Time inverted without inverted systems

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Thatch
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Time inverted without inverted systems

Unread postby Thatch » Tue Nov 21, 2017 10:40 am

Hi

I have two questions for those with experience.
I have a share in a RV4 which has a 150hp carburettor engine, FP prop and no inverted systems.
Busy with aerobatic training but not on this aircraft. So far only doing rolls on the RV4.

My questions are :

How long can one stay inverted in certain manoeuvres without inverted systems and without doing harm to my aircraft? Obviously not to stay inverted long but such manoeuvres like hesitation rolls, loops, Cubans, split S etc all involve some inverted time. I know that these can all be done without inverted systems but how long before the spluttering starts :shock:

Secondly are all aerobatics done with the electric fuel pump on all the time?

Thanks
Reilly
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Re: Time inverted without inverted systems

Unread postby jimdavis » Tue Nov 21, 2017 10:55 am

Thatch wrote:Hi

I have two questions for those with experience.
I have a share in a RV4 which has a 150hp carburettor engine, FP prop and no inverted systems.
Busy with aerobatic training but not on this aircraft. So far only doing rolls on the RV4.

My questions are :

How long can one stay inverted in certain manoeuvres without inverted systems and without doing harm to my aircraft? Obviously not to stay inverted long but such manoeuvres like hesitation rolls, loops, Cubans, split S etc all involve some inverted time. I know that these can all be done without inverted systems but how long before the spluttering starts :shock:

Secondly are all aerobatics done with the electric fuel pump on all the time?

Thanks
Reilly


Thatch, for the engine, there is a big difference between inverted and negative G. You can go over the top of a loop but still maintain positive G

Your engine will not produce any power under negative G. You must learn to throttle smoothly all the way back before you experience negative G. It's very bad for the engine to suddenly cut from full power to no power due to fuel starvation - which is what happens as soon as you lift off your seat.

It's vital you discuss this with your instructor - particularly if you are training on an aircraft that does have inverted power.

Jim
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Re: Time inverted without inverted systems

Unread postby Thatch » Tue Nov 21, 2017 12:23 pm

Tks Jim

Yes we are training in his extra which has inverted systems and a CS prop so still lots to learn about my plane which cannot really handle two up aerobatics with the aft CG.
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Re: Time inverted without inverted systems

Unread postby jimdavis » Tue Nov 21, 2017 1:52 pm

Thatch wrote:Tks Jim

Yes we are training in his extra which has inverted systems and a CS prop so still lots to learn about my plane which cannot really handle two up aerobatics with the aft CG.


Thatch - it's really important that your instructor teaches you to fly his aircraft as if it had no inverted power. It will be a terrible shock to you, and to your aircraft if you try to fly it the way you are learning now.

I have to ask - is your friend and instructor, There is no law to say he may not show you how to do aerobatics, if he is not, as long as he doesn't charge you for instruction, and you don't log it as dual. I am not really interested in that side of it. But he really isn't doing you a favour by letting you use inverted power, when you won't have any in your aircraft.

Good luck - and have fun. Aerobatics is all about learning to fly an aeroplane accurately in any attitude - and having fun.

And having more fun :D

jim
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Re: Time inverted without inverted systems

Unread postby jimdavis » Tue Nov 21, 2017 2:03 pm

Oh! I didn't notice in your initial post - you said FP prop.

That makes a hell of a difference - obviously to the performance - but far more importantly - to your engine handling.

For instance, with a fixed prop you won't be able to use full power to gain airspeed for a loop. You initially get the nose down and use full throttle along your line feature, then you watch the rev counter, and as your airspeed increases you have to throttle back a bit to keep the revs within limits. Then as you pull up into the loop and the airspeed and revs decay, you feed in just enough power to control the revs, you will probably only be using full throttle when you are through 45 degrees or so nose up.

You really have to live with your engine all the way through aerobatics with a fixed prop.

Learning aerobatics is almost as much fun as your initial flying training - you are a luck man. :D

Enjoy doll :lol:

jim
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Re: Time inverted without inverted systems

Unread postby StressMerchant » Fri Nov 24, 2017 4:37 am

With a non-type certified aircraft, it is often difficult to answer questions such as "how long until damage occurs", because the system details can vary considerably.

For a type certified aircraft, the engine installation has to conform to FAR 23.2425, which states:
"a) The installed powerplant must operate without any hazardous characteristics during normal and emergency operation within the range of operating limitations for the airplane and the engine."

This is actually the latest version, until recently there was a separate requirement for negative acceleration in FAR 23.943. The older version will probably still be used as a basis for many years, and is worth remembering because most of the light aircraft we fly were designed around it. The older text specifies:

"No hazardous malfunction of an engine, an auxiliary power unit approved for use in flight, or any component or system associated with the powerplant or auxiliary power unit may occur when the airplane is operated at the negative accelerations within the flight envelopes prescribed in Sec. 23.333. This must be shown for the greatest value and duration of the acceleration expected in service." (My underlining)

During certification the aircraft must undergo negative G manoeuvres to demonstrate that it can withstand the accelerations and durations. For a non-aerobatic aircraft, the standard test involves a series of push-over manoeuvres where the G-loading (or absence thereof) is maintained for a specified period. The FAA offer an Advisory with suggested times and accelerations, which is the common benchmark. In the work I have done (for a non-aerobatic type) we used -0.5G for 5 seconds. The lack of an inverted oil system meant that the oil pressure started falling immediately, and this had a knock on effect on prop RPM as the oil supply to the governor was reduced.

Remember that although you may be measuring the "acceptable" inverted time from a fuel feed perspective, the engine may already be sustaining damage if the oil system is not feeding properly. And it won't be as obvious as "cough, splutter".

For an aerobatic type, the designer is expected to specify a duration and value of the negative acceleration, and then demonstrate in test that the limits are achievable with damage.

FWIW, a late friend of mine was involved with the training aircraft at the European Space Agency. ESA had an Airbus A300 that was used for zero-G parabolic manoeuvres. According to my friend, they had major difficulties setting up the oil and fuel systems for sustained zero-G. Negative accelerations are fairly straightfoward in that "clunk tanks" or multiple pick-ups can be used, but under zero-G there is no obvious pick-up point!

I don't know if it is still the case, but the Sports Aerobatics Club used to offer a "classic" class, which was specifically designed around aircraft without systems designed for inverted flight.

Enjoy the aerobatics. It is great fun, and aerobatics crowd are a great bunch of people.
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Re: Time inverted without inverted systems

Unread postby Leon le Roux » Sat Nov 25, 2017 9:12 am

Very informative, thanks. Would love to sit in on a casual mentor/ lecture to gain more insight into aerobatics introduction - technical and benefits of specifically rated versus capable aircraft and the techniques/ limitations required how to approach either.
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Re: Time inverted without inverted systems

Unread postby happyskipper » Sat Nov 25, 2017 10:01 am

Leon le Roux wrote:Very informative, thanks. Would love to sit in on a casual mentor/ lecture to gain more insight into aerobatics introduction - technical and benefits of specifically rated versus capable aircraft and the techniques/ limitations required how to approach either.


Surely this should be part and parcel of your aerobatic training?
If you don't come right, PM me, and I will put you in touch with some of the "greats" of aerobatic flying - they will be only too happy to have a chat and a refreshment...
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Re: Time inverted without inverted systems

Unread postby Chalkie » Sun Dec 24, 2017 1:31 pm

Thatch wrote:Hi

I have two questions for those with experience.
I have a share in a RV4 which has a 150hp carburettor engine, FP prop and no inverted systems.
Busy with aerobatic training but not on this aircraft. So far only doing rolls on the RV4.

My questions are :

How long can one stay inverted in certain manoeuvres without inverted systems and without doing harm to my aircraft? Obviously not to stay inverted long but such manoeuvres like hesitation rolls, loops, Cubans, split S etc all involve some inverted time. I know that these can all be done without inverted systems but how long before the spluttering starts :shock:

Secondly are all aerobatics done with the electric fuel pump on all the time?

Thanks
Reilly

Reilly, whoever is instructing you in the Extra should drum it into you, that the RV-4 is an aerobatics-capable aircraft and not an aerobatic machine like the Extra. In an RV you need to do "Harvard-style Aerobatics" with max +3.5G and ALWAYS stay positive G. If you do that you will enjoy the RV.

Stress Merchant has given you a comprehensive answer, what I could add is any negative G manoeuvre, besides reducing oil pressure to zero, will result in oil being pumped out the breather. You really do not want to operate an engine without oil pressure as this will lead to bearing damage. Spluttering will occur with zero or negative G, but the engine should continue to run, rather try to avoid negative G in your RV and use the electric fuel pump any time you anticipate or practice an unusual attitude. (Belt and braces approach.)

The thing you need to be acutely aware of is lowering the nose in an RV with a FP prop will see RPM and speed climb rapidly and if you have the Sensenich metal FP prop, then this is limited to 2600 RPM and any excursion above that will result in cumulative stress to the propeller and should be avoided at all costs. So aeros in your RV will keep you busy maintaining all parameters 'in the green.'

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