Advantage of push-pull

I have been thinking what are the advantages and disadvantages of push-pull configuration. Everyone knows that push-pull has both disadvantages of pusher and tractor configuration but also implements a simple to control center line thrust operation for a critical single engine situation. However, there is more than that to it.

If you think one-of-a-kind aircraft, e.g. what Burt Rutan used to do during the early years. You want to build a twin on a shoestring budget. Then you realize that you have to buy two of everything. What if you have two engines already hanging around but they are not exactly the same make, model and horse power.

In case of center line thrust, no problem. Nothing requires the two engines to be the same. Not even weight and balance. Burt Rutan’s Voyager is an example. You can find that the front engine is different from the rear engine.

It might not be because of the reason described above, but if you are into auto conversions and designing a twin, how you plan to get two identical engines for not much cost at all (from totalled cars for example). Might prove to be a challenge, especially in a country like Finland where the population and the availability of engines might be poor. With center line thrust you can use different engines in the front and rear.

By the way: Merry Christmas and Happy New Year!

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    • dodlithr
    • December 28th, 2008

    Some car engine+PSRU ideas:

    I have been studying some car engines for small aircraft and found the Saab Aero's 250 hp turbo engine rather interesting:

    http://www.bildelsbasen.se/?index=stock&link=search&company_id=140&searchmode=1&listmode=3&vc%5B%5D=187103100&vc1=187&vc2=103&pc%5B%5D=120101100&pc1=120&sortfield=vehiclemodelyear&sortorder=1&page=1&PHPSESSID=3dd99b9fc05b339d6ce85e96e4cc30f5

    search:
    "saab delar B235R motor"

    You should be able to get those in Stockholm in large quantities and ship them easily to Finland.

    ————-

    The turbo car engine does not loose power in high altitudes and should run without modifications below 15 000 ft (?). All car engine modifications need the belt reduction unit (PSRU) and stronger bearings for the propeller.

    http://www.epi-eng.com/propeller_reduction_technology/implementation_overview.htm

    Janne.

    • dodlithr
    • December 28th, 2008

    Some car engine+PSRU ideas:

    I have been studying some car engines for small aircraft and found the Saab Aero's 250 hp turbo engine rather interesting:

    http://www.bildelsbasen.se/?index=stock&link=search&company_id=140&searchmode=1&listmode=3&vc%5B%5D=187103100&vc1=187&vc2=103&pc%5B%5D=120101100&pc1=120&sortfield=vehiclemodelyear&sortorder=1&page=1&PHPSESSID=3dd99b9fc05b339d6ce85e96e4cc30f5

    search:
    "saab delar B235R motor"

    You should be able to get those in Stockholm in large quantities and ship them easily to Finland.

    ————-

    The turbo car engine does not loose power in high altitudes and should run without modifications below 15 000 ft (?). All car engine modifications need the belt reduction unit (PSRU) and stronger bearings for the propeller.

    http://www.epi-eng.com/propeller_reduction_technology/implementation_overview.htm

    Janne.

    • Karoliina Salminen
    • January 2nd, 2009

    Auto turbos do not work as they are in aircraft without modifications, because the maximum spinning speed of turbo gets exceeded at high altitude. Aircraft use requires possibly computer controlled waste gate (old-fashioned aircraft often use manually controlled waste gate, but it is not good user interface).

    Also Below 12000 feet is not very high altitude, it is easily achievable without turbo.

    When I am talking about single turbo, I mean altitudes above 20000 feet, with two turbos, I mean altitudes over 40000 feet up to 60000 feet. 25000 feet cruise altitude already helps in getting over the weather a bit, but it is not over highest clouds. In fact in some cases it means flying in badly icing conditions inside cloud tops. Therefore a lot higher cruise altitude would be required to completely escape the weather and that can be accomplished with special two turbo arrangement with low pressure (low pressure turbo compesses air for the second high pressure turbo) and high pressure turbos and inter-cooler and after-cooler. Burt Rutan’s Raptor UAV has that kind of modification done to Rotax 912, and it can reach altitude up to 63000 feet. The turbos on that engine are as big as the engine by itself.

    Aircraft engine would benefit also from dry sump lubrication, normal flight causes situations (e.g. long steady turn to one direction) where standard car engine lubrication may not be enough.

    I am looking for engines at hp range 80-120, I don’t want more than that much of power per engine because otherwise fuel consumption becomes outragiously prohibitive. Economical aircraft goes fast because of the efficiency of the airframe, not because of a large horse power engine. There is no escape for the specific fuel consumption, and any engine, producing 250 hp, drinks a lot of fuel, no matter if they are old-fashioned Lycosaurs or modern car engines.

    There is also one more thing in the car engine which needs to be taken into account. The ECU is tuned for car use, and it interprets high power as an acceleration and the engine runs rich. To save fuel, the engine should be running instead lean of peak in cruise. This requires replacing the ECU with one that has aircraft settings which prevents the engine running in open loop mode at cruise power (which is in car terms high power, usually only used in acceleration).

    There are multiple challenges in adapting car engine for aircraft use, but it is doable. It just is not very easy and does not work out of the box just like that.

    • Karoliina Salminen
    • January 2nd, 2009

    Auto turbos do not work as they are in aircraft without modifications, because the maximum spinning speed of turbo gets exceeded at high altitude. Aircraft use requires possibly computer controlled waste gate (old-fashioned aircraft often use manually controlled waste gate, but it is not good user interface).

    Also Below 12000 feet is not very high altitude, it is easily achievable without turbo.

    When I am talking about single turbo, I mean altitudes above 20000 feet, with two turbos, I mean altitudes over 40000 feet up to 60000 feet. 25000 feet cruise altitude already helps in getting over the weather a bit, but it is not over highest clouds. In fact in some cases it means flying in badly icing conditions inside cloud tops. Therefore a lot higher cruise altitude would be required to completely escape the weather and that can be accomplished with special two turbo arrangement with low pressure (low pressure turbo compesses air for the second high pressure turbo) and high pressure turbos and inter-cooler and after-cooler. Burt Rutan’s Raptor UAV has that kind of modification done to Rotax 912, and it can reach altitude up to 63000 feet. The turbos on that engine are as big as the engine by itself.

    Aircraft engine would benefit also from dry sump lubrication, normal flight causes situations (e.g. long steady turn to one direction) where standard car engine lubrication may not be enough.

    I am looking for engines at hp range 80-120, I don’t want more than that much of power per engine because otherwise fuel consumption becomes outragiously prohibitive. Economical aircraft goes fast because of the efficiency of the airframe, not because of a large horse power engine. There is no escape for the specific fuel consumption, and any engine, producing 250 hp, drinks a lot of fuel, no matter if they are old-fashioned Lycosaurs or modern car engines.

    There is also one more thing in the car engine which needs to be taken into account. The ECU is tuned for car use, and it interprets high power as an acceleration and the engine runs rich. To save fuel, the engine should be running instead lean of peak in cruise. This requires replacing the ECU with one that has aircraft settings which prevents the engine running in open loop mode at cruise power (which is in car terms high power, usually only used in acceleration).

    There are multiple challenges in adapting car engine for aircraft use, but it is doable. It just is not very easy and does not work out of the box just like that.

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