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Sifo
| Posted on Thursday, July 11, 2019 - 07:41 pm: |
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Yeah, tip speed becomes a problem. It's not just RPM and prop diameter. You also have to add in forward speed. Big problem for rotary wing craft too. Jets solved so many problems! |
Aesquire
| Posted on Thursday, July 11, 2019 - 10:24 pm: |
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It's not nearly as bad as most people would expect though. In some ways you are right. The center area of a prop has a hub which provides no thrust, and ideally has a spinner to smooth airflow....but what about those radials which seldom ran a spinner? Cooling. Except for the Fw-190 which used a geared faster than the prop, fan, and a special cowling shape to match, spinners just added on tended to choke air flow for cooling. It takes an iterative and comprehensive design program to optimize cooling air flow. The Republic "Rainbow" recon airplane seems to have finally solved all the drag issues with radials, with no protruding cowl flaps, all the turbos & intercoolers hidden in the nacelle, and jet thrust from heating the air to recover the energy lost slowing it down to use for cooling oil, air, & feeding the engine. Consider the engines were the size of a small block Mopar... Plus four thousand cubic inches. https://en.m.wikipedia.org/wiki/Republic_XF-12_Rai nbow Then jets made the military give up on big piston engines. ( but the Skyraider hung on till the Vietnam war, it was that good at It's job ) And you are right about prop twist. That has to be optimized for a single speed. There are a few companies making variable twist props, taking advantage of composite blades. But no one today is selling props that vary in pitch and twist under pilot control. Ivoprop uses twisting blades to vary pitch with a light, simple hub system. Prince uses special tip shapes to vary the twist by reacting to load and rpm with no mechanical bits at all, automatically. However, in large part, the propeller is lousy near the center because the engine is blocking airflow there. No need to improve has been felt by designers, since it wouldn't help much. Going to a spinner size motor will force improvement. But it won't be huge, just worth it. And in pusher applications, without a heavy drive shaft, bearings, and structure to support them, a pusher has the engine, nacelle or fuselage, blocking airflow into the prop. This is why the claimed big advantage of pusher props, the reduction in drag by not blowing high speed air over the airplane from a front mounted prop, is seldom realized in the real world. Again, small diameter motors help with drag. The Alice plane takes full advantage in this category. However, they seem to assume a very high level of reliability when they stick props out at the wingtips. That offers some drag reduction by changing the tip vortex but also has to deal with variable in coming air angles. I'd love to see wind tunnel tests. The real problem with most wing mounted engines is asymmetric thrust when one quits. A whole lot of multi engine rating is learning to handle the sudden "desire" for the plane to yaw when an engine stops. A lot of the design of the plane is giving enough rudder control to do so. And since rudder control is airflow/speed dependent, on most multi engine planes there is a critical speed below which you can not stop the plane from yawing & rolling into the ground, without cutting power to the other engine(s). Props at the tips have the greatest leverage possible. That's why designers tuck wing mounted engines as close to the center as they can, despite noise and wing spar bending load. Thus I bet Alice would have to cut power to the other wing motor if one quits. And the regulations would demand the fuselage mounted motor to be capable of climbing on it's power alone. I guess the wing mounted motors might be able to climb with the fuselage motor stopped?? Still, I'm cynical. I'll always suspect that pretty websites and CGI airplanes in exotic locations, are scams until I see them actually fly. |
Gregtonn
| Posted on Thursday, July 11, 2019 - 10:44 pm: |
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...CGI airplanes in exotic locations... What is CGI? Not sure if you are referring to Prescott as an exotic location, if so you may want to know that it is one of the busiest general aviation airports in the country. Not sure if it still is but several years back it was the busiest. G |
Aesquire
| Posted on Friday, July 12, 2019 - 12:33 am: |
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Computer graphics. I see websites for the flying car electric magical physics busters all the time that never reach hardware. Raising funds for future planes that don't & never will exist. These sites inevitability have pretty pictures of imaginary airplanes flying past exotic locations like Paris, Bangkok, etc. often at altitudes and locations that a real airplane would get shot down at. However, occasionally, there IS hardware, and it DOES fly. Then I'm astonished & pleased. Doesn't keep me from being cynical. The odds are massively in my favor. The Pal-V for example. Until they flew, AND were able to drive public roads, I didn't think it real. It is, though, and while I seriously doubt there's enough rich guys who will spend the time to get the specialized training to fly one, to stay in business, I want one. Only a lotto ticket away! Blackfly, an electric VTOL multi copter with wings, is another real flyer. They didn't fly at Oshkosh, for legal reasons, but they also didn't first get my attention with pretty fake pictures, I first saw them when the nth prototype was ready to show off, and had enough hours to let other than test pilots try it. Another "I Want One", but I doubt they can hit the price goal of "luxury SUV". I also doubt I can budget that. |
Aesquire
| Posted on Friday, July 12, 2019 - 12:47 am: |
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In my experience, most new & exciting flying machines never get to anything physical that resembles an aircraft. Then there are a few that have a not yet flown prototype, often a proof of concept instead of ready for sale once we fix a few things. They sometimes are purchased by optimists when the original guy splits. Seldom do those leave the ground. The more exotic, the less likely to fly. And these aren't necessarily scams. The surest way to not fly is to combine an exotic shape, like a reverse sweep flying wing, with an untried engine, like a turbocharged Lexus with a novel prop speed reduction unit. ( gear box ) Hey! I'm rooting for Alice! I just won't be shocked if it's never ready. |
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