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Thread: Flight theory question of a Zenith CH 701

  1. #1
    Registered User Mike J's Avatar
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    Flight theory question of a Zenith CH 701

    Hi guys

    I'm really confused by that Zenith 701 STOL airplane. I've read all the articles on the Zenith website but I'm confused about something and was wondering if somebody could explain it.

    Power quits at say 2,000 ft AGL. So given this type of airplane and it's built in characteristics how does this play out? I would imagine it comes down pretty steep and fast yet it stalls slow and at unusual high AOA. Ok, so take the Sonex for example or maybe even a Midget Mustang. If the engine quits in these types of airplanes they also come down pretty fast but for different reasons. Glide ratio vs drag right?????

    Now lets take a Cub. Big wing flys pretty slow etc. The engine quits and a guy has a few more options compared to a Sonex or a Mustang.

    So as I look at all these different types of airplanes we have to pick from and then there's this CH701 with very unique capabilities I can't seem to figure out if a power out in a 701 would be worse or better than a different type? Maybe I just don't understand these STOL airplanes.

    If the engine quits which airplane would you want to be in?

    Mike

  2. #2
    Moderator addaon's Avatar
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    Re: Flight theory question of a Zenith CH 701

    There's three different (related) things to think about here:

    1) Glide ratio. How many feet forward you go for each foot you go down.
    2) Sink rate. How many feet down you go for each minute of time.
    3) Horizontal speed. How many feet forward you go for each minute of time.

    Consider a power off, steady-state descent at best glide. (That is, the engine quits, you trim for the published best-glide speed, maybe pull the prop control all the way back or pull the nose up for a bit to let the prop stop depending on engine, and then just sit there.)

    The better the glide ratio, the further you can go. This corresponds with being more likely to find a good landing field.

    The better (lower) the sink rate, the more time you have. This corresponds to more chances to restart the engine and better decision making. In a steady-state decent, this also controls landing gear loads, which partially controls whether you're landing or crashing. In practice, most off-field emergency landings are still controlled landings, and the pilot will flare to reduce vertical speed to near zero and horizontal speed to near stall at the moment of touch-down.

    The lower the horizontal speed, the less energy that has to dissipate once you're on the ground. [Strictly, this depends on the speed in direction of flight, not horizontal speed; but horizontal speed is dominant.] If you're landing more or less normally, this is braking distance and ground roll; how big a field you need to put it down. If you're landing in an abnormal attitude (oops!), this is how much energy is absorbed by the tree you hit / the structure of the aircraft / your body.

    A STOL design like the 701 will tend to have a worse glide ratio than other designs, a relatively typical sink rate, and a very low horizontal speed. Compared to a Sonex, it won't give you as many options of where to put it down, but it won't take nearly as much space, either. If I'm flying over a forest and just out of range of a runway in a 701, I'd much rather be in a Sonex. If I'm over small farms with lots of fences, I'd much rather be in a 701. If I'm over a road or runway, either is fine; and if I'm over non-landable terrain (mountain, etc) I'll take the 701 for the lower energy at landing.

    A Cub has a pretty typical glide ratio, a low sink rate, and a low horizontal speed. It's easy to cherry-pick situations where the 701 is safer in an engine-out situation, but the Cub is probably just a little more likely to find a nice place to end up. Both are slow enough that you'd expect to survive putting them in a tree.

    A high-performance design like a Lancair has a good glide ratio, a high sink rate, and a high horizontal speed. You have a higher chance than in a 701 or Cub to be in range of a runway or road, because of the increased glide; but if you're not, you're probably not going to have a good day.

    There's non-aerodynamic factors at play as well here, of course. A Cub or 701 on big wheels is going to be a lot safer to land on a soft field than a design on tiny low-drag tires, regardless of air speed. A 3000 lb aircraft is going to have a lot more non-meat structure to absorb energy than a 600 lb empty 701. Airbags exist, and work. And different designs have different strategies (or lack thereof) for absorbing crash energy, avoiding putting a yoke through your face, etc.

    That should give you a sense for what to think about... but as a personal preference, if I have to put down something when the fan stops, I'd much rather be in an aircraft with a 35 kt stall than a 55 kt stall. A perfect emergency landing is possible and safe in any airplane; a horribly awful one is more survivable in a slow one.
    Norman likes this.

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    Registered User Head in the clouds's Avatar
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    Re: Flight theory question of a Zenith CH 701

    Quote Originally Posted by Mike J View Post

    If the engine quits which airplane would you want to be in?
    Addaon's answer is fully comprehensive of course, but a simple one that I sometimes use to explain this, and allowing that it does depend a bit on whether you're flying over a huge smooth flat area or a rugged landscape, is -

    If the engine quits would you rather be flying a Learjet or a parachute ...?

    I'd take the low performance machine every time.

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    Registered User Mike J's Avatar
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    Re: Flight theory question of a Zenith CH 701

    Thanks for the reply, that's exactly what I was looking for.

    I tell ya, the more I read about those STOL 701/750/801's the more appealing they become. They really are great designs, it's hard not to like them. They look tough as well.

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    Moderator addaon's Avatar
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    Re: Flight theory question of a Zenith CH 701

    I'm (theoretically) building a 701, and have flown two, one on wheels and one on floats. Obviously I like it, given that I'm building it...

    But as a low-airtime pilot, the extreme STOL side of the performance envelope is outside of my comfort zone. The wrong side of the power curve is called that for a reason; you're operating in a condition where problems require immediate responses, and errors become accidents very quickly. As a result, while the high-alpha performance of the 701 is truly exceptional (there are other designs as good, but not many better), I fly it roughly the same as I fly a Champ or a Cub. If I didn't personally prefer to work in aluminum than wood and fabric, I'd have a hard time justifying it over more conventional designs. If I didn't want to put floats on it, I'd have a hard time justifying it over a Sonex.

    Just my thoughts.

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    Registered User Mike J's Avatar
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    Re: Flight theory question of a Zenith CH 701

    Quote Originally Posted by addaon View Post
    I'm (theoretically) building a 701, and have flown two, one on wheels and one on floats. Obviously I like it, given that I'm building it...

    But as a low-airtime pilot, the extreme STOL side of the performance envelope is outside of my comfort zone. The wrong side of the power curve is called that for a reason; you're operating in a condition where problems require immediate responses, and errors become accidents very quickly. As a result, while the high-alpha performance of the 701 is truly exceptional (there are other designs as good, but not many better), I fly it roughly the same as I fly a Champ or a Cub. If I didn't personally prefer to work in aluminum than wood and fabric, I'd have a hard time justifying it over more conventional designs. If I didn't want to put floats on it, I'd have a hard time justifying it over a Sonex.

    Just my thoughts.
    I've been in a Sonex a few times know, for what it is it's an amazing aircraft. I just can't get comfortable in it. If you have wide shoulders or have a thicker build it's a single place airplane. I have to contort myself to get in with two of us. Otherwise it's a killer performer. On YouTube there's a guy with videos doing major cross countries in it. He flew his Sonex from like NY to the Bahamas. It's a cool video.

    I prefer a high wing airplane. An affordable scratch built high wing in aluminium is a rare thing.

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