BeechTalk - BT

Username Protected · **Posted:** 04 Nov 2024, 11:42

Username Protected wrote:

One problem with configurations like this is that there are failure modes that can take out both engines.

While the LearFan had more failure modes (gear box and prop related) that would take out propulsion from both engines, essentially all twin-turbine airplanes have some failure modes that do this as well (rotor non-containment that impacts the other engine or a critical support system thereof). This is why 25.903(d)(1) says "minimize the hazard" instead of "eliminate" it.

One notable exception is the American Jet Industries (later Gulfstream American) Hustler (400 with a PT6/JT15D combo and 500 with a TPE up front). The rotor zones of these engine do not overlap.

The Hustler design (minus the front TP engine) was later used to develop the Peregrine, already mentioned in this thread.

--Phil

Username Protected · **Posted:** 04 Nov 2024, 15:36

Username Protected wrote:

One problem with configurations like this is that there are failure modes that can take out both engines.

While the LearFan had more failure modes (gear box and prop related) that would take out propulsion from both engines, essentially all twin-turbine airplanes have some failure modes that do this as well (rotor non-containment that impacts the other engine or a critical support system thereof). This is why 25.903(d)(1) says "minimize the hazard" instead of "eliminate" it.

One notable exception is the American Jet Industries (later Gulfstream American) Hustler (400 with a PT6/JT15D combo and 500 with a TPE up front). The rotor zones of these engine do not overlap.

The Hustler design (minus the front TP engine) was later used to develop the Peregrine, already mentioned in this thread.

--Phil

Username Protected · **Posted:** 04 Nov 2024, 15:48

There's a lot more acceptance of SETP today, replacing the LearFan's twin-pac and custom gearbox with just a bigger PT6 would save a bunch of weight and complication.

Username Protected · **Posted:** 04 Nov 2024, 18:48

https://www.simflight.com/2022/04/10/li ... ject-news/

Flight sim Learfan to try

Crash test report and photos from Langley research

https://ntrs.nasa.gov/api/citations/200 ... 087001.pdf

https://archive.org/search?query=subjec ... T+TESTS%22

Username Protected · **Posted:** 05 Nov 2024, 02:38

This is a good picture of Moya Lear and the LearFan on the first page of this article:
https://www.historynet.com/learjet-history/moya-lear/
Photo by Paul Harris/Getty Images

Username Protected · **Posted:** 06 Nov 2024, 21:09

Username Protected wrote:

An old friend of mine was an engineer on the Learfan and he has some great stories about blowing up fuselages for certification testing.

All turboprop engines have gearboxes, it's not really rocket science at that level to build one that's reliable. They're troublesome in piston engines because they have to build them cheaply. A PT6 gearbox costs as much as a whole IO-520, which makes it, in my opinion anyway, much easier to build something very solid.

From his telling of the tale, the fuselage testing required on the LearFan was a trial run for what the FAA did to Beech on the Starship. Lear died and the company eventually gave up before being able to certify a composite airframe. Beech nearly bankrupted themselves actually getting the job done.

At the end of the day, Cirrus, Boeing, and Airbus all owe a debt to Lear and Beech for the pioneering composite fuselage work. I doubt any of those would have had the dedication to get through the trials Lear and Beech had to go through.

It can be rocket science to show that the gearbox, combined with other failure modes such as propeller and prop control, dual engine failure, prop blade failure, and engine compartment fire meets the certification requirements for complete loss of propulsion. Depending on certification basis and airplane category, this can be once in a billion flight hours. It is helpful if an impending failure can be detected in advance, hence vibration monitors, chip detectors, etc. It is also helpful if there is some sort of backup, such as pitch stops or locks, for the loss of prop control. We did some advance design studies on some of this at Cessna.

Username Protected · **Posted:** 06 Nov 2024, 21:23

I think we're forgetting that the combined propulsion (two engines into one gearbox) in the Lear Fan had a few things in common with the Twin Huey helicopter/Bell 212. The 212 got its civil certificate about ten years before the Lear Fan first flew so this engineering wasn't something out of the blue.

Username Protected · **Posted:** 06 Nov 2024, 22:25

Username Protected wrote:

I think we're forgetting that the combined propulsion (two engines into one gearbox) in the Lear Fan had a few things in common with the Twin Huey helicopter/Bell 212. The 212 got its civil certificate about ten years before the Lear Fan first flew so this engineering wasn't something out of the blue.

The Bell 212/Twin Huey has the prop on the wrong part of the aircraft to be a SETP. It's not very fast, either. However, it's technology is amazing. Thanks for sharing the gearbox information! I was a 67N and 15C (candidate) and loved the Huey but never got any wrench or stick time on the twin version. Love those helicopters.

However, I absolutely LOVE the UH-1N aircraft that still operate in and around DC. Most are relegated to the US Park Service/Police. The whomp whomp of those 2 rotor blades is just amazing!

Username Protected · **Posted:** 07 Nov 2024, 00:10

Username Protected wrote:

I think we're forgetting that the combined propulsion (two engines into one gearbox) in the Lear Fan had a few things in common with the Twin Huey helicopter/Bell 212. The 212 got its civil certificate about ten years before the Lear Fan first flew so this engineering wasn't something out of the blue.

I think we went and talked with Bell people when we sister divisions, to get some insight. I think the issue comes down to regulations, preambles, advisory material, and interpretations being different.

Username Protected · **Posted:** 17 Nov 2024, 16:41

2002 through 2007 I owned and operated an Aerostar which I hangared at Love Field in Dallas. There was a dust covered strange looking airplane that I hazily recognized kept next to me in the big corporate hangar there. I asked the mechanic working on my airplane what it was... "the last existing Lear Fan" was the response. I was stunned at the disrepair it was in..

It is now two doors down at the Frontiers of Flight museum here at Love Field on display. I think there is another one in the Smithsonian Air and Space Museum.

Username Protected · **Posted:** 17 Nov 2024, 17:01

All three Lear Fan aircraft have been preserved. They are on display at:

- Museum of Flight in Seattle
- Frontiers of Flight Museum in Dallas
- In front of the Civil Aerospace Medical Institute at FAA, OKC

Username Protected · **Posted:** 04 Dec 2024, 01:16

I had a supervisor at Cessna who had previously worked at Cessna, quit to work at Learfan, and ended up coming back to Cessna.

Username Protected · **Posted:** 04 Dec 2024, 09:03

It’s almost always better to have one big engine than two small ones on a gearbox. More efficient, less weight and mechanical complications. Today it would have 1 big PT-6 or GE equivalent.

Pusher or tractor configuration? The market is used to a tractor appearance like the PC12 and Caravan, after 25 years that looks “right” to buyers, so no surprise Textron went that way with Denali.

A modern Learfan would be a pusher Denali, I guess, what would it have that the Denali does not?

Username Protected · **Posted:** 04 Dec 2024, 09:13

Username Protected wrote:

They're troublesome in piston engines because they have to build them cheaply.

the bigger reason is that piston engines have torque reversals. Which normally the designer would handle by just making the gearset bigger - but piston engines are also severely weight constrained.

It's why belt drives are (IMO) the more elegant solution for pistons

Username Protected · **Posted:** 04 Dec 2024, 11:51

Username Protected wrote:

It's why belt drives are (IMO) the more elegant solution for pistons

You’re right, but in the same vein that Jesse describes about how something “looks right” to consumers, there’s always somebody who’s convinced that belts are bad because his friend’s timing belt broke 30 years ago. Sometimes stubbornness is well-funded.

LearFan if built today