I've noticed that the bigger and heavier a plane is, the more likely it is to use jets, and the smaller it is, the more likely it is to use propellers. The most extremely large sizes seem to include no propellor planes at all, and the smallest sizes (unless you count the Jet-Man's winged backpack-suit) seem to include no jets at all. This kind of distribution between two different things usually is a sign that each of them just works well in one size range and doesn't work so well in the other's size range, whether for mechanical/geometrical reasons or due to costs or something else.

Is there a mechanical issue that makes each one more practical than the other for a given size? If there is, what allows the Jet-Man's tiny little jets to work and why would he use them instead of propellors (possibly with rims for safety)? Are jets just always more efficient/powerful but harder to pay for and thus prone to being used in high-cost planes but skipped in low-cost planes?

I swear, if I'd been as fascinated by aeronautical stuff at 18 as I am now at 31, I would have picked that as my major in the first place...

I've always thought of it that way. I'm not sure about efficiency or power, but it is cost effective to get the plane from point A to B in as little time as possible. Time is money.
I remember plans in the 70's for a jet version of the BD-5 (a one-seater IIRC). Never happened, but now we keep seeing plans for personal jets. And, what about the smaller Lears?

No answers, just things to think about.

__________________
Numbers are not case sensitive. (me)

I was 13 when Jim Bede was developing and promoting the BD-5. I wanted one! I believe there are still some flying. We had one at an airshow here about 15 years ago. Unfortunatele, there was a flameout on takeoff, and the pilot died in the wreck. Link to a video of a BD-5J

There have been prop planes that are quite large.

There have been jets that are quite small.

I don't think overall plane size is the major factor.

__________________
"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

A lot of the difference is that for small airplanes, jets tend to be quite expensive. However, once you get up to larger airplanes, almost all of the prop planes tend to be jet anyways (turboprop), and so the cost evens out. In addition, you have the higher cruising speed and greater efficiency of the jet. Also, in large sizes, the propeller tips start approaching sonic, creating massive amounts of noise and a whole host of new problems. Basically, for small airplanes, cost is the driving factor, and that factor is eliminated when it comes to large airplanes.

__________________
WANTED:

Schroedinger's Cat

Dead And Alive

Some very large airplanes such as the Hughes Hercules (he hated the name "Spruce Goose") were powered by propellors. However, turbine engines have a much higher power to weight ratio than piston engines. Aircraft piston engines pretty well topped out at around 3,500 horsepower. Even then, they tended to be fairly heavy, complicated, and needed a lot of maintenance. By comparison, turboprop engines that produce over 12,000 horsepower exist for a few aircraft.

Turbine engines run smoothly and tend to require a lot less maintenance than piston engines. They produce far more power for a given weight but tend to consume more fuel. Also, turbine engines tend to be much more expensive than piston engines. This is one of the biggest factors in why there aren't many small turbine powered aircraft.

The BD-5J still exists in kit form. You can buy a turbojet or turboprop version from this company. That's an example of a very small aircraft powered by turbine engines. Here are some more:

Helicycle single seat helicopter kit.
Viper two seat homebuilt jet.
Maverick 4-5 seat jets (some are kits)
Lancair turboprop 4 seaters (IV-Propjet and the Evolution).
CompAir has several turbine powered homebuilt aircraft.

Cessna also makes the Caravan and there are a host of Very Light Jets either in production or under development.

A couple of examples of large prop driven aircraft.

C130
http://www.theaviationzone.com/image...cules/main.asp

C133
http://www.theaviationzone.com/image...c133/page1.asp

__________________
"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

BD-5. You mean there's someone IN there?
I thought that it was a large model!

Isn't the shape reminiscent of the Nazi ME 163 rocket plane? Is that because both were/are very small? Though the BD-5 is even smaller than the Messerschmidt.

John

The Me-163 was a swept wing tailless fighter powered by a rocket engine. The BD-5 is a straight wing airplane with an all-moving horizontal stabilator. There might be some resemblence from the side view but that's mainly coincidental.

As for large propellor driven aircraft, the Antonov An-22 was very big. Each of its four engines was rated at 15,000 shaft horsepower. You'd have a very difficult time building a piston engine that powerful that is light enough to fly.

On the opposite end of the spectrum, there is a version of the beautiful Italian Alisport Silent motorglider that's powered by two very small model airplane jet engines. Bill Carlton flies an aerobatic airshow routine in the plane. Video links are available here. There are also gas and electric powered versions of their sailplanes. The gas versions are interesting because they use single bladed propellors.

Perhaps the smallest manned jet ever flown (certainly the lightest) is this version of the Cricri (Cricket) by French pilot Nicolas Charmont. I think it uses the same engines as the Silent motorglider mentioned above. Whether it's smaller than the BD-5J is a matter of argument.

Hi, You have to consider the mission statement for each aircraft, and the customer. Some people demand fast and HIGH. At altitudes of 40,000 feet,
Jet Turbines are the answer. At 100,000 feet, only the SR 71 will operate
.....at hugh costs and an elaborate tech team and with refueling etc.
Most small aircraft are not pressurized, so they are working in the altitude envelope of less than 14,000 feet . The cost of operation for high performance
piston aircraft which are pressurized , espaecially at todays aero fuel prices
is extravagant. My cousin formerly flew an Aerostar. You are looking at
flight levels of 25,000 feet, with fuel burns of 36 GPH @215 KIAS to
44 GPH @250KIAS . That's gallons per hour. At $4.50 a gallon for aero gas, it gets expensive. The people flying this kind of stuff are looking at convenience of flying vs reliability and down time/maintanance. Oh..by the way, this thing takes off /rotates at 92 kts, And lands at 100 kts.
In 1975, the Aerostar 601 broke the speed record for a piston-engine aircraft over a 1000-kilometer, closed course of 305 mph.
So...if you want to fly high and fast without going into a Lear, people fometimes fly an Aerostar. They do run into money. Full gas 200 gallons.
Do the math.
Now, our alaskan friends are taking Beavers and converting them from the radials that are getting hard to find parts for, and outfitting them with turboprops. Reliable, and powerfull for seaplanes, these are the alternative
to keep these great planes in and out of the far flung alaskan wilderness
with severe payloads , uncertain weather and eager customers. If the money is there, they will find a way. Non pressurized , brute force cargo and ferry.
Now, if you only want a 2 seat aircraft and a small briefcase, Burt Rutan gave us the Long-EZ , 175Kt. cruise on 100 HP. Pretty darn good, and they don't rust. But.....you build it.
By the way: They have done some pretty successfull experiments on some DC-9's altered to spin a large multi bananna looking propellers that delivered sparkling performance and were fairly quiet.

Today, the problems are changing. Jet A used to be substantially cheap by comparison with aero gas 100L . That's changing. And with changing fuel goes changing thinking.
The book is never finished.
It is....interesting.
Best regards, Dan

" In 1857 Henson and Stringfellow flew an rubber -powered model which flew over 600 meters before encountering an obstruction..."

You mean the ground?

Larry,
"Perhaps the smallest manned jet ever flown (certainly the lightest) is this version of the Cricri (Cricket) by French pilot Nicolas Charmont. I think it uses the same engines as the Silent motorglider mentioned above. Whether it's smaller than the BD-5J is a matter of argument."

Surely not?
How about Yves Rossy, the Flying Man?
His wing is neraly as big as the Cricket, but the rest of the airframe is him!
See: http://uk.youtube.com/watch?v=SHYXrqoS08o
And http://www.jet-man.com/actuel_eng.html

John

Well, since Mr. Rossy can't take off or land using his wings, I'd say his setup (while very very cool) doesn't really qualify as the smallest jet aircraft. I've read that he hopes to be able to take off and land in the rig one day. If and when that happens, he'll likely take the record for the smallest jet aircraft, at least in some dimensions. For example, his wingspan might have to be longer than a BD-5J's but his length will likely be shorter. It'll probably be lighter than either a BD-5J or the jet CriCri but that remains to be seen.

Just look at Howard Hughes' Spruce Goose flying boat.... as big as a 747 and all props baby (and it DID fly.)

The Cri-Cri is the worlds smallest manned jet, but as an R/C plane flyer I can tell you that tiny jets have been here for a while. You can get micro-jets for a couple of thousand dollars that'll push a 50lb plane to 300mph.

Here's a video of a production turbojet only a couple of inches in diameter:
http://www.youtube.com/watch?v=TiPl0AS-Sdg

You want smaller??? With electric motors driving a fan in a shroud (a ducted fan) we go down to units as small as 30mm -- yes just over ONE INCH in diameter that can fly planes a couple of ounces:

http://robotbirds.com/catalog/produc...oducts_id=1636

Probably a tree.

Hi, If he wanted to take off and land, he would be well advised to do it off of
a frozen lake on a warm day. Short skis or maybe a belly ski . I don't think skates are advisable, same way as aircraft tires have no tread.
Side slip ....don't fight it. I wonder what his V 1 would be? 70 KTS IAS ?
Dan

Update on Yves Rossy

Rocket man flies on jet-powered wings
In reference to the 360

I WANT TO SEE VIDEO.

__________________
Numbers are not case sensitive. (me)

The main reason is cost.

The B-36 peacemaker had the following engines:

4× General Electric J47 turbojets, 5,200 lbf (23 kN) each
6× Pratt & Whitney R-4360-53 "Wasp Major" radials, 3,800 hp (2,500 kW) each

It had the following dimensions:

Length: 162 ft 1 in (49.40 m)
Wingspan: 230 ft 0 in (70.10 m)
Max takeoff weight: 410,000 lb (190,000 kg)

That's a greater wingspan that an 8 jet engined B-25, and close to it's max gross takeoff weight.

The main reason behind larger aircraft having jets is that jet engines cost a lot more than pistons and propellors. However, they are a lot less maintenance, over time, than pistons and props, too.

The larger aircraft justify this cost because they fly far more often, usually as much as 20 times a month, whereas most smaller aircraft are privately owned and might fly an average of perhaps once a week or less. Thus, there is no justification for the additional cost.

The booming VLJ (very light jet) market has microjets that carry between four and seven pax is booming largely due to three factors:

1. Improvements in manufacturing have driven down the constant-dollar costs of jet engines and airframes.

2. Airlines adopted a hub-and-spoke scheduling a couple of decades ago, which solves some of their scheduling problems, but turns a 50-minute direct trip into one that takes 5 hours, in part due to a 2-1/2 hour layover.

3. The Internet is also booming with companies like NetJet, where several people can put in preferences for direct flights from point A to point B, particularly where they're willing to be a bit flexible on dates, and share a plane with others.

Result: Air Taxi. An inexpensive solution for the niche of direct flights to, from, or between obscure places. And because the flights are both full and frequent, they use jets instead of pistons.

__________________
I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol. Human.

Whoever says "perception is reality" is daft. It's merely an abstraction, and often not a very good one.

It all boils down to the airplane's flight parameters.

Whether jet or prop, the main factors involve the desired airspeed and expected flight altitude. Recip and turboprops are more effecient at low airspeeds and relatively low altitudes, which is why most of the short range regional air carriers use the ATR's, Saab, and Embraer turboprops.

High altitude/high airspeed and/or heavy lifting requires propulsion systems that will work better in those flight regimes, such as turbojet and turbofan systems.

There are exceptions (jet fighters and bombers that have to operate from ground level to 45,000 feet and the B-36 mentioned - which required engine turbosuperchargers on the recips to "breathe" at cruise altitude). As an aside, the B-36 also had an unfortunate tendency to catch fire due to the location of the engine's carb and a magnesium-alloy motor mount in the early models; jet fuel is MUCH less volatile compared to avgas, too.

__________________
"Any technology, no matter how primitive, is magic to those who don't understand it." - Florence Ambrose