Saunders JetHawk...Any builders?

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
The problem is "the propulsion system". There are "shrouded propellers" which is essentially what the "Mass Flow" folks are advocating. The inlet system for those type fans is only good for low speed 100 mph etc. For a "true fan" like what's on a high bypass turbo-fan,...you need a diffusing inlet, that slows and compresses the air flow, through the right "capture area", to deliver the right flow, at the right "axial Mach number" to the fan. THE FAN IS THE BOSS of the propulsion system. The type of fan that Mass Flow encourages has a converging inlet and a diverging exhaust, exactly the opposite of what is needed for high performance fan powered sport plane. That type of inlet results in what's known as "Spillage drag" air regurgitates out the mouth of the inlet,...rippling, turbulating,...down the sides of the aircraft. Mass FLow even goes as far as to state the design is supposed to prevent the fan blades from going supersonic. For a shrouded prop,...that's true, not for a genuine fan. I worked at Pratt & Whitney and GE,...fans on military and civil aircraft exceed the speed of sound and then some. The "relative mach number" on a typical fan is .8 at the hub, 1.0 at the mid-span,...and 1.4 or greater at the tip. Now,...on a commercial engine you want to keep "the fans axial mach number" below 1.0, but that's not the point.
Anyway,...the guy who put the "Mass Flow" inspired fan on the back of a Long EZ,...lost performance for all the reasons I sited. The proof in the pudding. I wish this stuff could get straightened out, enthusiastic people,...hard working and well intended,..end up wasting enormous time and energy by taking the wrong advice...
Very interesting, I may be wrong but I was thinking the "Mass Flow" book published after the JetHawk failure indicated one of the problems with jet look-a-like ducted fan attempts was that they WERE trying to mimic turbofan inlets by using a diffuser inlet instead of a convergent inlet like they (I think) now propose.

I don't suppose you'd have any math/spreadsheets you'd be willing to share on basic duct design along these lines?
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
I hate to sound brash or abrupt,...but the Mass Flow guys are flat wrong period. They have experience with ducted props,..shrouded props etc. Dirigible(sp?) vectored thrust devices which are only "useful" at static to low air speed. You need a diffuser for a fan-jet designed to operate at any significant speed 300+ etc. Take a look at the A-10 Warthog. It has two high bypass turbo fans,..and flys at speeds of 300-400 mph most of the time. It has a standard normal shock type inlet like a commercial airplane fan does. The "Fan" has a design axial mach number that has to met to be at its design point. I.e. you have to "diffuse" in the inlet,..trade free stream velocity for static pressure rise,...then the fan compresses from there.

The Mass Flow guys are just dead wrong. NONE,....of the commercial airliner guys do that. THe rational they use is "you want to keep the fan blades sub-sonic",...well,..that's total crap. For decades now,...a high bypass turbofans have mach numbers of about .8 at the hub,...mach 1 at the mid-span,...and well supersonic at the tip,..typically 1.4. Now,..the "axial mach number" needs to stay subsonic on a commercial engine,...but the relative mach numbers on the fan blades (i.e what the blade sees),..are well into the supersonic regime.
 

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
The Mass Flow guys are just dead wrong. NONE,....of the commercial airliner guys do that. THe rational they use is "you want to keep the fan blades sub-sonic",...well,..that's total crap. For decades now,...a high bypass turbofans have mach numbers of about .8 at the hub,...mach 1 at the mid-span,...and well supersonic at the tip,..typically 1.4. Now,..the "axial mach number" needs to stay subsonic on a commercial engine,...but the relative mach numbers on the fan blades (i.e what the blade sees),..are well into the supersonic regime.
I'm no engineer by any means, but I was thinking the diffuser inlet on jets had more to do with manipulating the pressure of the air entering the engine core than anything to do with the fan.
 

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
Wow...I originally posted this query in Sept. 2005, and had 89 responses and 6,282 reads so far. My original question remains though. Has anyone actually built a JetHawk? Has anyone assisted a builder during construction of a JetHawk? Anyone ever see one fly or falter back to Earth?
It seems to me, my post has only stirred up a whirlpool of speculation and controversy rather than any useable insight or information.
looseround
I DO NOT know this for a fact. But supposedly the prototype flew once. It barley got off the ground. It was discovered later that more than likely only one of the rotors was making power, meaning it flew with 50% of it intended power. I don't have nor have I seen plans. But I've heard enough negative comments from knowledgeable folks who have to conclude you don't want to copy the airframe as per the plans.
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,752
Location
Orange County, California
... I don't have nor have I seen plans....
The company was only in business for a very brief time back in the late '80s. Long gone. Occasionally a plans set surfaces on eBay. I have to wonder if it's the same set over and over - someone buys it, goes on this or another forum, finds out what a disaster the airplane really is, and then puts it up on eBay... ad nauseum.

AFAIK, only the prototype ever flew. I don't recall hearing of any completed customer examples.
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
Actually,...."I read" somewhere,..mid-90s I think,...that several were started,...maybe 5 or 6,...reached various levels of completion and not flown,...exept one (other than the original). The guy flew it,....was very disappointed,...mainly cuz of the fan inefficiency. THe plane is a real floater,....almost a glider,...so it does not need much thrust to get off the ground. ANyway,...this one other guy it was claimed in the article,....took the fan out,...stuck some small turbojet in there and go performance that was "acceptable" using his words,....but fuel consumption was high of course. I have a set of plans,....and have used them for ideas,...cockpit sizing etc. But they are of very limited use.
 

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
I'd love to hear some more informed opinions on a convergent vs divergent inlet performance for a ducted fan.
 

orion

R.I.P.
Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
The one that was modified to jet was, i believe in the Portland, OR. area. The jets used were TRS-18 turbojets but if my info is right, instead of using actual flight engines, he used converted smoke generators. One of my contacts indicated that the build quality was very poor and apparently the project was never finished. Probably saved his life this way.
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
"I'd love to hear some more informed opinions on a convergent vs divergent inlet performance for a ducted fan."

I worked as a project Engineer at Pratt & Whitney for 5 years. Worked with some of the best compressor aerodynamics folks in the world. Learned streamline analysis from them etc. etc. ..maybe if you could be more specific with what you want to know??

What I can tell you is this. A true ducted fan,..i.e. one that has had proper a streamline analysis done on it,..its a low pressure (relatively speaking) compressor stage. Its designed to compress air and expand it through a nozzle at high velocity producing thrust. A shouded prop,..(The Saunders Jet Hawk) is not a fan/ compressor. Its just a crude flat bladed propeller with no stators. Its very "lossey" to say the least.

"IF" one is using a true fan/ compressor stage, it has a design point, optimal pressure ratio and flow rate. The fan has an axial mach number (at the fan face). The purpose of the inlet is to "maximize" inlet pressure recovery, deliver this to the fan, e.g. (at the design cruise mach number), with minimum drag. Just take a look at every single SUB-SONIC commercial or military aircraft,...they all have diffusing inlets. A converging inlet on a commercial airliner would be a disaster; huge spillage drag, reduced airspeed, excessive fuel consumption.

Now,...if you want to talk supersonic aircraft,...that's another kettle of fish entirely. They come in a variety of types. Normal shock inlets are the simplist,...and are used up to Mach 2. Mixed compression inlets are used above that. Planes like the F-15, F-14 have a sophisticated variable ramp system to control oblique shock position,...etc. The SR-71 has the teliscoping spike, along with bleed doors. They are used to control the shock at the throat of the inlet which is convergent,...then divergent. Takes supersonic air and compresses it outside the inlet,...then further compresses it inside the converging section,..(that is a supersonic diffuser by definition),..then,..the flow passes the throat where a standing normal shock exists,...after that,....further pressure rise is done in the last divergent section with is a conventional subsonic diffuser.

So,..there are many, many inlet diffusers,....for all flight regimes. For Subsonic flight, like an airliner or even the A-10 Warthog,...(which is pretty slow),...a simple diverging, diffuser is the best choice. Ram recovery can approach 100% with minimum complexity, cost and weight.

For shrouded propellers the converging inlet offers some advantages but mainly in the low airspeed regime.
 

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
..maybe if you could be more specific with what you want to know??
I'm interested in speeds up to about 250 KTS.

...A true ducted fan,..i.e. one that has had proper a streamline analysis done on it,..its a low pressure (relatively speaking) compressor stage. Its designed to compress air and expand it through a nozzle at high velocity producing thrust.
Is this because it's part of and overall engine, that is a "first stage" of a multi-stage compressor. In which case higher pressure ratios mean, if I'm not mistaken" better fuel economy of the overall engine?
Or is a low pressure compressor stage simply more efficient at producing thrust than a shrouded prop?

"IF" one is using a true fan/ compressor stage, it has a design point, optimal pressure ratio and flow rate.
Do I need a super computer and an engineering degree to do this? You have any math or words of wisdom on determining optimal pressure ratio?

So,..there are many, many inlet diffusers,....for all flight regimes. For Subsonic flight, like an airliner or even the A-10 Warthog,...(which is pretty slow),...a simple diverging, diffuser is the best choice. Ram recovery can approach 100% with minimum complexity, cost and weight.
Again any guidance on pointing me in the right direction in this regard would be very interesting and much appreciated.
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
Take these one at a time I guess.

1) 250 kts; there is one design on what is technically a shrouded prop,...I know the designed very well,...he ran it,...and got the thrust numbers he was predicting,...essentially,...it was a hoop with the blades centrifugally loaded. As I talked to him about it,....he said it was really closer to a "high disk loaded propeller),...unlike the stuff Mass Flow was pushing,...his has only a small hoop,...and is carefully designed to accelerate a particle continuously all the way to the fan exit. It had no stators. But "he" did it right. Its an elegantly simple inexpensive design. He said he study shows he could go 300+ mph,...in a small plane with 100 hp. So,..for 250 kts,..it would be an excellent choice. Unfortunately he never proceed past running it as a rig. The way to think of it,...a very stubby bladed well design prop,...close the aircraft fuselage.

2) On a high by-pass turbo-fan,...the gas turbine core is the HP generator for the fan. ANd yes,...the fan is the first "compression stage for the whole engine,..it feeds the core. A modern commercial engine gets 85-90% of its take off thrust from the fan. At cruise altitude it still gets 70+%, if fact,..the newer ones may well be a lot higher. SO,...for low noise,...best fuel consumption,...you want to flow as much air as possible through the fan, with the Pressure ratio the mission cruise speed requires,...keeping mind maintainability,...practical size,...necelle drag etc.

3) It pains me to say this,...but to get a real fan right,...your best chance of success,....would be to have both an Engineering degree,...experience at a gas turbine manufacturer,...(so you get to pick up tips that only THEY know),...even going to a PHD at a University,...you'd get something functional, but not optimal. I know of a bunch of guys who worked in maintenence on F-15s,...they managed to get the 3rd stage fan blades,...and stators,...built a rotor hub,...and houseing etc. THey allegedly used a Jaguar V-12 car engine to drive it. THey being mechanics,..(however smart),..did not know,...the leading edge of the 3rd stage takes its airflow at an angle coming off the exit of the 2nd stage that is not "AXIAL". which through off the aero,...off its design point. ANd that's assuming you could get the rpms up,...high precision ball bearings,...so,...short of being able to design the fan from the ground up,....and "getting it right" its very, very, very hard. They only reason I have succeeded is my manufacturing tooling knowledge,...and making friends with one of the best compressor aero guys in the world. He taught me "streamline analysis". To be clear,....I'm not saying a "non gas turbine engineer" couldn't get it right,...I'm saying the chances for messing even one thing up,...would be higher,...and more likely. ANd,..I'm talking aero right now. Having some sense of the blades resonant frequency,...is another concern,...but there are plenty of mechanical designers that can help with that. But to combine all that,....along with the manufacturing (KNOW HOW AND SHOW HOW) is a tall order indeed.

I have a viable aero package,...(its been rig run,..and met all the flow and pressure ratio targets),...I successfully created the tooling,...to make "RTM" (resin transfer carbon blades for it). I have not run the carbon fiber version yet,..but I'm 100% confident,...it would work as well. Then comes the plane,....I thought many times about retrofitting a Rutan Long EZ maybe,...(it could probably be done),...but finding someone to be willing to make significant changes (putting an inlet system one etc.) is problematical,...and probably as much work as designing and building a basic plane intended to have a fan to start with. Build the plane to match the fan,..not the other way around.

4) I guess on the question of "Guidance" the above is the best I can do,..though for most folks it would sound discouraging. The guy who did the "Smitty Hair Plane fan" did what I would have done,...(had I not been an Engineer and had access to awesome folks in a propulsion brain trust like Pratt & Whitney.

5) Here is what I would reccomend for someone who has some good manufacturing skills,...knows how to take measurements with "U-tube" Manometers,...has some composite skills,...and was just DETERMINED to do it. DO this,...find a good turbo charger,..(take the centrifugal compressor),...take splashes off the vane passages,...get the compressor map for the unit,....pick the best efficiency point of pressure ratio,....scale up the aero with a method like a copy machine,...and some clever simple tooling. With that sort of unit with straight radial diffuser vanes,...the "flow" scales to the square of the "inducer diameter". I.e. double the diameter,...quadruples the flow and halfs the rpm for that design point. You could set up a spread sheet to work this out,...but it would be a lot of work. You have to match the power required curve with the power curve of the engine. Being Centrifugal you'd have to have elbows or something to turn the flow back axial.

Centrifugals are rugged,...less succeptible to FOD (foreign object damage),...easy to build in principle,..(much easier than an axial flow),...and most importantly for you,...you'd get the Pressure ratio you need with a real chance of success. Your thermal efficiency would be lower than an axial,...but if you did a really maticulous job of copying the turboe compressor,...(the aero guys at the turbo vendor have done that job for you). You could do it,...but it would not be easy,...For a non-propulsion Engineer,...my opinion is that a simple centrifugal would give the best overall chance for success.

MAtt
 

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
For a non-propulsion Engineer,...my opinion is that a simple centrifugal would give the best overall chance for success.
Matt
Interesting opinion. I have to admit I would have thought they have to spin at very high speed to get any real pressure ratio. Which might be difficult with an IC engine.

I'm surmising then from your comments combined with everything else I've read that a convergent duct would improve very low speed thrust, by improving mass flow, but create too much spillage drag to be suitable for higher speeds.

Thanks for taking the time to make your comments.
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
That is "correct" but incomplete. The exit area of the propulsion system is the mass flow throttle "so to speak". In a static thrust condition,...there is no spillage drag obviously,....the converging duct does not inhibit operation. But the flow is also controlled by the exit area. If you keep reducing it,...mass flow goes down,...and velocity goes up,....static thrust on a (per/hp) basis goes down. But all this is assuming the fan can pump against the adverse pressure gradient. If its a prop,...in a pipe,....it will stall very quickly. If your fan is designed to produce a particular pressure ratio at a given flow,....you throttle the exit to test "that point". If you have not seen it,...my youtube videos show us changing the fan exhaust exit area,..to "explore its performance". Its a crude variable area nozzle which is only used to map the fan,...and nothing else. ALso notice,....the video labled 90% speed,...was when we closed it down to near the stall line. If you watch my friend marking the height of the u-tube manomenters,...you can see the pressure is higher as one approaches the stall line. That point was 90 % speed as some reduced flow,...the whole rig was hp limited by the 2.0 liter datsun truck engine. To be at 100% speed and 100% flow,..at the design point, (e.g. pressure ratio of 1.1 to 1),...you'd need 185-190 hp,...at 5,000 rpm. THat works out to nearly 400 lbf thrust.
 

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
That is "correct" but incomplete. The exit area of the propulsion system is the mass flow throttle "so to speak". In a static thrust condition,...there is no spillage drag obviously,....the converging duct does not inhibit operation. But the flow is also controlled by the exit area. If you keep reducing it,...mass flow goes down,...and velocity goes up,....static thrust on a (per/hp) basis goes down. But all this is assuming the fan can pump against the adverse pressure gradient. If its a prop,...in a pipe,....it will stall very quickly. If your fan is designed to produce a particular pressure ratio at a given flow,....you throttle the exit to test "that point". If you have not seen it,...my youtube videos show us changing the fan exhaust exit area,..to "explore its performance". Its a crude variable area nozzle which is only used to map the fan,...and nothing else. ALso notice,....the video labled 90% speed,...was when we closed it down to near the stall line. If you watch my friend marking the height of the u-tube manomenters,...you can see the pressure is higher as one approaches the stall line. That point was 90 % speed as some reduced flow,...the whole rig was hp limited by the 2.0 liter datsun truck engine. To be at 100% speed and 100% flow,..at the design point, (e.g. pressure ratio of 1.1 to 1),...you'd need 185-190 hp,...at 5,000 rpm. THat works out to nearly 400 lbf thrust.
In your opinion would a variable exit area be more "useful" as a duct/fan tuning device, over a wide range of airspeeds, than a variable inlet area? Would incorporating both be even better?
 

shadow

Active Member
Joined
Sep 23, 2008
Messages
26
Matt,

I have a couple questions for you if you don't mind answering!

1) How much thrust would your coldjet produce at say 400kts?
2) Any thoughts on mixed mode fans?

Thanks,
Serge.
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
In your opinion would a variable exit area be more "useful" as a duct/fan tuning device, over a wide range of airspeeds, than a variable inlet area? Would incorporating both be even better?
For the speeds,...(300 mph) we're talking about,...but more importantly the pressure ratios we can achieve with an auto-engine rpm,...6,000 continuous for a Mazda rotary, a variable inlet or exit would most likely not be worth the weigh penalty, cost, etc. Remember,...every "add-on-trick" must earn its way onto the plane,...not just be a gimic.

For us,.......determining your best OP-LINE that has the optimal combination of flow, efficiency, and then thrust,....fix the nozzle at that point,...then develope an inlet that delivers the right "corrected flow" with maximum pressure recovery,...(or put another way,..min Total Pressure loss),....and that has to be determined by the speed capability of the plane. What I invision eventually doing,...it taking my best estimate of perfromance,...compare it to the fans capability,...then build a little extra area in the inlet to do a final adjustment from flight data. If one were to put "tufts" around the inlet,...the tufts would tell you if you were incurring spillage drag. Then,...one could put a streamlined plug at the center point of the inlet to reduce its area,...if that was still not enough,...make another one a little bigger or smaller,..which would be fairly easy.

Matt
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
Matt,

I have a couple questions for you if you don't mind answering!

1) How much thrust would your coldjet produce at say 400kts?
2) Any thoughts on mixed mode fans?

Thanks,
Serge.
Serge,...

I'll run my aircraft deck,...with my current fan flow,...and PR,....vary the flat plate area that gives that speed. I'll just use max VFR altitude 18,000 feet,...and tell you the thrust and the hp required.

Now as to the "mixed-mode" fan,...I have an idea of what that means,...you're may be different. So,..if you could define that as you feel about it,...I can better answer the question.

Matt
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
Matt,

I have a couple questions for you if you don't mind answering!

1) How much thrust would your coldjet produce at say 400kts?
2) Any thoughts on mixed mode fans?

Thanks,
Serge.
Serge,...

I ran two cases:

Sea Level max speed:

Pressure ratio of 1.15, Flow = 36 lbm/ sec corrected flow

I kept lowering flat plate area to reach 460 mph. I also ran a case for using the waste heat of the engine to boost enthalpy and thrust.

At Sea level you can go 460 mph with 380 HP, Thrust is 240 lbf (without heat added) Flat plate area was .44 sq feet.

With heat,...FLate plate area can rise to .58 sq. feet. Thrust is 315 lbf.

So,...if you design what amounts to manned missile, with perfect execution on the inlet system,..etc. etc. you can go 460 mph,...but the flat plate areas are simply not reasonable. .58 is rediculous let alone .44.

I have been told by "folks who have done it",...that a plane with a flat plate area of 1.0 is not easy but it is do-able.

At 18,000 feet you can do the same just on less horsepower, but you still need an unrealistically low drag plane,...that you'd have to "shoe-horn" yourself into :))

Now,...if you had more altitude capability,...higher pressure ratio or a larger fan,...one can achieve quite amazing speeds on modest horsepower with a really slick plane, i.e. a flat plate area of 1-1.2. The heat addition of waste heat helps considerably. Using the cooling system to boost thrust,...you also eliminate the typical cooling drag which is typically about 6% of total aircraft drag.

Matt
 

Lucrum

Well-Known Member
Log Member
Joined
Jun 10, 2008
Messages
956
Location
Canton, GA
...I also ran a case for using the waste heat of the engine to boost enthalpy and thrust...
Matt
I had planed on dumping the engine exhaust directly aft but not within the duct. Would dumping the exhaust in the duct generally be "better"?
 

rocketboy105

Member
Joined
Oct 24, 2010
Messages
16
Location
Greenville SC
That is my opinion; what I favor is using a section of stainless steel honey comb in a cylindrical shell. THe exhaust would enter a volute manifold,...to evenly distribute the exhaust through out the honeycomb. The exhaust would heat the honeycomb,...and then is leaked between honeycomb cells. this method should result in the lowest pressure loss,...minimum weight,...and even distribution of the heat into the airflow.
 
Top