Rotary engine and blip-switch again

[CGE]

Hello to everyone,

I am the new guy. I'm a student of aerospace engineering at the university of Stuttgart, Germany. I am very keen of learning more about the first steps of aviation and i already learnd a lot from this forum thanks!

But I don't get a thing: Why does a rotary don't have a throttle or just a very limited one? Why dosen't it have a normal carb?

Did the other non-rotary engines like the BMW IIIa had a normal carb with a full range throttle? Did they also have a blip switch?

Thank you for any respond ;-).

cheers

Robin

PS: I used the search funktion but didn't find a answer to "why"....

[Doug Bigler]

The Le Rhone and Bentley rotary engines had carburetors. I've read that the blip switch is necessary during landing because the rotary engines still provide a lot of thrust even at lowest operating RPM (it's on record that the Sopwith Pup flew 25 mph above stalling speed at 600 RPM). Here is a link to the 80HP Le Rhone Instruction Manual in case it's helpful:
http://www.uwm.edu/Libraries/special...ne/LeRhone.pdf

[Greybeard]

Originally Posted by CGE

...Why does a rotary don't have a throttle or just a very limited one? Why dosen't it have a normal carb?

Indeed, I too couldn't find in literature an "official", "scientific" and "shared" reason for that after years of study, reading and research about aviation and especially Otto cycle engines.

My opinion is rotary couldn't have a carburettor and, more precisely, a throttle because it aspired mixture from its crankcase, at least Gnome types, under the effect of centrifugal forces. This required induction valves counterweighted and a pressure into it with littlest possible disturbes. On the contrary, a throttle makes of course a vacuum; this latter would have caused poor volumetric efficiency and, as a consequence, low power output. Le Rhone and Clerget did have external induction pipes (as well as valves) and this allowed to have a carburettor.

Did the other non-rotary engines like the BMW IIIa had a normal carb with a full range throttle? Did they also have a blip switch?

Yes, all stationary types did have a carburettor but not a blip switch, as far as I know.

GB

[CGE]

Thanks you two!

I read your replys carefully, also this manuel. I came to the point that the later rotarys where equipped with a (limited) throttle.

The later rotarys had a inlet-pipe on top of the cylinder, the air / flue mixture will have to flow against the rotary force, this causes a pressure loss. So you can't close the carb-valve to far, the sum of all pressure losses will be to big. You need a blip switch to reduce power under that point.

That would explain why the linear engines had a fully operating throttle and no need for a blip switch.

Do you agree with that explanation?

Cheers

Robin

[shackleton]

In a way it depends on how you define a carburettor. If it is defined as a device for mixing air and fuel to provide a charge that can be ignited for an engine you can say that the crankcase of rotary engines meets that definition. Some form of throttle valve is necessary so that the mass (not the volume) of air mixed with the fuel is correct at different altitudes.

[Greybeard]

Originally Posted by CGE

Do you agree with that explanation?

Not yet.

First of all, can be said that all rotaries did have a carburettor (which scope and definition is to provide a constant air to fuel rate mixture to engine, otherwise everything could be named carburettor) but Gnome types (from early Omega to late Monosoupape N).

Early Gnome did have an inlet valve on their pistons head, balanced to counterweight centrifugal force, but that must open under slight pressure difference caused by piston's induction stroke.

Monosoupape had openings in their lower side of cylinders to allow rich and slightly pressurized mixture from crankcase to enter cylinder, where mixed with fresh air aspirated from just the single valve (i.e.: mono soupape).

In both instances must have happened that to reduce pressure in crankcase by throttling a carburettor would affect seriously power output.

On the contrary, all other types that aspirated mixture from a dedicated inlet valve, being this latter operated at will by mechanical devices (push rod and so on) were not slaved to complex effects of centrifugal forces and pressure differences, to feed theirselves.

[shackleton]

On the contrary, all other types that aspirated mixture from a dedicated inlet valve, being this latter operated at will by mechanical devices (push rod and so on) were not slaved to complex effects of centrifugal forces and pressure differences, to feed theirselves.[/QUOTE]

Interesting, but other types were affected by centrifugal force altering the effective opening and closing time of valves. This is a quote from Engines, carbs and magnetoes.

"Le Rhone engine. When this engine is revolving
at moderately slow speeds, the exhaust valve closes
at forty degrees before top dead centre, while at the
normal rate of revolution for this engine the exhaust
closes five degrees past the top dead centre. The
reason for this is that mechanically the valve closes
at forty degrees before top dead centre, but owing
to the influence of centrifugal force at high speeds
the tappet rod is hurled outwards, thus depressing
the exhaust valve and keeping it open until five
degrees past top dead centre. Similarly, in the
case of the Monosoupape engine, the exhaust valve
is made much heavier than mechanical strength
demands of it, purely to counteract the influence
of centrifugal force on the tappet rods. (push rods) Thus, by
making the valve heavier, the necessity of having
to fit balance weights to the tappet rod,(We English would probably prefer to call that a push rod) as is done
in the Gnome, is obviated.

From the particular examples given, it is ap-
parent that it is absolutely essential, in replacing
any part of an engine which is affected by centrif-
ugal force, that the weight of the new part should
be of equal weight with the one it is replacing. "

He does mention that blip switches could lead to problems if the petrol supply was not closed off while using them - I assume he means if the engine is shutdown using the blip switch in a long approach glide it would continue to windmill around with excess unburnt fuel accumlating in it and a fire risk or difficulty in restarting the engine in the air. Some blip switches were arranged to cut the ignition to only some of the cylinders allowing a measure of control over engine speed.

[Greybeard]

Originally Posted by shackleton

Interesting, but other types were affected by centrifugal force altering the effective opening and closing time of valves.

Yes, I know. But operated valves are still better than devices mentioned about Gnomes, since you've more and better parameters to act with.

He does mention that blip switches could lead to problems if the petrol supply was not closed off while using them ...
Some blip switches were arranged to cut the ignition to only some of the cylinders allowing a measure of control over engine speed.

Yes, ignition cut-off must be limited in time (say, ten seconds) to avoid serious problems (even fire).
Gnome Mono type N did have different combo of cylinders to cut-off, approximating regulation by throttle.

...But CGE is still listening?

[shackleton]

...But CGE is still listening?

I'm not sure but I find your contributions very interesting :-)

[The PIPE]

Dear Fellow Rotary/Umlaufmotor Fans:

The PIPE Here...don't worry about the term "umlaufmotor"...it's Deutsch for "rotary engine", as in WW I aircraft....that's all!

I was reading this thread, and remembering about how Cole Palen's pilots at ORA in the "good ol' days" were flight-demonstrating how the 160 hp Gnôme Monosoupapes had a "selector" switch that stopped the ignition going to a certain number of cylinders (I THINK it was in "multiples of three", firing either only three, six OR all nine cylinders when the blip switch was released)...they did this in the Palen/deFlavia Camel, which has used one, and Brian Coughlin, who did so in possibly BOTH of the Fokker D VIIIs he built, and which were both powered with 160 hp Gnôme Monos...

...and to my surprise the Germans also had very similar selector switches available for their Oberursel umlaufmotors, as evidenced from reading some of my back issues of Leo's WW I AERO quarterly periodical, quite some time ago.

Also, when the question of a "proper throttle" ever existing on a rotary engine came up as the main thrust of this thread, I distinctly remember what I read in the Siemens-Schuckert D III and D IV Aircraft in Profile monograph about the Siemens-Halske Sh.III eleven cylinder, counter-rotating (crankcase, cylinders & prop going counterclockwise, the "innards" going the other direction) umlaufmotor that those late war fighters had for a powerplant, being the ONLY one having a "proper throttle", as mentioned in the old A.I.P. monograph.

ALL the rotary engines used in WW I (and before) to my knowledge, had some sort of variable sets of controls to both adjust the spark timing (advanced/retarded), and the fuel mixture strength (lean/rich), but for a "throttle" of ANY sort, well, that's what the blip switch AND selector switch were supposed to be for...

...could that unusual ELEVEN cylinder Siemens-Halske Sh.III series of umlaufmotoren be REALLY the only one that had the same sort of "throttle" as any in-line engine (Hisso, R-R, Liberty, Mercedes, Hiero, BMW, SPA [Italy], etc) would have had in World War I ???

"Just CURIOUS" and "asking", that's all...!!!!

Yours Sincerely,

The PIPE!