What's her email address?

Having built an experimental aircraft, with inspections by an EAA Tech Counselor/DAR at every major phase of construction, documenting the build carefully (reviewed by DAR), and undergoing an airworthiness inspection I can assure you this is not the case for most experimental aircraft builds. I drilled out rivets (many hundreds) and re-made or replaced parts that would have been acceptable in production (repaired) - I even re-built some entire assemblies for relatively minor imperfections (one elevator I re-built because I wasn't happy with a mounting ring for an inspection plate - purely a cosmetic issue - on the bottom of the elevator where I was pretty much the only one who'd notice).


Of course there are exceptions, but most in the experimental world are very serious about the pursuit. The "bad" actors tend to get culled from the herd in one fashion or another.

I’m trying to digest Pete’s latest design, but my grasp of physics isn’t quite what it used to be, so I have a question. I build a plane that requires a 400hp ICE driving a prop to cruise at “X” airspeed, then I remove the engine and install two electric ducted fans. If everything else stays the same, won’t I still need 400hp from the ducted fans to cruise at “X” airspeed. And if I do, and I’m using Pete’s engine/generator/motor set-up, won’t I need an engine with more than 400hp to turn my generator due to the loses of going from mechanical energy, to electrical energy, and then back to mechanical. I must be missing something. It’s been a long week. My brain is fried. Is there some way to get 400hp to the ducted fans while driving the generator with a 200hp engine?

Shhhh!! We want to get at least another couple seasons out of this reality show.

Short answer is no, the ducted fans are less efficient so you will need more hp than with the prop before you get to the generator loses.

That said, the Audi engine was likely making 250 hp or less, so who knows.

What's her email address?

hotstuff@weatherbabe.com[/quote]

Todd,
You know the rules. Pictures or it never happened.

Phil

Phil

There are a couple of ways to "do" ducted fans, neither of which are overly suitable to prop and turboprop speed airplanes.

You can have a ducted propeller, which gains a little bit of efficiency from the shroud eliminating tip effect, but loses a lot of static thrust because it's... a small propeller. And you have a certain tradeoff with the extra structure and complications from that, plus some more efficiency losses because the air blasting through the shroud has some wetted area drag going on too. A 300hp ducted propeller doesn't really get you anything that a good 76~80" constant speed propeller doesn't already do well.

You can also make the duct a converging nozzle. This is exactly how the fan/bypass air in turbofan engines works. The nozzle does what nozzles do, it squeezes the fast, high pressure air through a tighter space and then you get more thrust than if you just blew the air along through a plain duct. Here's the catch though- putting ~300hp into a well designed fan in this way is going to produce about 250 pounds of static thrust. It will produce about the same thrust on up to about 200-300 knots airspeed, but keep going faster and that thrust will drop off. If you put the same horsepower into a smaller fan then it will make less thrust but it will also maintain that thrust (approximately) to a higher speed.

That's how the basic laws of fluid mechanics, turbomachinery, and propulsion work.

Ducted fans seem very popular in CGI airplane, but I haven't see any in low speed real airplanes.

My understanding is that as you get to a noticeable fraction of the speed of sound, you need to efficiently decelerate / compress he inlet air to a speed a fan / prop can handle efficiently (usually .3-.5 mach, though there are prop designs that operate at higher speeds). The duct is designed to do this.

At low speeds I don't think it helps. It does terminate the pressure across the prop blades, but you have extra drag from the air moving across the surface area and you need a huge duct if you want a big prop.

Has anyone see a practical use of a duct on a plane that cruised below mach .5?

These laws don’t apply to Peter and are therefore not relevant to this thread.

These laws don’t apply to Peter and are therefore not relevant to this thread.

Which is why you don't see them in practice. It's not because of groupthink by engineers or by airplane manufacturers.

The question was “practical use below Mach .5.” For low and slow operations near people, low noise is a very practical concern that must be considered in the design. If the question had been “use in a fast traveling machine like Peter is trying to build” then maybe not.