Turbine Conversion

[Aré van Eck]

Hi Everyone,new guy here. I just love Mooneys and saw an article on here of RotorheadUK who was busy doing a turbine conversion. Has there been any progress after the last post? Am rather curious to have more information on this as its something I've been very interested in.

 

Regards

Aré

[LANCECASPER]

9 minutes ago, Aré van Eck said:

Hi Everyone,new guy here. I just love Mooneys and saw an article on here of RotorheadUK who was busy doing a turbine conversion. Has there been any progress after the last post? Am rather curious to have more information on this as its something I've been very interested in.

 

Regards

Aré

This subject comes up every few years on Mooneyspace. The limiting factor in the Mooney airframe is the tail. There have been STCs that go up to 310hp and 2700 RPM, but the Type Certificate for our airframes have only gone up to 280hp. Mooney engineers have commented that in flight tests at higher speeds that tail flutter was observed. It's not a good day when your tail separates from the airframe.

To get a turbine engine conversion approved you'd need to re-design the tail and the landing gear for higher gross weight since you would need to be able to carry more fuel to feed the turbine, since they burn more. Then you would need to convince people to fly on oxygen on every flight since the only place that turbines are efficient is up in the flight levels. Rather than develop this, it would be a lot less expensive just to buy an early Piper Meridian that's pressurized and already has a PT6 turbne.

[Austintatious]

I agree with the fuel part... Even the added tanks don't add enough fuel to make a turbine make any sense... You will have no range.  Just look at the Meridian, terrible range with the turbine, because it was a piston aircraft they adapted to turbine.  That is nearly always the case.

 

I don't agree about the speed concerns or weight concerns.... That is to say that there would be nothing about a turbine that would result in operating at speeds in excess of what the airframe is safely capable of.   The Rocket has 305 HP and it can be flown at full power.... Any turbine that would fit into the mooney nose will likely be less than 305 HP.

Furthermore, even if you could somehow add enough fuel to make it make sense, I doubt that fuel would be more weight that the weight savings from switching from a heavy piston motor to a turbine.  The weight savings would be substantial.

 

 

[exM20K]

A turbine conversion will be airspeed limited to the top of the green, which is pretty close to where the acclaim cruises at/below 10,000.

while excess power = better climb, the rudder trim is almost full right as is in a full power acclaim climb, so it is not obvious that the airframe and rudder would take to substantially more power.

this project might be fun to do if that is your thing, there are better options available now (LX7, Jetprop, Meridian, Silver Eagle, Bonanza’s) all of which have known pro’s and con’s and a somewhat active secondary market in which to buy or sell.

-dan

[christaylor302]

I agree with the fuel part... Even the added tanks don't add enough fuel to make a turbine make any sense... You will have no range.  Just look at the Meridian, terrible range with the turbine, because it was a piston aircraft they adapted to turbine.  That is nearly always the case.
 
I don't agree about the speed concerns or weight concerns.... That is to say that there would be nothing about a turbine that would result in operating at speeds in excess of what the airframe is safely capable of.   The Rocket has 305 HP and it can be flown at full power.... Any turbine that would fit into the mooney nose will likely be less than 305 HP.
Furthermore, even if you could somehow add enough fuel to make it make sense, I doubt that fuel would be more weight that the weight savings from switching from a heavy piston motor to a turbine.  The weight savings would be substantial.
 
 

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[Fly Boomer]

5 minutes ago, christaylor302 said:

 

 

Sent from my iPhone using Tapatalk

 

 

Chris, you quoted Austintatious, but somehow your comment didn't come through.  Please give it another try.

[A64Pilot]

9 hours ago, Austintatious said:

I agree with the fuel part... Even the added tanks don't add enough fuel to make a turbine make any sense... You will have no range.  Just look at the Meridian, terrible range with the turbine, because it was a piston aircraft they adapted to turbine.  That is nearly always the case.

 

I don't agree about the speed concerns or weight concerns.... That is to say that there would be nothing about a turbine that would result in operating at speeds in excess of what the airframe is safely capable of.   The Rocket has 305 HP and it can be flown at full power.... Any turbine that would fit into the mooney nose will likely be less than 305 HP.

Furthermore, even if you could somehow add enough fuel to make it make sense, I doubt that fuel would be more weight that the weight savings from switching from a heavy piston motor to a turbine.  The weight savings would be substantial.

 

 

I don’t know of a less than 300 HP turbine, the old Allison now Rolls baby turbine was 317 SHP, but very quickly grew to 420 SHP and now is much higher

We got 2.5 hours with reserve from them in the OH-58 helicopters that held 71.5 gls of fuel

The best use of turbines in airplanes is to hugely derate them, the TBM’s for example have cores capable of 1700 SHP but are derated to about half that, that means they can make rated power way up high which of course means speed. It also means smaller and lighter gearboxes etc.

From a flutter margin etc that’s indicated airspeed, so if we can assume a Mooney would be good for 250 kts, then that has to be over 400 up high, but I have not done that math

I do agree that an unpressurized turbine is likely to not be very desirable.

A bigger problem as I see it is cost a turbine STC would likely cost way more than even a newer Mooney is worth.

 

[exM20K]

4 minutes ago, A64Pilot said:

From a flutter margin etc that’s indicated airspeed

Flutter is a function of TAS, not IAS.

[Hank]

20 minutes ago, exM20K said:

Flutter is a function of TAS, not IAS.

But it's a driver of Redline, which is Indicated Airspeed.

[exM20K]

1 hour ago, Hank said:

But it's a driver of Redline, which is Indicated Airspeed.

Vne is a huge buffer over flutter speed.  the flutter margin at 2000’ at Vne is way bigger than the margin at FL250.

Is Vne solely a flutter margin? I recall Mooney factory test pilots diving until flutter starts as part of certification, of course. Presumably the TAS that induced flutter is then applied to the IAS at service ceiling plus buffer to set Vno.

At 2000’ standard day, 190KIAS is about 195 KTAS

At FL250, standard day, 190 KIAS is about 290KTAS.

kinda a little less margin.

Thinking flutter margin is a constant function of indicated airspeed is common. And wrong.

-dan

[aviatoreb]

Reminder: There was a liquid rocket by rocket engineering. -a much rarer bird.  I hear there were exactly 5 built.  It was built on top of the long body airframe.  The M20M or was it the M20 PFM?  It involved fitting a TSIOL550 - the liquid cooled piston and I have read ads over the years for the very few that came for sale one describing 335HP and another describing 350HP.  I have read it was capable of 260TAS up high.  So an interesting thing about these is that the airframe itself went through some strengthening.  I hear that some gussets were added, but I dont know the details.  Presumably somehow where the wings and also tail feathers meet the airframe.  I am really surprised this got by the FAA and presumably it wouldnt in today's FAA, but anyway airframe strengthening would surely be needed on a turbine mooney.

[A64Pilot]

Flutter is just one driver of VNE, there are many others, hopefully if well designed flutter isn’t even remotely close to VNE. Crop duster I built had an over 100 kt flutter margin, reason is that over the years flight control surfaces gain weight from painting and repairs and even insect nests and as the majority of the surface is behind the hinge line over the years if not rebalanced you lose flutter margin, therefore in my opinion while not required a responsible manufacturer designs for a large flutter margin.

Engine failure concerned me, flutter scares me.

Crop Dusters are I believe much worse with owner repairs than most GA aircraft so it was prudent to take that consideration into the design. I even had owners changing the wing angle of incidence of both the wing and tail. I know this because several told me they did and were trying to get me to incorporate their “improvements”

Some other drivers are gust loading and even windshield structure, several others that don’t come immediately to mind as I am no Engineer, but structure and aerodynamics both drive VNE, but in a Velocity Dive test it’s flutter in most cases that your validating, and on light aircraft that VD test is the most hazardous requiring a parachute, boots being worn and a chase plane etc. I had a door quick release installed when I did one.

I misspoke about flutter being IAS, but point is without any analysis I believe the Mooney tail has enough margins for a Turbine installation. I base that on my belief that if it didn’t then over the years there would be several accidents of Mooney’s losing their tails and I don’t believe there has been. You can test forever but the actual acid test is a fleet with years of operation.

I agree that having the whole empennage being hinged like it is, is weaker than if it were one piece, but apparently it’s strong enough. 

Mooney’s of course have their issues, all aircraft are compromises, just structure doesn't seem to be one of Mooney’s weak links.

I just don’t think the market exists for a small four pax non pressurized turbine myself, in truth it seems the market doesn’t exist for a 4 pax piston complex aircraft.

Edited Wednesday at 01:18 PM by A64Pilot

[A64Pilot]

I seriously doubt any post WWII era test pilot would dive until flutter onset, reason is sometimes flutter can completely destroy an aircraft almost instantly. The WWII era guys were very brave or very foolish, many didn’t survive.

When Rockwell Commander was doing a VD test flight of the 112 the aircraft disintegrated, they recreated the event in NASA’s wind tunnel and the time frame from flutter onset to complete disintegration was less than 1 sec. 

I really wouldn’t want an aircraft to disintegrate on me https://asn.flightsafety.org/wikibase/2731

Edited Wednesday at 01:35 PM by A64Pilot

[Pinecone]

12 hours ago, A64Pilot said:

I don’t know of a less than 300 HP turbine, the old Allison now Rolls baby turbine was 317 SHP, but very quickly grew to 420 SHP and now is much higher

We got 2.5 hours with reserve from them in the OH-58 helicopters that held 71.5 gls of fuel

The best use of turbines in airplanes is to hugely derate them, the TBM’s for example have cores capable of 1700 SHP but are derated to about half that, that means they can make rated power way up high which of course means speed. It also means smaller and lighter gearboxes etc.

From a flutter margin etc that’s indicated airspeed, so if we can assume a Mooney would be good for 250 kts, then that has to be over 400 up high, but I have not done that math

I do agree that an unpressurized turbine is likely to not be very desirable.

A bigger problem as I see it is cost a turbine STC would likely cost way more than even a newer Mooney is worth.

 

Since my plane carries 104 gallons, that would be 3.6 hours with reserve.  250 indicated at 18000 on a standard day is 330 KTAS. So right around 1000 mile range.  Except with turbines, Vne is reduced to the top of the green, so only 174, giving a TAS of 230.

Also, I understand that the small Allison/Rolls is not great a high altitudes, so might actually work fairly well.

And yes, cost would be a HUGE factor.  But so would the cool factor.

The Silver Eagle uses that engine.  From AOPA article:

"Best speed versus economy for the Silver Eagle comes in the mid-to-upper teens, where owners report speeds of 195 to 210 KTAS (Vne is 167 knots, added by me) on 20 to 24 gallons per hour, depending on altitude and temperature. Above FL200, the repurposed helicopter engine loses power precipitously. In fact, nearing the P210’s service ceiling of FL230, the piston version is faster than the turbine."

[A64Pilot]

There is a small Pratt PT-6 that I think would be better. The Baby Allison was designed for the US Army to be used in either a fixed wing or Rotary wing scout.

PT6-21, Bill Hatfield does a C-206 conversion with one, plus there are shed loads of used -21’s available, which is the most important part of a turbine conversion (affordable engines) Old King Air engine, I think 550 SHP, but you derate it, derating keeps power way up high and adds mucho life to an engine too

https://turbineconversions.com/conversions/turbine-206-cessna/

So far in our history it takes Government bucks to fund engine development, I believe the small jets only exist because they essentially use cruise missile engines?

Maybe the Government will pay for a baby turbo prop to be developed for drones, just based on the Ukraine war it’s my belief that the future will see way more drones than we see now, way more, the Terminator is closer than we think. It’s been my belief that the day of the manned fighter ended last Century just too many in the AirForce don’t want to see the end of it. I base that on conversations I had at the Edwards O club with pilots that were doing Air to Air testing against a very rudimentary remotely piloted vehicle. They said you got two turns, the first at the merge when “fights on” and you got killed in the second turn every time. An unmanned aircraft has an unbelievable maneuverability whereas any manned modern fighter I believe the pilot is what drives the maneuverability limit.

This is what they were flying against, it was just essentially an FPV drone, it wasn’t very advanced, yet it ate manned fighters lunch.

https://en.wikipedia.org/wiki/McDonnell_Douglas_X-36

 

 

Edited Wednesday at 02:08 PM by A64Pilot