# experiamental fighter jet style aircraft

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#### DangerZone

##### Well-Known Member
HBA Supporter
Scimitar blades are efficient, in part, because they are quiet. A new C-130 is almost silent from a surprisingly short distance if it's taxiing directly toward or away from you. When I was in the service, there was a designated smoking area barely fifty feet off the wing tip. The old paddle blades would make you abandon your cigarette and go inside if you didn't have over-ear protection. After the switch to the J model, we could smoke and carry on a conversation just by raising our voices.

This is the reason the un-ducted fans being studied by industry have scimitar shape. Noise abatement is serious business in the commercial fleet.
Excellent point, the more blades there are the more noise the prop/fan produces. By using the appropriately shaped scimitar blades the noise level goes drastically down yet it is still considerable when compared to those hubs with a smaller number of blades. A typical prop has two, three or four blades, at most 5 or 6. A typical fan has from 9 to several times more blades, the more blades the more noise. So if the fan blades are straight the noise really becomes a problem at these rotational speeds. Yet when the shape of the blades are scimitar, curved feather or other shapes the noise level can be reduced quite a lot.

The tip of the blade travels at a higher speed than the middle or the root. By designing the tip accordingly to the speed and the angles of attack most appropriate for that use the noise level and efficiency can be adjusted precisely. It makes sense, a typical efficient airfoil for low speeds can be thick and swept forward for good efficiency, so is the root of the scimitar blade. A typical airfoil for medium subsonic speeds is straight and slightly laminar, so is the middle of the scimitar blades. A typical high subsonic speed airfoil is thin and swept back, so is the tip of the scimitar blade. So if designed properly the blades can contribute to both better efficiency and less noise production.

So you are right, it is not just noise reduction, it is also efficiency improvement that is a bit better. I've seen a blade manufacturer at a fair advertise their shape on a smaller prop model, a good straight prop blade was producing around 2kg of thrust per HP while a scimitar shaped one was making more than 2.5kg. Which is quite a significant difference and it would surely affect range and fuel consumption. Using such an unducted fan for propulsion of a small fighter style experimental homebuilt 'jet' aircraft could assure better range and fuel economy at the sacrifice of speed. If anyone has more ideas or thoughts on this subject it would be nice to read them here...

#### DangerZone

##### Well-Known Member
HBA Supporter
If you roughly double the torque the engine produces (otherwise it won't make that power at 6000 rpm), it'll break down in minutes. These are drag racing settings, not to mention that the fuel consumption will be appropriate for a drag racing engine too..

Not at all. Boxers are used for one primary reason in motorcycles; a center of gravity that's a lot lower than any other type of engine, though the weight is indeed slightly higher. Boxers stopped being troublesome about 4 decades ago...
These are sportbike engines and their torque is adjusted to the need, they do not have to haul 10 ton trucks so all excess torque would be unnecessary.

The GSXR1100 and the Busa on the other hand were/are reliable even after overpowering them, which to me as someone who needs BOTH an efficent AND reliable engine is a smarter choice. If you find a more reliable and powerful engine at up to 70kg weight please be so kind to point out in that direction, I'd be very thankful.

##### Well-Known Member
These are sportbike engines and their torque is adjusted to the need, they do not have to haul 10 ton trucks so all excess torque would be unnecessary.

The GSXR1100 and the Busa on the other hand were/are reliable even after overpowering them, which to me as someone who needs BOTH an efficent AND reliable engine is a smarter choice. If you find a more reliable and powerful engine at up to 70kg weight please be so kind to point out in that direction, I'd be very thankful.
Good to know that they (BMW) are just a bunch of idiots earning on a hyped overmarketed product, as are mates of me that race them, or myself for that matter. I guess they would do well to taste a bit of your infinite wisdom and real-world experience. Why don't you apply as new chief engineer for them, or start teaching us how to really race a motorcycle?

#### Detego

##### Well-Known Member
... the engine is set HIGHER, which improved handling of the bikes at track.
... A lower center of gravity is not needed, it is even counter productive.
... the best place to put the engine is at the right place where gyroscopic moments and torque occur, not in the lowest center of gravity.
... the reasons boxer motors are obsolete for racing, they cannot match the competition

"The boxer engine was first patented by German engineer Karl Benz."
http://www.life123.com/cars-vehicles/repair-troubleshooting/car-engines/boxer-engine.shtml

Engine Comparison | Subaru Boxer Engine

SRT USA Wins 2012 Rally America National Championship at NEFR

Subaru Rally Team USA Wins X Games Rally Medal

All-Wheel Drive; 170-hp SUBARU BOXER engine.

#### DangerZone

##### Well-Known Member
HBA Supporter
Good to know that they (BMW) are just a bunch of idiots earning on a hyped overmarketed product, as are mates of me that race them, or myself for that matter. I guess they would do well to taste a bit of your infinite wisdom and real-world experience. Why don't you apply as new chief engineer for them, or start teaching us how to really race a motorcycle?
Is there a point calling BMW, your mates or yourself idiots? Frankly, there's no need to teach them infinite wisdom, they taught it themselves: the last motorbike engine that BMW produced (S1000RR) for racing is NOT A BOXER but an in line four.

"The boxer engine was first patented by German engineer Karl Benz."
http://www.life123.com/cars-vehicles/repair-troubleshooting/car-engines/boxer-engine.shtml

View attachment 21434
Engine Comparison | Subaru Boxer Engine

SRT USA Wins 2012 Rally America National Championship at NEFR

View attachment 21435

Subaru Rally Team USA Wins X Games Rally Medal

All-Wheel Drive; 170-hp SUBARU BOXER engine.
Are you sure you want to say that these Subarus lean into curves as motorcycles do? Don't you think that there are quite a few differences between motorcycle CGs and car CGs..?

Interesting is the fact that before Karl Benz's engine there were electric cars. A lower CG could be obtained by simply installing the batteries bellow the axles, if that would be necessary. In fact, the first car to breach the 100km/h limit was an electric car, La jamais contente from Belgium. It achieved a record at 105km/h in 1899 which no other gasoline engine could at the time. Even today, more than a hundred years later, electric cars can outperform ICE cars easily. Yet we don't see many of them on the tracks, right? And speaking of endurance and longevity internal combustion engines are nowhere near electric motor vehicles. So please correct me if I did not understand your point well, did you want to say that the boxer engine is better than all other electric motors since there aren't any electric winning the Rally America National Championship. Or that Subaru's boxer engine is better than all other internal combustion engines because it has a lower CG?

The biggest problem would then be this: if the lower CG in a boxer engine is THAT important then all Rotaxes, Lycomings, Continentals and other boxer engines would suffer a lot because they are nowhere near the CG of an aircraft. And BMW would not produce a high tech motorcycle engine that is in a four line but would stick to the good ole boxer concept. Hell, they would even keep producing the boxers in the seventies and would not go to in line fours, sixes and V8 engines that they use today. Right?

Guys, I don't get you. Why are you always trying to fight someones words on the forum and drift off topic, isn't it becoming tiresome? Quite frankly, would it not be nicer to discuss the topic and bring ideas together about homebuilt experimental jet style aircraft? Because that subject is way more interesting than whose is bigger, the Beemer's or the Subaru's. Engine, of course, not the IQ.

So I quit. You're right, the boxer engine rules, yeeey, meeee liiike it long tiiiime. Could we cut the crap now and get back to the topic from the subject of the thread?

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##### Well-Known Member
Guys, I don't get you. Why are you always trying to fight someones words on the forum and drift off topic, isn't it becoming tiresome?
Because not everyone has the knowledge or experience to see some statements for the blathering BS they are. Having a "counter opinion" at least gives them the opportunity to judge both points of view and draw their own conclusions.

But indeed, back OT now.

#### DangerZone

##### Well-Known Member
HBA Supporter
Because not everyone has the knowledge or experience to see some statements for the blathering BS they are. Having a "counter opinion" at least gives them the opportunity to judge both points of view and draw their own conclusions.

But indeed, back OT now.
Very true. Yet there's a flip side to that coin. You see, I have a V8 BMW in my driveway for more than 9 years now with 350k km and I know exactly all the good sides and the bad sides of it, it was me who had to pay for all the repairs and maintenance during all these years. Not someone else's opinions, the company or god knows who, it is my money which kept the car running. And kept the company running. An ocean is made of tiny little drops of water, my money also contributed as a drop of water. So I will not bull**** that it is the best car in the world just because I drive it, or just because the company is fancy or prestigious. Even though some admire such a car with awe. The car is good, period. Not the best, not the worst. Yet I will certainly not close my eyes on all the repairs/problems I went through with it all these years. And I would never put it in an aircraft for flying. While a GSXR1100 engine modified for aircraft use, that I would. I could rely on that engine a lot more for safe flying. You can call it an opinion or experience, I call it 'my money'. And when I sum it all up through all these years and all the models I had, it comes to a nice bucket of water. A bit more than expected from many high end product companies in all those years I guess. So if someone wants to learn from other peoples' experience (my experience) to save a bit of money, then good for them.

Back OT, what do you think about homebuilt experimental fighter jet like aircraft? Any preference, ideas, which airplane would be closest to your taste of flying and why? Any opinions, thoughts or knowledge on the BD-10 or the ViperJet?

#### Detego

##### Well-Known Member
1.) you want to say that these Subarus lean into curves as motorcycles do?

2.) before Karl Benz's engine there were electric cars.

3.) Subaru's boxer engine is better than all other internal combustion engines because it has a lower CG?

4.) if the lower CG in a boxer engine is THAT important .. they are nowhere near the CG of an aircraft.

5.) Why are you always trying to fight someones words on the forum and drift off topic, isn't it becoming tiresome?

1.) YES, a car like a Motorcycles leans into curves and the less weight up top the better it handles in said curve.

2.) Were not discussing electric motors.

3.) YES, in a racing application keeping the weight lower in the frame/chassis, increases stability and speed in the turns.

4.) We have in aircraft design what we call the 'Thrust-Line of the Engine'. We must also blend the Engine Cowl with the Rest of the Fuselage.

5.) I've given you my opinion, if you don't like it I have others.

The question never asked is never answered: Why is it that the Japanese do not manufacture Boxer Engines for their Motorcycles?

o Royalties paid to Benz.

o The ability to lean the Bike Further into Turns; forgoing the improved C/G provided by the Boxer Engine (ground clearance).

o Their engine looks better; gives it that go fast look.

"Aerodynamics are for people who cannot build engines." - Enzo Ferrari

#### DangerZone

##### Well-Known Member
HBA Supporter
1.) YES, a car like a Motorcycles leans into curves and the less weight up top the better it handles in said curve.

2.) Were not discussing electric motors.

3.) YES, in a racing application keeping the weight lower in the frame/chassis, increases stability and speed in the turns.

4.) We have in aircraft design what we call the 'Thrust-Line of the Engine'. We must also blend the Engine Cowl with the Rest of the Fuselage.

5.) I've given you my opinion, if you don't like it I have others.

The question never asked is never answered: Why is it that the Japanese do not manufacture Boxer Engines for their Motorcycles?

o Royalties paid to Benz.

o The ability to lean the Bike Further into Turns; forgoing the improved C/G provided by the Boxer Engine (ground clearance).

o Their engine looks better; gives it that go fast look. View attachment 21438

"Aerodynamics are for people who cannot build engines." - Enzo Ferrari
1. Motorcycle lean into curves because they can. Cars cannot, with acting centrifugal and centripetal forces they need that lower CG mass positioning.
2. Completely true, we were discussing experimental fighter jet style aircraft. Until you and some others started adding Subarus and boxers into the equation.
3. Take at look at racing beyond the mentioned rally and you may find many other concepts of engines besides boxer ones. Winning.
4. Of course. Yet that will not change the fact that an engine in the aircraft does not have to be in the CG range.
5. Your opinions are great yet I miss the point of relevance to the topic.

Royalties to a patent seize to exist after 20 years because the concept becomes common good. Karl Benz will definitely not sue you if you produce a boxer engine, which existed even before his invention as a steam engine.
The ground clearance was achieved by Moto Guzzi years ago by positioning the opposed pistons in a V so the clearance allowed leaning into curves. Yet then people discovered that there is a thing called physics which introduced them to the gyroscopic moment and homokinetic rotation forces.

Guys, I give up. Have your EXPERIMENTAL FIGHTER JET STYLE AIRCRAFT thread and feel free to blast it with off topic opinions. I think there are better ways to use precious time when interested in experimental progress. Have fun and good day.

#### djschwartz

##### Well-Known Member
Back OT, what do you think about homebuilt experimental fighter jet like aircraft? Any preference, ideas, which airplane would be closest to your taste of flying and why? Any opinions, thoughts or knowledge on the BD-10 or the ViperJet?
The BD10 was a very bad design. Came apart in flight and killed the test pilot. Jim Bede has a long reputation of over-hyping and under developing his designs. Some of his designs, like the BD4, are OK but never lived up to the claims he made. In the end I think he began to believe his own hype about what a great designer he was and that led to the 10 and its flaws. The airplane was simply under designed with far too many short-cuts taken to try to get it done with the effort and money he had available. Designing a fighter-like jet is a very complex and challenging task. It takes a team of experienced engineers and a huge budget. Without that not only are the chances of success low, but the chances of killing some one are high.

The Viper also did not live up to its initial expectations. Major design changes were required between the first and second prototypes. It's not as fast, costs far more, and burns more fuel then was claimed by the designers initially. That's been true of all of the attempts at a homebuilt or personal jet and is the reason you don't see very many of them flying around. As has been said, you can buy a jet trainer like the L-29 or L-39 for far less money than it would cost to design and build something that even approached their performance.

#### Topaz

##### Super Moderator
Staff member
Log Member
Does someone have some study data about propfan? Would be cooler on cold jet than a prop. I have been reading just some non-technical mambo-jambo about the topic which has concentrated mostly on noise and about the turbine engine that turns the propfan rather than actual interesting topics. I am not interested in the turbine nor noise, but would like to see some factual numbers if compared to traditional prop on low to medium subsonic speeds (not reaching even nearly transsonic). I have been doubting that propfan would be (much) less efficient at low speed (from 80 to 300 mph) than larger traditional propeller, but very much would like to be proven wrong on that. If the speed this propfan is not a problem, just a design requirement for the electric motor number of poles and wire length. But that could be a separate topic.
Heck of a design challenge, from what little I've read on the subject. Blade shape tailored, multiple and non-linear spanwise sweep angles tailored to the local Mach value at design speed and RPM, highly tailored twist distributions along the span, very thin blades (structural issue), and so on. And then most of them are variable pitch, too. Not impossible, and I understand that if you don't care about noise, the problem becomes much easier, relatively speaking. Efficient and quiet still baffles the aerospace primes. Not something I think a homebuilder could resolve.

Jan would probably know more on the subject. Looks like one of those "big team, lots of calculation, modeling, and then crossing your fingers during the tests" sort of things.

#### Topaz

##### Super Moderator
Staff member
Log Member
...Back OT, what do you think about homebuilt experimental fighter jet like aircraft? Any preference, ideas, which airplane would be closest to your taste of flying and why? Any opinions, thoughts or knowledge on the BD-10 or the ViperJet?
I think at one time or another, every single one of us has dreamed of such an aircraft. But the realities of such a project are completely daunting, when you look at them. This is orders of magnitude harder than something like a Glassair or RV-x aircraft. And orders of magnitude more expensive. Our own late Orion (who certainly had the industry experience to speak with knowledge and authority on the subject) estimated that developing and building such an aircraft would require funding well into the seven-figure range, and I don't doubt him on that. ViperJet and Javelin (ATG) have both spent, individually, several million dollars on their respective projects. ATG, declared bankruptcy in 2008 after failing to obtain $200 million in financing to fund further development. Are you really prepared to invest that kind of funding in a project like this? As for the BD-10, my understanding is that one of the crashes occurred when the flap drive partially failed on approach, driving one flap down and not the other. The ailerons were not powerful enough to overcome the resulting roll moment. I don't know about the other crash(es?), but the aircraft had an abysmal record in the flight test program. AFAIK, none are still flying. The whole project (another multi-million-dollar effort) seemed a bit slap-dash and underdeveloped. Jim Bede is a clever aircraft designer (but a lousy businessman and a worse marketing man), but in this case, it seems he was probably in over his head. #### jlknolla ##### Well-Known Member Heck of a design challenge, from what little I've read on the subject. Blade shape tailored, multiple and non-linear spanwise sweep angles tailored to the local Mach value at design speed and RPM, highly tailored twist distributions along the span, very thin blades (structural issue), and so on. And then most of them are variable pitch, too. Not impossible, and I understand that if you don't care about noise, the problem becomes much easier, relatively speaking. Efficient and quiet still baffles the aerospace primes. Not something I think a homebuilder could resolve. Jan would probably know more on the subject. Looks like one of those "big team, lots of calculation, modeling, and then crossing your fingers during the tests" sort of things. If there were performance gains to be had, IF they could be had economically AND reliably, AND if there were not concerns about public acceptance, UDF's would already be the norm. It is a massive design challenge to get right, and several huge and well funded/deep pocket OEM's have looked at it but the technology in practice does not meet the need in the right ratio of performance/acceptance/cost/complexity. At 402 KTAS, the Piaggio Avanti is the fastest production propellor driven airplane on the planet, faster in fact than many light jets. It is also hands down the most efficient/'green' business aircraft ever conceived, and from a propulsion standpoint it is a basic garden-variety PT-6/Hartzell setup, albeit in a pusher orientation. As a turboprop, it is also significantly more efficient than any jet, although best economy is achieved at a more pedestrian 320 or so KTAS. For my money, a basic turboprop is still the most efficient propulsion setup for most common weights, and to Karoliina's question, that suggests a slightly exotic prop (maybe 4-7 blades depending on solidity needed to absorb power and allowable prop dimensions) would be a better choice than something like a UDF that adds a whole new design/integration/aerodynamic effort on top of say an electric motor and the requisite storage and control systems. #### jlknolla ##### Well-Known Member I actually have met and done business with Bede on a couple projects which occurred after the loss of the Falcon CEO and testbed, and I am friends with an engineer who supported the BD-10 program in St Louis - have had many intersting conversations about that particular accident. Without going into details here, there is a lot of misinformation about the program, part of which Topaz corrected with respect to the flap failure - the loss of the Falcon aircraft is an interesting story and I will try and pull together what I can and post it if I can figure out how to write it up without violating any info shared in confidence. I'll also second Topaz's observation of Jim, very clever designer/engineer, but not a particularly good businessman. And yes, I was eventually paid for all the work I did (I consider myself quite lucky). #### DangerZone ##### Well-Known Member HBA Supporter The BD10 was a very bad design. Came apart in flight and killed the test pilot. Jim Bede has a long reputation of over-hyping and under developing his designs. Some of his designs, like the BD4, are OK but never lived up to the claims he made. In the end I think he began to believe his own hype about what a great designer he was and that led to the 10 and its flaws. The airplane was simply under designed with far too many short-cuts taken to try to get it done with the effort and money he had available. Designing a fighter-like jet is a very complex and challenging task. It takes a team of experienced engineers and a huge budget. Without that not only are the chances of success low, but the chances of killing some one are high. The Viper also did not live up to its initial expectations. Major design changes were required between the first and second prototypes. It's not as fast, costs far more, and burns more fuel then was claimed by the designers initially. That's been true of all of the attempts at a homebuilt or personal jet and is the reason you don't see very many of them flying around. As has been said, you can buy a jet trainer like the L-29 or L-39 for far less money than it would cost to design and build something that even approached their performance. Yeah, you're right, the structure of the BD-10 was not adequate to the needs. And it did kill the test pilot. But didn't many other good concepts with bad construction do that also in the past...? The BD was a brilliant idea and concept yet the construction and engineering was problematic and had quite a few structural design flaws, lethal ones. When it comes to experimental aircraft we have to be aware of the fact that many good concepts started that way. Take a look at the Ambrosini SS4 which was built by Sergio Stefanuti in the '30s of last century. A great concept, a brilliant idea at the time, yet the test pilot was also killed during initial testing. Based on this airplane the Japanese built the Kyushu Shinden, the Americans built the Curtiss Wright Ascender adn Burt Rutan built the VariViggen/EZ. So it could be just a question of time until someone redesigns the concept and builds a better prototype. In fact, isn't the ViperJet just an upragde of the BD-10 idea with many of the flaws resolved and built the proper way? My point is, if the construction or implementation of an idea is bad that does not have to mean the concept is bad, as a concept per se. In time, someone might improve all the bad design flaws, or even completely redesign the concept. The BD-10 had just too many issues, but the idea of a private jet fighter weighting bellow one ton and flying at more than 1Mach seems like a feasible one with the technology that we have today. I think at one time or another, every single one of us has dreamed of such an aircraft. But the realities of such a project are completely daunting, when you look at them. This is orders of magnitude harder than something like a Glassair or RV-x aircraft. And orders of magnitude more expensive. Our own late Orion (who certainly had the industry experience to speak with knowledge and authority on the subject) estimated that developing and building such an aircraft would require funding well into the seven-figure range, and I don't doubt him on that. ViperJet and Javelin (ATG) have both spent, individually, several million dollars on their respective projects. ATG, declared bankruptcy in 2008 after failing to obtain$200 million in financing to fund further development.

Are you really prepared to invest that kind of funding in a project like this?

As for the BD-10, my understanding is that one of the crashes occurred when the flap drive partially failed on approach, driving one flap down and not the other. The ailerons were not powerful enough to overcome the resulting roll moment. I don't know about the other crash(es?), but the aircraft had an abysmal record in the flight test program. AFAIK, none are still flying. The whole project (another multi-million-dollar effort) seemed a bit slap-dash and underdeveloped. Jim Bede is a clever aircraft designer (but a lousy businessman and a worse marketing man), but in this case, it seems he was probably in over his head.
I agree, I bet many of us had the same dream at some point in our life. And I am neither prepared nor willing to invest millions in such a project, because I haven't got them. And even if I had them there is a question whether it would be worth it because money invested would never be returned. So from that perspective I guess only some ministries of defense or drug lords would profit from such a concept.

From what I read one of the BD-10s had a tail flutter problem which broke in flight and another had this issue that you mentioned. Among other things, of course, it seems there was a mixture of design solutions to get a compromise. Millions wasted on a product that did not live to it's expectations. At the same time there were people who wanted to achieve the same with less money, take for example the Smitty Hairplane and the JetHawk II. Both were failures not due to lack of money but to basic understanding of the ducted fan and shrouded principles.

The Fantrainer had some results in the seventies and later on as a good product yet it is more of a shrouded prop than a ducted fan. The conclusion is that many have the same concept yet so far not many (if any) have achieved to have a very fast airplane that is not too thirsty fuel-wise. In other words, the concept and idea are good yet the construction and production are the problem.

I actually have met and done business with Bede on a couple projects which occurred after the loss of the Falcon CEO and testbed, and I am friends with an engineer who supported the BD-10 program in St Louis - have had many intersting conversations about that particular accident.

Without going into details here, there is a lot of misinformation about the program, part of which Topaz corrected with respect to the flap failure - the loss of the Falcon aircraft is an interesting story and I will try and pull together what I can and post it if I can figure out how to write it up without violating any info shared in confidence.

I'll also second Topaz's observation of Jim, very clever designer/engineer, but not a particularly good businessman. And yes, I was eventually paid for all the work I did (I consider myself quite lucky).
Such a story would be very interesting, along with the details why they chose such wing and tail airfoil profiles. It seems they envisaged the airplane would be cruising at transonic speed or above and sacrificed good properties at lower speeds with such an approach. Yet the airplanes was flying at lower than transonic speeds, it makes no sense. I've seen the BD-10 presentation video and your insight would demistify a lot of things. Cause from a bystander's point of view, there definitely seems to be too many mistakes for such a high cost project.

#### karoliina.t.salminen

##### Well-Known Member
I was just reading about SubSonex. Seen articles about it before many times, but now checked out some videos too. It seems the cost of PBS TJ100 is almost reasonable. Poor efficiency and poor thrust from one engine, but composite a single seater of size of SubSonex could be an interesting concept, just for fun. not to travel somewhere with the machine, but just for local fun flights. Would fit in similar category as my KTM 250 EXC-F which is for only riding enduro trails or mx track for fun or for racing or both, and not for traveling anywhere further (on road) than to get to this said trail or motocross track because of the very frequent maintenance and overhaul periods. But I saw the light of justification for investing in this kind of bike despite it is highly unpractical for anything else. Similarly a little jet, just for flying [jet aerobatics] for fun at lowish altitude something like 30 minutes at a time, would not be so bad idea when all traveling is taken out of the equation. That 30-60 minutes will cost the same as getting to 1000 km away with a Diamond due to the very poor efficiency of the turbojet engine. but a lot of people fly for not getting somewhere but just for fun locally, and shorter flight time if it was rewarding enough could be worth it. And it is better to burn a lots of Jet-A with the price one could shoot to sky in fireworks in the new year eve in one blow - imo, better price-what you get ratio to burn it (the money) as Jet-A in a jet engine. The jet should be very small and lightweight to offer awesome power to weight ratio. I am suspecting though that two of these jet engines could be needed of have more jet-like performance (that would be usually expected from a jet) allthough this would double the fuel cost and halve the endurance, as this 44 gallons per hour would be a quite steep figure, especially for the "get nowhere" little single seater.

#### Toobuilder

##### Well-Known Member
Log Member
Jets are hard enough to justify in the best case, but to take all practicality out of it and fly it as a simple "fun" machine for local flights would really be a limited market, I would think. Using something like the sub Sonex for example, many airplanes have a higher T/W ratio and would fly circles around it. The only real appeal then, is the jet engine. Seems like that would only be fun until someone in a Harmon Rocket blows past you on half the fuel burn.

#### highspeed

##### Well-Known Member
And the Rocket will probably cost the same to build when all is said and done.

#### karoliina.t.salminen

##### Well-Known Member
Indeed, a fun jet would mean high thrust/weight ratio and light weight will help but also lots of thrust is required to get the ratio great. And then comes the price of the engine or engines.
And soon we arrive at a concept similar to Viperjet, engine costing something like half million and will carry 1000 kg fuel. The only remaining nice thing if the jet engine would be a low performance one pretty much is that the jet engine is smooth and cool - "it is a jet" after all. And there is no prop obstructing the view. And the engine is fully aerobatic, no problem to run it completely upside down or at negative G loadings as long as is needed.

I don't think jet engines should be used in personal airplanes that have intention to travel somewhere. Getting somewhere is more efficient at low subsonic speeds with using propeller. I would not have flown the trip I did with the Diamond using a Viperjet even if I would have had one. It would be quite odd to travel around the world with a plane like Viperjet. It would be certainly possible, but filling the fuel tank would cause the credit card balance to get used very quickly and it might prove to be tricky to transfer money from your bank account to your credit card account while on trip on the exotic places of the northern route. Such as in Kuujjuaq without internet, without cellular connectivity and credit card getting dried out already there (with a Viperjet or similar). You might get to sleep on the floor of the little catholic church's guesthouse, like we did (for other reason, our reason was that the other place to sleep was totally full and we were unlucky enough to not have packed a tent with us because we had plan to inflate the life raft to function as tent in case of emergency landing to e.g. Greenland icecap), when you could no longer afford the nearby roughly zero star motel grade place that calls itself a hotel and costs the price of a suite in quality hotel elsewhere. Or maybe the people that can afford a Viperjet have limitless creditcards that will ease the frequent expensive fuel fill-ups.

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