out on one wing), we use rudder to counter it, but the rudder is not a "pure" control. It primarily affects yaw (which is what we want), but also causes some roll (which is not what we want, but it is a caused because a conventional rudder/VSTAB is above the aircraft's longitudinal axis). The amount of roll will vary among aircraft (largely influenced by the amount of dihedral the wing has and the differential lift caused when only one wing has the slipstream of an engine running over it). To counter this roll, we need to apply ailerons. If we need to ask for a lot of lift from our wing, we'll approach the wing's critical AoA with our aileron's deflected differentially, and one wing will stall before the other. This is a recipe for rapid spin entry. And, as macrae points out, a third engine on the centerline doesn't change things: We still need rudder to counter the asymmetric thrust, and that still causes roll. Also, the plane will still have a VMC (which a centerline twin will not have).
    Here's the practical argument against three engines or a non-centerline thrust twin for this project: The goal is a safe, inexpensive, simple-to-build plane with good performance. If we had a lot of money to spend, heavy industrial engines would not be very attractive. Most private pilots do not have a multiengine rating, they have an SEL rating. For a pilot with a SEL rating, getting a multiengine rating costs many thousands of dollars. With a single-seat centerline thrust configuration, it is more likely that the plane can be flown without a multi-engine (multi-kilobuck) rating.

    I'm not against a trimotor plane using small industrial engines, it might work pretty well and it could probably be a two-seater (I was chiming in on the idea favorably when it came up). But it would be a different beast than this one.

    I don't believe this is accurate, and in any event it does not apply to flights in this airplane. With regard to aircraft power loss, the NTSB must be notified if a plane that has an MTOW of 12,500 lbs or more experiences inflight "sustained loss of the power or thrust produced by two or more engines" (and it apparently doesn't matter if you have two engines or ten. In fact, by the way this particular subparagraph is written, if you had just one engine and it quit, you wouldn't have to report it.) There are other reporting requirements, but none I've seen require reporting to the NTSB is a light twin loses one engine and no damage/injuries occur.
     
    Last edited: Jan 1, 2019
    mcrae0104 likes this.
  10. Jan 1, 2019 #510

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,330
    Likes Received:
    588
    Location:
    capital district NY
    From a lost an engine many times perspective, it isn't the big deal in reality that it sounds like in black and white if you have plenty of them in the first place.
     
  11. Jan 1, 2019 #511

    rotax618

    rotax618

    rotax618

    Well-Known Member

    Joined:
    Nov 1, 2005
    Messages:
    735
    Likes Received:
    287
    Location:
    Evans Head Australia
    The Jabiru powered Boorabees do exhibit a small pitch up on applicatation of power as the thrust line of the Jabiru is lower, they don’t have a gearbox to raise the thrust line. The engine thrustline is at the same incidence as the tailplane which is 2.5 deg less than the wing.
     

    Attached Files:

  12. Jan 1, 2019 #512

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,483
    Likes Received:
    2,524
    Location:
    Port Townsend WA
    The primary concern of multi-engine is extra pilot management complexity. That' s why Sport Pilot forbids multi operation and that's why the FAA tries to limit EA-B twin operation without a multi rating. Where do you get a centerline rating?

    My experience, limited as it is, is that it is hard to know how much power is available on takeoff. Can't see the rear engine or know if they are up to full power or not. The tach will be dancing all over. So the twin pilot is tempted to continue and struggle around the pattern where a single pilot would see and hear his only engine when losing power as it overheats and then the single pilot will hopefully dump it down immediately. Many single pilots can't even accept that the single engine failed , though it is obvious, and they still often stall. Much less likely a twin pilot will put it down when needed, I think.
     
  13. Jan 1, 2019 #513

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,330
    Likes Received:
    588
    Location:
    capital district NY
    I don't know, maybe I'm just sensitive but I always knew when I had an engine out. Sometimes if it was just a coasting jug you wouldn't know there was anything wrong with an engine until the mechanic told you but with 72 cylinders one of them underperforming it isn't always going to be self evident. The idea you are going to suddenly become stupid because you have redundant power is an odd idea don't you think? Options is what we are taught as pilots, and more engines give you more options and more time to exercise those options. It is not altogether infrequent that we hear about some unfortunate landing off airport in a single engine aircraft and barring the fuel starvation stories many would have probably enjoyed another engine or two.
     
  14. Jan 1, 2019 #514

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,330
    Likes Received:
    588
    Location:
    capital district NY
    Complexity of a fixed gear, fixed prop, two 28hp engine VFR aircraft? Seriously? I'm checking my calendar and … yep it is Jan 1st not April 1st.
     
  15. Jan 1, 2019 #515

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,483
    Likes Received:
    2,524
    Location:
    Port Townsend WA
    Was your four engine transport single pilot?
     
  16. Jan 1, 2019 #516

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,330
    Likes Received:
    588
    Location:
    capital district NY
    Of course not, but implying that two 28hp fixed propeller engines is to much airplane for someone to handle is a little over the top.
     
  17. Jan 1, 2019 #517

    Vigilant1

    Vigilant1

    Vigilant1

    Well-Known Member Lifetime Supporter

    Joined:
    Jan 24, 2011
    Messages:
    4,240
    Likes Received:
    1,973
    Location:
    US
    A Revisit to Induced Drag

    I want to clean up a loose end, sorry to regress a bit.
    Quick recap: In a previous series of posts we discussed induced drag--specifically, whether it was a function of wing area. This came after lr27 proposed using a high-lift airfoil as a way to reduce wing area and weight. I said that induced drag goes go up as Cl is increased and wing area is reduced. Pictsidhe and lr27 did not agree, and maintained that, for a given weight, induced drag varied only as a function of span and speed, and was not affected by Cl or wing area. I said I'd need to review my spreadsheet and the logic behind the equations make sure my calculations were correct (more here). Then we started discussing Oswald Span Efficiency ("e").

    I've had the chance to review things, and I reached some conclusions:

    1) My spreadsheet for computing induced drag is correct, so the estimates I've done for the Micromaster so far are okay (whew!). It uses the Raymer and Hiscocks equations correctly, it computes the Oswald Span Efficiency Factor ("e") correctly according to the equation provided by Raymer (though, as Topaz notes, this might not be right for planes of very high or low AR. More on that below).

    2) Regarding wing area, wing span, Cl, and induced drag: I think "everyone was right" about something.

    lr27 provided the needed equations:
    I can't take issue with this math or this approach. The fly in the ointment is Oswald, which does vary by aspect ratio (which, if we keep wing span constant, is like saying it varies with chord and wing area) . As posted previously:
    Here are some worked examples to see how Raymer's method of computing "e" affects things. I also back-calculated some of the numbers using lr27's equation in red above, inserting the value of "e" calculated by Raymer's equation, the results are shown in red below. They are very close to my spreadsheet results, differences are probably just due to rounding:

    Common to all: Aircraft weight (lift required) is 766 lbs. Airspeed is 70 knots, SL std day conditions
    Case 1 (base case): 26' wingspan, 2.7' chord (no taper). Wing area: 70.2 sq ft. AR: 9.6 Cl: 0.665 Value of "e": 0.77 Induced drag: 21.98 lbs (21.88)
    Case 2: 35' wingspan, 2.006' chord (no taper). Wing area: 70.2 sq ft. AR:17.4 Cl: 0.665 Value of "e": 0.58 Induced drag: 16.02 lbs (16.04)
    Case 3: 26' wingspan, 3.0' root chord, 2.4' tip chord (tapered wing). Wing area: 70.2 sq ft. AR: 9.6 Cl: 0.665 Value of "e": 0.77 Induced drag: 21.98 lbs
    Case 4: 52' wingspan, 1.35' chord (no taper). Wing area: 70.2 sq ft. AR: 38.5 CL: .665 Value of "e": 0.18 Induced drag: 23.29 lbs
    Case 5: 26' span, 1.495' chord (no taper). Wing area: 38.87 AR: 17.4 Cl: 1.202 Value of "e": 0.58 Induced drag: 28.97 (29.06)

    Observations:
    Case 1 vs Case 2: Increasing span moderately while keeping wing area constant decreases induced drag (this was expected)
    Case 1 vs Case 3: Keeping span, AR, and area constant but using a more efficient tapered wing did not improve "e" and did not reduce the calculated induced drag.
    Case 1 vs Case 4: Increasing span a lot (very high AR) produced a very poor value of "e" and an increase in the induced drag value (this is counterintuitive, but is consistent with what Topaz posted earlier)
    Case 1 vs Case 5: Keeping span constant but decreasing chord (so, smaller wing and a higher required Cl) resulted in a reduced value of "e" and increased induced drag. This leads to:
    1) Given constant airspeed and lift, span is not the only determinant of induced drag (because of the impact of "e")
    2) Using a high-lift airfoil to produce the same lift with reduced wing area will increase induced drag. Is this really due to the higher Cl, or is it perhaps due to a decreased AR? Well . . .
    Case 2 vs Case 5: Keeping AR constant but decreasing wing area (thus decreasing span and also increasing required Cl) leaves "e" unchanged, but it increases induced drag (a lot).

    The wing with the smallest area and highest required Cl had the highest calculated induced drag.

    Now, the unresolved question is whether an AR of 17.4 (Case 2 and Case 5) falls into the "bogus zone" for Raymer's equation used to find "e."

    Enough fun with numbers for now.
     
    Last edited: Jan 1, 2019
    Topaz and blane.c like this.
  18. Jan 1, 2019 #518

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    12,483
    Likes Received:
    2,524
    Location:
    Port Townsend WA
    Blane, The statistics for twin operations are worse than single. Not implied, just fact.
    How many centerline twin lawn mower powered airplanes have you seen?
     
  19. Jan 1, 2019 #519

    Tiger Tim

    Tiger Tim

    Tiger Tim

    Well-Known Member

    Joined:
    Apr 26, 2013
    Messages:
    2,972
    Likes Received:
    1,673
    Location:
    Thunder Bay
    I somehow missed out on flying Skymasters for a summer though most of my professional pilot friends have. They’ve all said that on takeoff the procedure was to advance the rear throttle first to ensure power was available, then the front. Apparently it was possible that the prop wash from the front could windmill the rear up to enough RPM that you might think it was producing power. Once confirmed operating in a quick static run up I think it would be pretty obvious if an engine quit on the takeoff run any time after brake release.
     
  20. Jan 1, 2019 #520

    blane.c

    blane.c

    blane.c

    Well-Known Member HBA Supporter

    Joined:
    Jun 27, 2015
    Messages:
    3,330
    Likes Received:
    588
    Location:
    capital district NY
    Well maybe we should fly in the face of statistics and with more people flying twins and hopefully triples and doing more flying just change those old tired statistics how about that idea?
     

Share This Page



arrow_white