Turbine power for self launch and sustained flight.

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Cozyflier

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Hello to all sailplane enthusiasts,
 
I am presently building an older HP-18 sailplane. This is a Schreder design from the 70's which has a glass fuselage pod, aluminum wing skins, tail boom and V tail. Besides an airfoil modification I am considering twin Microjet AT-450 turbine engines. The jets produce 45 pounds of thrust each and are intended for RC jet aircraft. Bob Carlton used these power plants on his Silent Sailplane and performed throughout the country with his aerobatic sailplane routine.
 
I am considering a similar installation and am looking to see if anyone may have some history of information with there own personal experience.
 
The HP-18 is a compact sailplane and the Microjets are the only option that seems viable. Each engine is just 5" in dia by 10.5" long, weighing just 5 pounds each.
 
I understand that a self launch can be achieved using a hard surface runway. 1000' to become airborne, another 1000 in ground effect to gain climb speed and then a sustained 450' / minute to altitude.
 
In addition the engines can be started in flight if lift is weak to get back home or to a suitable landing site.
 
The CG and drag concerns are minimal since the extend and retract operation would be a vertical mast directly behind the wing area with minimal frontal area.
 
I see these engines as a viable alternative to piston or electric power, especially when the area for installation is compact and limited.

Any and all comments are welcome,
 
Thanks,
Chris
 

autoreply

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Hello to all sailplane enthusiasts,
 
I am presently building an older HP-18 sailplane. This is a Schreder design from the 70's which has a glass fuselage pod, aluminum wing skins, tail boom and V tail. Besides an airfoil modification I am considering twin Microjet AT-450 turbine engines. The jets produce 45 pounds of thrust each and are intended for RC jet aircraft. Bob Carlton used these power plants on his Silent Sailplane and performed throughout the country with his aerobatic sailplane routine.
 
I am considering a similar installation and am looking to see if anyone may have some history of information with there own personal experience.
 
The HP-18 is a compact sailplane and the Microjets are the only option that seems viable. Each engine is just 5" in dia by 10.5" long, weighing just 5 pounds each.
 
I understand that a self launch can be achieved using a hard surface runway. 1000' to become airborne, another 1000 in ground effect to gain climb speed and then a sustained 450' / minute to altitude.
 
In addition the engines can be started in flight if lift is weak to get back home or to a suitable landing site.
 
The CG and drag concerns are minimal since the extend and retract operation would be a vertical mast directly behind the wing area with minimal frontal area.
 
I see these engines as a viable alternative to piston or electric power, especially when the area for installation is compact and limited.

Any and all comments are welcome,
 
Thanks,
Chris
I would think thrust is a bit on the low side (but far from unrealistic). A 18m/open class glider has roughly 2 to 3 times that thrust during ground run and they're still pretty marginal on T/O, especially on grass.
Another big advantage of slightly bigger engines is a much better climb. Your glider has something like 20-25 lbs of drag during T/O, so climb on one engine is probably pretty minimal. (120 fpm or so, but a healthy 350 fpm on both)

My guess is that, with the above set-up your engine performance will be comparable to that of the big open class gliders with a 55hp Wankel. Do realize that performance at higher speeds is considerably better, compared to a prop, 2 engines will likely give you over 100 kts cruise.

Whether that performance is acceptable is highly dependent on where you fly. In the mountains, anything that climbs with less than 600 fpm at 10,000 ft is considered a deathtrap (as a sustainer), in flat terrain and with a sufficiently large home field, this kind of performance is plenty.

You seem to have addressed all major issues that arise with turbines (reliability, in-flight restart, type of fuel), so no other remarks.

If you're interested in other (more powerful) engines, you might have a look here:
AMT Netherlands
 

skeeter_ca

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Not sure if it true but i have heard the TBO's on those model turbine engines are really low. Like in the 25 hour range. It could get very expensive quickly.
 

autoreply

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Not sure if it true but i have heard the TBO's on those model turbine engines are really low. Like in the 25 hour range. It could get very expensive quickly.
Typical use is 5-10 minutes per 5 hours of flight, or only a couple of hours a year (with 200 soaring hours). In both turbines and 2-strokes, conservation during winter and annuals are the most expensive issues.
 

goldrush

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I realise you will only be using the engines intermittently, but one thing to remember is that these engines, like all small turbines, are THIRSTY.
2 of these will consume a gallon of Kerosene every 3 minutes. (almost 7 pounds)
 

Topaz

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A few other issues can arise. Hot gas impingement on the tails, radiant heating of the upper fuselage during taxi or run-up, etc. IIRC, a fair portion of Schraeder's structures are bonded. Watch out for heat on any bonded joints, depending upon the adhesive used. It might do to purchase one engine and do a heat survey at the distance between your intended installation and the tail structure. How hot will it get back there, at full thrust, sitting static on the runway on a hot day? (worst case scenario) I know you have a V-tail and that, nominally, the hot gasses will go between the tails. But it doesn't take much of a cross-breeze (easy to find in a runup area) to blow the efflux over onto the tail.

Since you plan on retracting the engines, you need to go through the structural implications of cutting a hole in the upper fuselage (if any - I don't know how heavily that area might be loaded), and cooling of the hot engines once they're retracted and the doors closed. EDIT: Not only is there potential for heat damage to the aircraft, but it's going to get mighty hot around those flexible plastic fuel lines you'll have to use to accomodate the retraction. /edit

Double-check the weight and balance anyway, both extended and retracted. The installation isn't far from the CG, but it's probably going to be aft of it, and that will move your CG towards the aft limit.

Make sure the landing gear can take the additional force of a bad landing with full fuel (aborted takeoff), plus the weight of the engines, mast, and retraction mechanism, engine bay doors, and their retraction mechanism.

Define the new useful load limits. That now includes full fuel. Can you still carry yourself and remain under MTOW?

Make sure the tanks have adequate venting and drainage, including test ports. This is a full and genuine fuel system. Needs all the bells and whistles.

I think the idea is neat, and I've seen videos of Carlton flying. Looks like a blast. :)

You mentioned "an airfoil modification". That's potentially a bigger set of issues than the engine installation. Out of curiosity, what are you doing with the wing?
 
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Topaz

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Interesting. And that's assuming the aircraft hasn't been painted, of course...

The only two Schraeder HP's I've seen personally (both HP-11s) had painted fuselages. The owners didn't like the look of the glass cockpit area being a different color than the tailcone.

In this case, though, it might not be a bad idea, like you say.
 

autoreply

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A few other issues can arise. Hot gas impingement on the tails, radiant heating of the upper fuselage during taxi or run-up, etc. IIRC, a fair portion of Schraeder's structures are bonded. Watch out for heat on any bonded joints, depending upon the adhesive used. It might do to purchase one engine and do a heat survey at the distance between your intended installation and the tail structure. How hot will it get back there, at full thrust, sitting static on the runway on a hot day? (worst case scenario) I know you have a V-tail and that, nominally, the hot gasses will go between the tails. But it doesn't take much of a cross-breeze (easy to find in a runup area) to blow the efflux over onto the tail.
I don't share your concerns about the tail. The German guy with the ASW-20 tried it and noticed only a couple of degrees higher temperature on the fin leading edge (directly downwind of the turbine), when running full-throttle in a hangar. You can stand 5 ft behind such an engine, without getting unconfortable (at full throttle)

I do share your concerns about the upper fuselage and (even more so) the huge thermal energy you lock in when retracting the engine. Most "conventional" sustainers are allowed to cool down for a couple of minutes (half retracted, to reduce drag), but even after that you have to provide some insulation in the engine bay. I recall most sustainers have heat shields in the engine bay.
 

Topaz

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I don't share your concerns about the tail. The German guy with the ASW-20 tried it and noticed only a couple of degrees higher temperature on the fin leading edge (directly downwind of the turbine), when running full-throttle in a hangar. You can stand 5 ft behind such an engine, without getting unconfortable (at full throttle)...

Oh, likely so, but I don't think it's something that can simply be ignored. It should be checked, and carefully. Like as not it won't be an issue, but the potential consequences are such that it shouldn't be ignored.

I think the biggest heat-related concern I have is the one I edited in to my earlier post: Heat damage to the flexible fuel lines over time from the retracted (but still hot) engines. If the lines split and the fuel ignites, you've got a major fire right at the most-stressed part of the (thin aluminum) fuselage, directly against the wing spar and just aft of the pilot. If that engine bay isn't kept well-ventilated and cool under all conditions, those lines are going to eventually be heat damaged and things are going to get mighty "interesting" on some flight, someday. No matter how well the cooling is accomplished, close inspection of those lines should be a mandatory preflight item for every flight.
 

Cozyflier

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Thank you all for your comments, I will do my best to answer some of your questions and address some of the concerns.​

I will be operating in the Mid West at 900' MSL from an airport that has a hard surface runway of 3000'. In addition there is a 2250' grass strip but I do have concerns using the turf for takeoff considering the added drag.​

The advertised TBO on these engines is 200 hours, a service or inspection interval of 50 hours is recommended.​

Fuel consumption is high with any single stage turbine. The compact size and minimal weight is offset by fuel consumption. By using the engines for takeoff and climb followed by shutdown after reaching altitude would be the normal cycle, I anticipate the time to get airborne and reach altitude to be in the 10 minute range here in the Mid West. Using this estimated time frame at full power would consume just under three gallons. If five gallons of fuel was carried this would allow for a self launch and climb to altitude, the remaining fuel would be available for in flight sustainer emergencies. One other consideration is that the glider is aloft and the trust required to maintain altitude will not require full power and will require less fuel.​

The heat concerns on the tail or aft fuselage from thrust have been addressed by Bob using these exact engines and also his new PBS TJ-100 turbine. Extensive testing has shown it is a non issue.​

Heat from the engines is another concern and the turbines would have to go thru a cool down period before it is retracted into the fuselage. The plastic fuel lines are at risk if there is excessive heat, standard aircraft quality hoses could be used to plumb the system. These are military spec which can withstand heat exposure to an acceptable limit.​

The HP-18 has a glass pod and the area for installation would have to be modified. By adding additional glass, carbon fiber and structural gusset ribs I believe it could be done. I have a vast background using composite material with my previous three powered aircraft builds ( all Rutan designs) so this modification is fairly straight forward.​

The Silent sailplane gross takeoff weight with the twin turbines, airshow smoke system, and pilot was around 700 lbs. The HP-18 will come in around 500 lbs empty and my weight is 180 pounds, add twin turbines and fuel and the takeoff weight are somewhat close for comparisons, one other note is that the silent is a 13 meter and the HP is a 15 meter. The added wing area in the HP should help with the takeoff and climb performance.​

Last item is the airfoil change, the HP-18 has a FX-67 from the seventies, I am modifying the gemetry to the Udo 15.3 . This is a unique airfoil developed by Udo Rumph who has built two HP-18's . This airfoil has been proven by Udo and has an improved climb and cruise polar compared to the FX-67. Udo is known to be competitive in the 15 meter class while competing against some of the 150K glass ships.​

Thanks again for all of the comments,
Chris​
 

autoreply

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I will be operating in the Mid West at 900' MSL from an airport that has a hard surface runway of 3000'. In addition there is a 2250' grass strip but I do have concerns using the turf for takeoff considering the added drag.
Usually static friction (how to start rolling) is the major problem. Gliders that don't have the necessary thrust to accelerate from a standing start (too much resistance from the grass) are often pushed the first 10'. Even they get airborne within 2000'.
One other consideration is that the glider is aloft and the trust required to maintain altitude will not require full power and will require less fuel
Fuel burn of those microjets is virtually continuous, you can only vary thrust, not fuel burn. Running "idle" isn't of so much use.
The added wing area in the HP should help with the takeoff and climb performance.
While it certainly does help, it doesn't come close to it's importance in reciprocating engines, since the ideal climb speed with a jet is higher. As the thrust of the jet increase (actually, if the thrust-to-weight ratio increases) the best climb speed also increases.

Let's have a look at a random polar:
ask23-polar-800x.gif

Assume 30 pounds of thrust and a weight of 600 lbs. That's a 1:20 ratio, or "negative" glide ratio. We could directly plot that:
i6xe6o.gif

And I did the same for 40 lbs of thrust.
You will notice that the best rate of climb (which is the biggest vertical difference between polar and "engine polar" occurs @ 90 km/h, being 0.6 m/s.

For the 40 lbs case though, maximum rate of climb is 1.1 m/s @ 120 km/h. That's a big difference, and the higher your thrust, the better it gets. Your climb speed increases progressively, hence it's beneficial to have a higher thrust level, even more so in a glider (like yours) with a really "flat" profile.
 

Cozyflier

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Good explanations and graph depiction, thanks for the further information. I now have a better understanding regarding the importance of the required thrust to climb.

I can see that it is of utmost importance to launch from a hard surface. Launching from grass or a rough surface would most likely not work for the space limitations at our glider field, on moderate to heavy cross wind days I will most likely have to get a tow.

Regarding fuel burn I see full throttle is the way to go, the more thrust the better. My fuel capacity may be better suited with a 7 gallon tank for unexpected added climbs and emergency sustainer operations.

Best regards,
Cozyflier
 

autoreply

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I can see that it is of utmost importance to launch from a hard surface. Launching from grass or a rough surface would most likely not work for the space limitations at our glider field, on moderate to heavy cross wind days I will most likely have to get a tow.
Don't consider my stated numbers as fact (or reliable source), they're very rough numbers and highly dependent on conditions. (We had a bit of up and downslope plus high grass).

How about this:
Take a comparable glider (with equally sized main wheel) and tow it by car. Put a spring balance in the rope and measure the average force you need to pull the glider and the maximum force to start it rolling. That should give you much more realistic numbers, you can calculate by hand :)

(Thus, v^2/a=s, where V is the take-off speed, a the acceleration (engine thrust minus rolling resistance and that divided by aircraft MTOW) and s the covered distance.)

Regarding fuel burn I see full throttle is the way to go, the more thrust the better. My fuel capacity may be better suited with a 7 gallon tank for unexpected added climbs and emergency sustainer operations.
Don't know whether you normally fly with ballast, but most turbo-pilots just consider the fuel (and engine) weight as just that, ballast. In a comp/XC you also dump ballast or pull the turbo, so the performance edge is minimal. For local "floating" it might be a consideration, but then of course you can simply leave the tank (almost) empty.
 

Cozyflier

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Well I am committed now, I have purchased the 45 lb thrust AMT 450 engines and will be installing two of them on my current HP-18 project. I recalculated the numbers and it will be sufficient for my self launch and sustainer operations (not great but adequate).

Present progress report: Wings are completed with the modified FX67 airfoil (Udo 15.3 geometry). The flaps and ailerons are nearing completion and the fuselage construction will be next on the list. Now that I have the turbines I should have all that is needed to complete the project besides paint, finish, instruments and avionics.

Chris
 

flyvulcan

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Hi Chris,

A guy in Waikerie, South Australia has installed 3 of these on his sailplane and it has flown in this configuration for the last 6 months or so. I can get more details or a contact of you want to compare notes.

Cheers,

Dave
 
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