Why was the IO-720 a failure?

[Tim Jodice]

By failure I mean it was/is rarely used in any airplane OEM or STC. Looking at high power applications with my non engineering education level I would think it would have been a great engine. These are a few examples that come to mind..

Why is pulling 375HP out of only 520 cubes then add the complexity of gearing it good for a 421? A 720 would seem great, slow it down a little to get 375 and use just a little boost to pressurize the cabin and hold power at altitude. 

Why was it better to spin a 541 in a beech Duke at ear piercing 2900RPM boosted to (I think) 42"MP to get 380HP. Again slow a 720 down add a touch of boost for cabin and altitude performance. 

There are posts on Beechtalk about getting rid of unsupported ancient lycoming and Adding 720s to Twin Bonanzas. with the exception of the opinion of many that think the old engines sound better the 720 is better in every way.

Those both assume that you don't want all the power that they are rated for. which makes sense for longevity. 

There is  a STC for piper malibus to remove the factory 310HP 520 and install a 310HP 550. The main if not only reason it exists is because of the poor service life of a high climbing heavily boosted engine. They traded the boost for more cubes that resulted in a cooler running longer life engine.

Speaking of heat, making power makes a certain amount of heat. In a water cooled engine you would size the radiator accordingly. you can add only so many fins to a cylinder and that is all you have. Adding a 4th row of cylinders just increased your cooling capacity 25%. More power? Cooler running? Or a little of both.

Any constant load application it seems a bigger engine asked for less of it max power has a better,  longer life and sometimes is even more efficient.

Weight? Price? Parts? Looking at one it would seem the only unique parts are the crank, cam and cases.

What your thoughts?

@M20Doc I figured this is right up your alley. 

 

[N201MKTurbo]

The IO-720 is almost 200 Lbs heavier than an IO-550

[Tim Jodice]

3 minutes ago, N201MKTurbo said:

The IO-720 is almost 200 Lbs heavier than an IO-550

What model 550? 

[A64Pilot]

The 720 is heavier, more expensive and in some installations had cooling problems.

‘At some point a turbine makes more sense, and a 720 is pushing that point, too bad there wasn’t a turboprop cruise missile or maybe we would have had an excellent little turbo prop.

‘Piper’s Brave used a 720, and the turbine conversion for the Brave really makes it a good airplane, wakes up a lot of potential.

 

Just about every 520 airplane there is has an STC to install a 550.

Edited April 25, 2021 by A64Pilot

[carusoam]

The IO720 is a brilliant device!

It has the HP of two IO360s...

With the drag of only one prop...

 

It is soooo big and sooo powerful... and thirsty too....

It is beyond the economic grasp of most ordinary pilots...

 

Wait...

We have one around here...

We have a few turbines as well...

I would like to hand the mic over to @M20Doc to expound on all the great qualities the IO720 has to offer... 

 

Wait a second...   Tim, are you working with Doc on this thread?   Something seems a bit fishy...

Stand by to hear how big the UL is on a plane that has a laminar flow wing....

Go IO550! 

Best regards,

-a-

[Tim Jodice]

What can I say? It arouses me. 

 

[carusoam]

I like the Nitrous system he plumbed in...  0 - T/O in under 2 seconds...  

How about the cylinder lockers..?

Mr. Patey knows a bit about generating HP!

Still waiting to see the wings...

Expect variable wingspan with that...?

-a-

[Guest]

What can I say, an IO 720 isn’t for everyone, if you own a Mooney the fuel burn will make your eyes water and the torque on take off will take out your knees!

Very few airframes used them from birth, a number more were converted by STC, a company called Mr RPM did quite a number of conversions.

Clarence

[RJBrown]

My next door neighbor has a piper Comanche 400. Uses more gas but goes way slower than a 300hp Mooney. 

[aviatoreb]

5 minutes ago, M20Doc said:

What can I say, an IO 720 isn’t for everyone, if you own a Mooney the fuel burn will make your eyes water and the torque on take off will take out your knees!

Very few airframes used them from birth, a number more were converted by STC, a company called Mr RPM did quite a number of conversions.

Clarence

Its quite a neat and unusual engine.  Is it right to think of it almost like two IO360's bonded nose to tail?

[carusoam]

Great for somebody that likes the power of a twin...

And the centerline thrust of a single...

 

To add a IO720... to my plane...

Would probably require a longer cowl, and moving the wing forward 12”

 

PP guessing only...

-a- 

[RLCarter]

I’ve always said “It’s better to have it and not need it than to need it and not have it” 

[aviatoreb]

Just now, carusoam said:

Great for somebody that likes the power of a twin...

And the centerline thrust of a single...

 

To add a IO720... to my plane...

Would probably require a longer cowl, and moving the wing forward 12”

 

PP guessing only...

-a- 

Maybe you could extend the tail out another 18 inches behind you?  The super stretch long body?  So that's some MAJOR Body mod.

It would be quite a think, a 400hp Mooney.  I bet a normally aspirated but 400hp Mooney would be FAST.

[Guest]

Just now, aviatoreb said:

Its quite a neat and unusual engine.  Is it right to think of it almost like two IO360's bonded nose to tail?

Basically it is a pair of 360’s end to end.  Mine is a narrow deck and uses the same cylinders as my old E models IO-360, just twice as many.  It’s like running a pair of E models.  It holds 17 quarts of oil, it burns 36-38 GPH on take off and 18-20 in cruise.

Mooney owners seem shocked at the fuel burn, Cirrus owns just nod and smile when looking at the fuel burn.

Clarence

[Niko182]

1 minute ago, RJBrown said:

My next door neighbor has a piper Comanche 400. Uses more gas but goes way slower than a 300hp Mooney. 

Depends which 300hp model. An Acclaim is a good chunk faster. But NA to NA, I think the Comanche is actually faster than an Ovation. Clarence said he gets about 190knots in his. In my Screaming Eagle, I saw 182 at 13.3 gallons on my way to Vegas in Cooler temps. Lower fuel flow in the Mooney, but I believe in a real world application the Comanche beats it. I seem to do better speed wise with people in the back seats and the CG further back, even though I'm heavier.

[aviatoreb]

4 minutes ago, M20Doc said:

Basically it is a pair of 360’s end to end.  Mine is a narrow deck and uses the same cylinders as my old E models IO-360, just twice as many.  It’s like running a pair of E models.  It holds 17 quarts of oil, it burns 36-38 GPH on take off and 18-20 in cruise.

Mooney owners seem shocked at the fuel burn, Cirrus owns just nod and smile when looking at the fuel burn.

Clarence

I sport a thirsty TSIO520.  Your fuel burn doesn't make me blush.  Unfortunately.  ;-O

[Guest]

13 hours ago, RJBrown said:

My next door neighbor has a piper Comanche 400. Uses more gas but goes way slower than a 300hp Mooney. 

Then there is something wrong with the airframe or the mixture control.

[RJBrown]

In three different websites the 400 Comanche 75% power 20-23 gph and 185 knots.

Information I read the Ovation 20 gph 190 knots. 
no time in either plane.

my Rocket was faster by far on the same fuel.

all at 12,000’ Rocket gets fast from there as it climbs while the NA Comanche and Ovation get slower and burn less fuel. 

Edited April 26, 2021 by RJBrown

[Guest]

Even with a larger, heavier engine my Comanche weighs less than a long body Mooney and has a higher gross weight to boot.

While it may not be as aerodynamically efficient as a Mooney it does well for a late fifties design, 190KTAS is pretty normal below 10K and I’ve seen as high as 199KTAS.

When I can’t afford to run it I’ll look at a Mooney again.

Clarence

[aviatoreb]

3 minutes ago, M20Doc said:

Even with a larger, heavier engine my Comanche weighs less than a long body Mooney and has a higher gross weight to boot.

While it may not be as aerodynamically efficient as a Mooney it does well for a late fifties design, 190KTAS is pretty normal below 10K and I’ve seen as high as 199KTAS.

When I can’t afford to run it I’ll look at a Mooney again.

Clarence

As I described again - WOW the IO720 would make for an exciting Mooney!  But I am sure you couldn't just bolt it on like they did the rocket bolting on a clearly too large engine - the balance would be so far out of whack even on a long body surely it would require a real structural change - a stretch - a foot maybe?  A major recent and probably would have to be a purpose built airplane based on an M20.  I know - current economics it won't happen but wow would that be an exciting Mooney M20 ultra stretch 400hp IO720.

[aviatoreb]

I remember I once found an article by a guy who turbo-normalized his IO720 on his Comanche 400.  He was claiming he was getting 260 TAS at 25,000.  I can't find that article again.  It was quite a fun read.

 

Edit: I found it!  I remembered my numbers wrong - he was claiming 281mph TAS.  And he super charged not turbo normalized.

[Nick Pilotte]

13 hours ago, carusoam said:

Great for somebody that likes the power of a twin...

And the centerline thrust of a single...

 

To add a IO720... to my plane...

Would probably require a longer cowl, and moving the wing forward 12”

 

PP guessing only...

-a- 

You and your 8 cylinders!!!!   It’s not a Firebird.  Just kidding. My wife’s late father flew a Brave 400.  He flew it fast and low.  
 

if you were to fit a 720 in the O, you’d need to add STC’d Charlie weights made out of iridium or osmium. Safer than lead, but heavier for the same volume. 

[Hank]

I can only think of two reasons that the -720 wasn't widely adopted:

1. It's too freaking large and heavy to fit onto many airframes without destroying the W&B.
2. It's too thirsty, and many airframes don't have room or lift reserves for 100-gal tanks.

But it's sure pretty to look at . . . . .

[KLRDMD]

14 hours ago, RLCarter said:

I’ve always said “It’s better to have it and not need it than to need it and not have it” 

That sounds like an argument for a twin.

[Tim Jodice]

30 minutes ago, Nick Pilotte said:

You and your 8 cylinders!!!!   It’s not a Firebird.  Just kidding. My wife’s late father flew a Brave 400.  He flew it fast and low.  
 

if you were to fit a 720 in the O, you’d need to add STC’d Charlie weights made out of iridium or osmium. Safer than lead, but heavier for the same volume. 

Depleted Uranium