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  1. Oct 19, 2003 #1

    W.M.Patrick

    W.M.Patrick

    W.M.Patrick

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    Location:
    Madison, Mississippi
    i am a mustang 11 builder and pilot, and have thought over what I
    would like in my next project. improve visability-by ssitting ahead of the wing- reduce noise level- auto engine - and it boils down to two
    designs. a pusher design with tandem seating and twin booms to the
    tail. frontal area 28" x 42". engine hp. 300 at 500#engine wt.
    The other option was a tractor design with auto engine the same and
    frontal area 48" X 42". Engine behind the spar like P-39.
    Question is where the prop inefficiency of the pusher would be made up for by less frontal area. Or the tractor design would outperform the
    pusher. Can anyone give me their ideas on this ??
     
  2. Oct 19, 2003 #2

    John Slade

    John Slade

    John Slade

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    Hi,
    First off, I see you're in Madison, MS. I think there's a fly-in at the Slovakia (sp?) airport just north of Jackson this weekend or next where you could see some experimentals. I may have read this in the canard forum or on the Cozy list.

    I can talk to the pusher a little - A lot of the prop inefficiency in EZ type canards is due to lack of air to the prop, but RV's and Cozy's seem fairly close on speed with the same power. With an automotive engine you can remove the "cowl cheeks" and get a lot more air to the prop. 48 inches is pretty wide, however. Cozy is more like 38. The wider Velocity looses a little in speed.

    The pusher design certainly gives good visibility and lets you leave the noise behind you. As for "twin booms to the tail", I have no idea. I'm no airplane designer, just a builder from someone else's plans.
     
  3. Oct 19, 2003 #3

    orion

    orion

    orion

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    This is not really an easy question as it depends on a whole slew of variables connected with the design of the airplane, the design of the propeller and the control of the air flow to and aft of the prop (depending of course on whether its a pusher or tractor configuration).

    In the most general sense, the efficiency of the prop itself tends to be about the same. The thing to discuss then is what the airflow does before or after the propeller. In a tractor, the air coming in is clean and undisturbed. However, as it exits the prop, the high energy air is directed into the cooling system and across the fuselage. This resulting high energy contact with the sizeable amount of wetted area results in a loss of energy and thus the configuration tends to be considered not as efficient as the aft mounted case.

    For the pusher prop however, the "efficiency" is strongly affected by the incoming flow. This means that the behavior of the prop is a function of the fuselage shape, the disturbing effects of cooling or exhaust, and the flow off the wing or other flight or control surface. Ideally, the prop shold be located at least one half of the diameter from any flight surface (a whole diameter is preferred) or in the case of fuselage mounting, the taper to the prop should be gradual, with no additional flows (such as cooling or engine exhaust) disturbing the streamlines or incoming flow density.

    The benefit of the pusher configuration though is that the low pressure field the prop creates in front of the prop disk actually acts to suck the flow inward, thus smoothing out the incoming flow, even on the less than ideal installations.

    The thing to watch out for on pushers however is the effect of the incoming flow on the propeller itself. By this I mean the structure. If the flow is disturbed and nonuniform, there is a chance that the prop could be affected structurally as it rotates through the varying streamlines. In the past this has led to several in flight prop failures or in the worst cases, even engine failures (prop flange or engine crank failure). Non-uniform flow could be cooling or engine exhaust as mentioned earlier, or even a small effect such as downwash off a wing or control surface. Even the change in flow density from a fixed wing or fairing could affect the prop in this manner.

    However, the final performance of the aircraft is more than a function of just the prop and it is difficult to make direct comparisons for what would be very different configurations. For this reason, comparing an EZ to an RV really does not make sense since the comparison is of two very different configurations with different design requirements. For example, the RV presents more frontal area and has more fuselage wetted area than the EZ aircraft - this decreases the overall efficiency due to the higher surface friction drag resulting from the larger contact area. It however tends to have a higher wing loading (as compared to an equivalent canard design) thus having higher lift efficiency there (better L/D).

    For the same minimum speed requirement though (fixed weight), a canard configuration will generally carry substantially more wing area than the conventionally configured airplane and so it generally flies at a lower L/D ratio. But the fuselage on the EZ is substantially smaller and it is configured in tandem, so that part of the drag equation is smaller than for the RV.

    The other thing to consider is the scale of the aircraft. Most of the "efficienncy" differences on general aviation airplanes is on the order of a few percent. Translated to the real world, it is unlikely that an average pilot would be able to tell the difference anyway. Therefore, when picking a design, choose one that meets your likes and preferences. If that's based on aesthetics, that's OK. After all, you'll want to be happy with your selection and what appeals to you will most likely make you a content builder/owner.
     
  4. Oct 19, 2003 #4

    John Slade

    John Slade

    John Slade

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    Interesting analysis, Orion.

    The solution to the prop stresses you mention is wooden props which seem to handle the varying airflow patterns admirably.

    >comparing an EZ to an RV really does not make sense
    From an aerodynamic perspective, perhaps. However, from the perspective of someone who wants to build an airplane I submit that such a comparison is valid. There are two of the most popular designs and each have their advantages and disadvantages. When refering to EZ designs I was thinking more of the 4 place ones like Cozy IV and Velocity which have similar width to the larger RVs.
     

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