# Lifting Tails?

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#### StRaNgEdAyS

##### Well-Known Member
That cozyjet looks fantastic!

#### pylon500

##### Well-Known Member
http://www.bluemountainavionics.com/greg/aircraft.html
WOW
Still, with the fuel burn that little baby will give you, I'd climb it to 20,00ft !, you can glide a lot further from there once empty!! :gig:
Just a thought though, are those inlets big enough?, and how 'smooth' is the air going in to them?
Arthur.

#### orion

##### R.I.P.
Just a quick correction on a couple of items above:

Although a jet engine will produce a relatively constant level of thrust to high speed, if you look at the perfomrance curves of most jet engines, the thrust level actually does drop off as the aircraft increases its speed. The drop is of course nothing near that which we see with a prop but it does drop.

Also, the thrust is very much a function of altitude. I don't know if the author above mis-spoke but the gist I was getting from the post is that the engine is somehow delivering more proportional power at altitude than a prop. This is not the case. Jet engine thrust levels are proportional to air density, just as is the power level of a recip.

This is actually the principal reason we have such large turbines installed on most turboprop airplanes. To get a specific altitude cruise, you need a specific amount of power and prop efficiency. However, when you install an engine that delivers the necessary power that you need at 30,000 feet, at sea level it generally seems like a massive case of over-kill (makes for great climb-outs though).

This is why a turbocharged recip can actually maintain more power to altitude than a turbine of the same sea level power. The turbine's power drops off while the recip's can be boosted to normallize to sea level MP, well above 20,000 feet. (And no, you don't turbocharge a turbine.)

There is also a bit of questionable information on the jet Cozy page. If your 300 horsepower Mazda engine installation only generates 300 pounds of take-off thrust, then you've got the wrong prop or your engine is putting out nowhere near 300 hp. Even with a mediocre prop, a 300 hp engine should be putting out about 800 pounds of thrust, and with a good constant speed prop it should be well over 900 pounds.

Finally, while the turbine Cozy will make great holes in the sky, my main question would be regarding the usefullness of the installtion. The converted engine has an SFC at sea level of about 1.1, give or take a bit. So, for 850 pounds of thrust, it consumes 935 pounds of kerosene per hour at low altitude - this translates to just short of about 150 gal. per hour! Talk about a toilet bowl for a carburator!

The only way to fly then is to get as high as possible as fast as possible. At cruise the thrust will go down with altitude and thus so will the fuel burn. So, assuming you get to about 20,000 feet, the density ratio is .53, and so you will generate about 450 pounds of thrust at full throttle. To extend the life of the engine, you will most likely throttle back. How much you throttle back depends on the airplane and the engine - some turbines do not like to cruise throttled back too far. For this discussion though, let's say you reduce power back to about 80% - that puts you at about 360 pounds of thrust.

For fuel flow, you will most likely find that the SFC actually goes up with the altitude gain, and goes up some more when you throttle back. Here I'm not sure where the numbers will go under these cnditions but historically for turbojet engines I've seen the SFC range from about 1.5 to nearly 2.0 at altitude and at partial throttle. Let's say then it's about 1.5. As such, you are now burning about 540 pounds per hour, which is about 85 gallons per hour. What this all translates to is that now your Cozy needs to be a flying fuel tank, or it will be an airplane that never really gets too far from your home airport.

Finally, in making this type of a conversion, I wonder if they've addressed the structural and aerodynamic effects? There are too many examples of this type of conversion where the individuals doing the work did nothing but install the engine. Since the airplane was not designed for the new performance envelope, the thrill of the jet install tends to be cut very short due to the airplane's and/or airframe's inability to handle the resulting changed operating envelope. Not good.

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#### mtorzews

##### Member
Orion,

I think Greg Richter put the jet on his Cozy mostly because he wanted something different and he could. Being different and being the first is important to some people. I don't think range was a big concern of his.

Also I don't think he has ever posted details of his performance. If it were mine and the performance was good, I would be itching to brag to everyone. He hasn't seemed to do this. Its very possible that the performance after the modification wasn't as expected for the reasons you stated. But I sure don't know.

I agree with you on your note about 300 Hp on the rotary and 300 lbs of thrust. First most rotaries will not produce 300 Hp. Most applications get around 200 HP or less. Second even at 200 Hp thrust should be higher than 300 lbs. Maybe Greg had a slip of the fingers when typing.

Anyways there definetly isn't a rush of people wanting to copy him. The cozy design is a ecomincal long range cruiser. Not a thrilling 1 hour rocket that his jet cozy tried to be.

I think its interesting to see what others attempt and experiment with, and thought others would enjoy it too.

Us canard enthuisiats are not trying to say that the canard design is the ultimate design. Yes there are others designs that can and do surpass its performance, but the don't do so at the same cost. The point we try to make is the canard is the best COMPROMISE of cost, speed, range, contruction technique, seats, and payload for us.

Our choice is a composite 4 seater that cruises near 200 Knots, has a range of 1000 Nm, and can be flown for under $40,000. Now if there were a different design with all of these characteristics plus allows one to bring a couple of bags along with four people then we would run for the new design. #### StRaNgEdAyS ##### Well-Known Member Orion noted: Finally, while the turbine Cozy will make great holes in the sky, my main question would be regarding the usefullness of the installtion. The converted engine has an SFC at sea level of about 1.1, give or take a bit. So, for 850 pounds of thrust, it consumes 935 pounds of kerosene per hour at low altitude - this translates to just short of about 150 gal. per hour! Talk about a toilet bowl for a carburator! This seems to be the BIG trade off for those of us wanting to extract the highest posssible performance. For me, I'm happy to trade off duration for performance, but then in order to achieve my goals, I have to deal with a phenominal 3lb/lb/hr fuel burn. Work THAT out at 610Lbs thrust and...... :ermm:... I think I better buy more shares in Ampol... :gig: Then we Aussies are never truly happy unless we do at least 6 impossible things before breakfast. (But we save walking on water for the truly special occasions. : ) #### Largeprime ##### Well-Known Member I guess the canadians have you beat there We used to walk on water FOR our breakfast. #### Midniteoyl ##### Well-Known Member #### mtorzews ##### Member The express planes are a great plane, but the cost to fly is much higher than the plans builts canards. (Although similar to kit built canards) Now if the express could be built from plans then it may be an option. #### malbo downunder ##### Member 4-place cmposite alternative to Cozy To quote mtorzews,- "our choice is a 4 seater that cruises near 200kts, range 1,000m, under$40G, etc, etc".
I love the Cozy but there IS an alternative,- go to www.visionaircraft.com then to "Builder's web Pages", & look at the 4-seaters under construction.
Not wanting to put down the Cozy in any way but I reckon the more choices we have the better & we should consider them all before comitting to any.
Just a thought.

#### BDD

##### Well-Known Member
I would avoid the lifting tail idea because it leads away from positive stability. Most tails have negative lift to counteract the pitching moment of most positively cambered airfoils. They do this to stabilize the plane.

A lifting tail is becoming more like a tandem wing design and they have less pitch stability than a conventional layout. The tail would also have to be redesigned to be larger to carry however much of the load you want it to carry. The main point though, is that when the tail becomes a lifting one....the stable concept of the original layout has been completely changed.

It does appear that the canard is an extreme morphing of the lifting tail concept. Extreme because the c.g has been moved far back towards the tail which is now the main wing in fact. The original concept has again been completely changed so that the front lifting surface is now an elevator and is at more positive angle of attack than the main (now rear) wing is. It's been morphed into a completely different concept and relies on a completely different set of relations between lift curve, etc. for it's stability.

It's interesting to look at these things this way though. Gives a new perspective and relates the main different layouts.

All things occur on a continuum. The acorn is the Oak tree but in a different form.

#### rtfm

##### Well-Known Member
Hi guys,
I'm coming to this (now dead) discussion very late - but it's all part of my due diligence in researching the archives of this great forum. So, if anyone has time to answer a question on this subject, I'd appreciate it.

If my understanding is correct, one of the reasons the H-Stab has to have a negative lift is to counteract the negative pitching moment of the wing (and presumably, of the body of the aircraft also). And as the airspeed increases, so does the pitching moment of the wing, and correspondingly, the negative pitching requirements of the tail.

It seems logical, therefore, that if one uses a very low pitching moment airfoil (eg: NACA747a315) it would be possible to reduce the drag requirements of the tail. And if one used a similar airfoil on the tail, it might be possible to arrive at a really low drag solution because of the inherently low grag/high lift of the airfoil. AND if one made the tail incidence variable...

Regards,
Duncan

#### rtfm

##### Well-Known Member
Sorry guys, but I'm on a roll at the moment...

If drag is an issue with a large tail, and if being close coupled is producing a twitchy aircraft in pitch - why not draw a design cue from the dinosaurs? Rhamphorhynchus in particular? Add a long tail extension. Due to the long moment arm, the actual tail surfaces need be very small - as evidenced by the flying dinos.

It might look a bit weird, but hey - who cares?

Regards,
Duncan

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#### pwood66889

##### Well-Known Member
There are wing airfoils with smaller coefficients of moment, RTFM. Harry Riblett designed a flock of them - laminar and turbulent flow. I can get the contact information if you are inteerested.
And there is a concept in aero design called Tail Volume. It is the area of the control surface (horizonal or verticle stab) times the length from the 1/4 chord of the surface to the 1/4 chord of the wing. There is an accepted range of figures if I recall correctly.
Percy in NM, USA

#### rtfm

##### Well-Known Member
G'day,
Yanking your chain a bit here - but would you say the tail extension of the Rhamphorhynchus fits into the acceptable range?

Cheers,
Duncan

#### BDD

##### Well-Known Member
Reducing the wing pitching moment would reduce the required tail volume and therefore the horizontal stabilizer area and/ or distance of the tail from the wing a.c.

Unfortunately, it also seems to me that most airfoils with high lift also have high camber and high pitching moment.

A rare exception to this is, I believe, the NACA 23015 which at certain Reynolds numbers has an excellent coefficient of lift with a very low, almost zero, negative pitching moment. The 23015 though has a sharp stall but at lower Reynolds numbers that might be very much improved.

I would think that a plane with a wing like this would also stay in trim better at different speeds and what tail you do have would be that much more effective in producing pitch stability since it is not also having to constantly keep the wing from pitching downward.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
If what you are attempting to reduce is drag, and you recognize the important issue of drag beng the composite of the wing, the tail, and the fuselage, you have made a lot of the trip. When you lengthen the tail, the down force to offset the pitching moments is made smaller, but you still have add in the skin drag of the long fuselage. And then it shifts your CG aft while not really shifting your neutral point. So you get to deal with the compromises in airplane design.

Now, once you consider that two tsails can give the same meoment with different lengths, you have one other feature: Damping. Tail moment goes with tail volume (area time arms length has units of L^3, which is volume), and tail damping goes with tail area times arm squared. So, the longer fuelage with the smaller tail may not only be less draggy, it will have more damping , which means that it will usually cause the airplane to settle down sooner after a disturbance, which is also good.

Now why a bug or bird or dinosaur needed a high tail volume, I do not get. Almost all of our flying species are statically unstable, being stabilized by the beastie's nervous system, with sensors and many control inputs available... There were a number of flying dinosaurs that were apparently statically unstable, yet they had highly developed wings, so they were apparently flying species. I wonder if these slender tail booms and tiny tails were actually secondary to flying, and had more to do with signaling within the species, attracting food/prey, or whatever.

Sorry, my folks were both biologists, and my father an airplane nut as well.

Anyway, rtfm, you keep up the chase,a dn the bigger picture will come into better focus along the way.

Billski

#### BDD

##### Well-Known Member
For the long tail examle, the longer the tail, the heavier the plane and the more it would tend to become tail heavy.

A tail boom (similar to the flying reptile example) would be the lightest way to do this and it would result in a smaller tail being required but the longer tail would also tend to deflect more and would become more flexible (would tend to be vibrated by the engine, etc.) so there would be a reasonable limit to tail length.

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#### rtfm

##### Well-Known Member
HeHeHe,
I couldn't resist it.

Cheers for the inputs, however.

Duncan

#### Midniteoyl

##### Well-Known Member
There are long tail booms out there, but not really on high performance planes...besides, with a tail boom like above, you'd need a wheel on it so it didnt rub on the flare... could see it now, 'wheelie landings'...

#### Topaz

##### Super Moderator
Staff member
Log Member
You see long tail booms and smaller tails on airplanes where trim drag and the induced drag of the tail are critical - in short, sailplanes.

The smaller tail lifts 'down' less than a bigger one on a shorter arm, so there is less 'extra' load added to the wing's required lift. That extra load results in more induced drag from the wing, and also induced drag from the tail itself. For most aircraft, the benefit isn't worth the extra penalty of longer landing gear and more difficult storage from the increased overall size.