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Motorglider definition...huh?

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cluttonfred

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I do a lot of back-of-a-napkin conceptual design and I was wondering how hard it would be to get a homebuilt light airplane classified as a motorglider. The FAA definition seems to be: 1) one or two seats; 2) 850 kg (1874 lb) max gross weight; and 3) 3 kg/sq m (0.62-lb/sq ft) max span loading (weight/span squared).

What strikes me as odd about that is that with a gross weight of 650 kg/1433 lb and a span of 13.74 m/45' 1", a classic motorglider like the Fournier RF-5 does not seem to qualify because of its span loading of 3.44 kg/sq m (0.7 lb/sq ft).

What gives...?
 
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jedi

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I do a lot of back-of-a-napkin conceptual design and I was wondering how hard it would be to get a homebuilt light airplane classified as a motorglider. The FAA definition seems to be: 1) one or two seats; 2) 850 kg (1874 lb) max gross weight; and 3) 3 kg/sq m (0.62-lb/sq ft) max span loading (weight/span squared).

What strikes me as odd about that is that with a gross weight of 650 kg/1433 lb and a span of 13.74 m/45' 1", a classic motorglider like the Fournier RF-5 does not seem to qualify because of its span loading of 3.44 kg/sq m (0.7 lb/sq ft).

What gives...?
I disagree with the statement of the original post. It is not a requirement for US motor gliders. You have given the JAR definition.

The FAA references the JAR defination suppliied as one certification standard that will be accepted but does not require the a USA motor glider complies with that defination. This has been discussed in other prior threads.
 

cluttonfred

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That’s good to know, but could you please point me to whatever other standard is acceptable to the FAA? All I found was AC 21.17-2a (1993) which says:

b- Additional Criteria for Powered Gliders.
(1) Powered fixed-wing gliders may be type certificated under Section 21.17(b) if:
(i) The number of occupants does not exceed two;
(ii) Maximum weight does not exceed 850 kg (1874 pounds); and
(iii) The maximum weight to wing span squared (w/b2) does not exceed 3.0 kg/ma (0.62 lb./ft.1).
NOTE: These criteria originated from JAR-22.

Cheers,

Matthew

I disagree with the statement of the original post. It is not a requirement for US motor gliders. You have given the JAR definition.

The FAA references the JAR defination suppliied as one certification standard that will be accepted but does not require the a USA motor glider complies with that defination. This has been discussed in other prior threads.
 

Turd Ferguson

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It should not be difficult to get a homebuilt certificated as a motorglider as they are not type certificated under Section 21.17(b) (section revised since 1993). I think it depends on what boxes the builder checks during registration.
For example, Sonex advertises the Xenos can be registered as a glider or airplane (and flown as an LSA).The FAA registry confirms this.
 

Hot Wings

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The ASTM definition of an LSA glider:

1.2 This specification is applicable to the design of a light
sport aircraft glider as defined by regulations and limited to day
VFR flight.
1.3 A glider for the purposes of this specification is defined
as a heavier than air aircraft that remains airborne through the
dynamic reaction of the air with a fixed wing and in which the
ability to remain aloft in free flight does not depend on the
propulsion from a power plant. A powered glider is defined for
the purposes of this specification as a glider equipped with a
power plant in which the flight characteristics are those of a
glider when the power plant is not in operation.


To me it wouldn't be too much of a stretch to apply this definition to a part 23 certified glider? Of course the highlited part applies to - any - aircraft with the engine shut off.
 

cluttonfred

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Well, to make my questions more precise....

1) Is it possible to register an experimental, amateur-built aircraft as a motor glider in the USA?

2) If so, what are the criteria used to determine if an E-AB is, in fact, a motor glider?
 

Turd Ferguson

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Still think it's the box checked by the person registering the aircraft. Again, Sonex Xenos can be checked as either glider or airplane without any physical measurements to a standard. It would be a glider, which includes powered gliders.

The Operating Limitations may require the pilot to meet certain standards.
 

cluttonfred

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I have not studied every word of the seven documents posted by blane.c, but I find it hard to believe that “because I said so” is going to be good enough for the FAA. By that logic *anything* could be register as a motor glider.

Did you read post 6?
There is no criteria other than owner choice.
 

blane.c

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In the following definition I think the words "depend principally on" are important.

Glider:
A heavier-than-air aircraft, that is supported in flight by the dynamic reaction of the air against its lifting surfaces and whose free flight does not depend principally on an engine.

Classes:
None.
 

Tiger Tim

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I have not studied every word of the seven documents posted by blane.c, but I find it hard to believe that “because I said so” is going to be good enough for the FAA. By that logic *anything* could be register as a motor glider.
It’s probably a rare enough case that the FAA figures their inspectors can filter out the obvious cheaters and wanted to save endless complexity in the regs. Like a fat UL you just make it look like you’re trying to follow the rules and you’ll probably get away with it forever. If it were me I’d want to see long wings and a means of starting the engine in flight. IIRC Rutan’s Ski Gull was registered as a glider.
 

cluttonfred

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OK, but what if somebody says that they have modest performance goals and tries to register a Volksplane as a motor glider? If there are no criteria, on what grounds does the FAA deny the application?
 

blane.c

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I would expect that at the minimum an expectation of an ability to glide would be reasonable even if not specifically called out. What is reasonable? So as not to mess up this privilege is a good question. I would not suggest that a craft without an ability to glide try to be registered as a glider as it could mess it up for everyone else with legitimate cause.

Perhaps an attitude to police ourselves would be prudent.

At least a guideline for minimum sink rate?

Ideas?
 
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