Puller vs Pusher Aircraft - Which is More Efficient?

one of the first decisions when designing an airplane like the darker one is where to place the engine and propeller and that's usually driven by the mission of the airplane the darker one is intended to fly fast while maintaining high efficiency and our prop is at the front of the airplane even though it's long been theorized that placing the prop at the back so it pushes the airplane would be better for Speed and efficiency so why is our prop at the front or would the darker one actually be better off with the prop at the back let's get into it before we get too far in our comparison of Pusher versus puller we have to establish some assumptions so that we're making a fair comparison we're exclusively going to be looking at conventional configuration aircraft that are single engine and single propeller which will look like this the arrangement with the conventional configuration and engine and propeller at the front of the airplane is probably what you've seen most commonly in light general aviation aircraft so the popular lancers and glass airs and the Cirrus Sr series aircraft all look like this as well as the Vans RVs and the dark Arrow one less common but interesting for the context of this discussion is the conventional configuration with the propeller in back probably the most notable Arrangement like this is the bd5 the Cirrus vk30 their first aircraft design followed this Arrangement and the Solara 500L a new design that's just come out looks like this so we're just going to be focusing on these Arrangements we're not going to be evaluating Canard configuration or multi-engine Arrangements you can think of this discussion as can we make the darker one better if we put the propeller at the back of the airplane probably the biggest performance metric that's going to be impacted by propeller placement is the propeller efficiency for propeller efficiency is defined by this formula Ada our prop efficiency equals TV over p t is propeller thrust V is our air speed and P is power at the prop shaft tractor Arrangement propeller in front is going to give us the best shot at maximizing propeller efficiency because the prop is spinning through smooth undisturbed airflow in this Arrangement and I've tried to show that with these straight blue streamlines coming into the prop a reasonable number to expect for prop efficiency in this Arrangement is around 85 percent so if we had 100 horsepower at the prop shaft we could expect to see about 85 horsepower delivered to the air in propulsive power we want to maximize the amount of power delivered to the air from the propeller shaft and that's why having a high efficiency is important if we were to move the propeller to the back of the airframe in a pusher Arrangement we'd expect to see a decrease in propeller efficiency that's because as the airflow passes over the airframe the airframe disturbs the airflow and changes the direction of the airflow a little bit and I've shown that here where our stream lines which are now squiggly because they're turbulent are curving in a little bit following the taper of the empennage so this disturbance to the airflow as it enters the prop disc has a negative impact on Prop efficiency that hit would be around five to ten percent loss in efficiency so now instead of 85 percent we're looking at 75 to 80 percent efficiency and instead of 85 of our 100 horsepower being delivered to the air we're looking at 75 to 80 horsepower delivered into propulsive Power based on this analysis alone we would conclude that the tractor or puller configuration is best for the dark Arrow one but this isn't the entire story there are more variables that we have to bring into our analysis to see the complete picture even though the tractor Arrangement has better propeller efficiency compared to The Pusher Arrangement it actually has worse airframe drag and that's because the propeller is sending a wake of disturbed accelerated airflow over the rest of the airframe which has a negative impact on drag if you watched our video on Wing aerodynamics you'll remember we said that drag is proportional to Velocity squared so if we have accelerated faster air passing over the airframe can have more drag compared to unaccelerated flow and because the propeller is disturbing the airflow it's going to trip up or would otherwise be laminar flow on the wing and increase drag that was the other thing we said in our wing aerodynamics video is we want to maximize laminar flow we want here going over the wings in smooth laminar sheets turbulent flow like this has more drag we can minimize any airflow disturbance over the airframe by putting the propeller at the back of the airplane like this now the airframe is flying through clean undisturbed air and that's going to have less drag compared to our tractor Arrangement how much is this worth in terms of a performance increase or drag reduction that's a little bit harder to calculate compared to our propeller analysis because this prop wake is actually a transient effect as the prop blades pass by they create pulses of disturbed accelerated air that pass over the airframe and then once the prop blade passes by the flow Smooths back out the prop blade wake looks like this as it's passing and then in between the blades it's smooth like this that makes the analysis tricky still we can make some assumptions and make some rough calculations to evaluate the dark Arrow one versus a hypothetical Pusher dark Arrow one when we run the drag analysis for a constant velocity we see that the drag is less in The Pusher Arrangement how do we now compare this to prop efficiency or power how do we get between these two because over here we have efficiency and power numbers and over here we have drag in cruise on accelerated flight thrust is equal to drag knowing that we can take our drag value and plug it into this equation for t to see what impact a drag reduction would have on requiring less power or requiring less prop efficiency when we run that check on our hypothetical dark Arrow one The Pusher arrangement in a specific economy Cruise scenario putting out 100 horsepower we see the airframe requires about five horsepower less to push through the air compared to our existing design so even though we're getting less power delivered to the air with less efficiency in our Pusher Arrangement the airframe requires less power to push through the air we see from this analysis we can't just look at the propeller efficiency we can't just look at the airframe drag we have to evaluate both the prop and the airframe together as a system because they work together and impact each other's performance running that analysis we see it's actually pretty close between the two at least close enough that the error in our assumptions creates overlap and these evaluations so we need to bring in more variables to determine which is better Pusher or puller for the dark Arrow one we'll head over to the prototype to discuss what else we need to bring into account engine cooling will be impacted by the propeller placement and it's normally considered a bit easier to achieve adequate Cooling in an air-cooled engine if the propeller is at the front of the airplane that's because we have this big fan blowing air over the engine and carrying heat away that's going to be helpful in any challenging cooling scenario like taxiing on the ground or in a climb where you have a high power setting but low air speed it's still possible to achieve adequate cooling with the propeller at the back of the airplane but it normally requires a little bit more attention and refinement of the cooling setup either way you go tractor or Pusher Arrangement the cooling normally requires iteration and testing to get everything dialed in and we're anticipating that with the dark Arrow one our cooling setup uses these aluminum plenums to duct airflow over the cylinder heads and carry heat away from the engine and they're paired up with these 3D printed inlets that were made on a pretty fancy industrial 3D printer that we don't have so we teamed up with the sponsor of this video isometry to get these manufactured one of the things we really like about zometry is their online quoting tool if you have a 3D model of a part that you want manufactured you can upload it to their website and get an instant quote on how much it will cost to manufacture you can also play around with different variables like the part quantity the dimensional tolerances in the manufacturing process to see how that's going to impact cost we use this tool quite a bit both for 3D printed parts and machine parts and we anticipate we're going to be using it a bit more as we go through flight testing and refine and iterate on the cooling setup I'll leave a link in the description of this video if you want to give zometry a try for yourself the general conclusion is that it's easier to achieve adequate cooling with the propeller at the front of the airplane so for our performance analysis is saying it's a wash where we place the propeller we might as well pick the location that gives us a leg up in terms of cooling another Factor we need to consider is accessibility and serviceability of the engine we have really good access to the engine with the propeller and engine mounted together on the firewall like you see here so if we need to get at the engine to change the oil or just to take a peek at it in testing all we have to do is pull the column off and we can see the engine really easily we can also pull the entire engine off the airframe if we want to mess with any of this Hardware or change anything on the firewall that's not necessarily the case for a pusher arrangement with a convention no configuration aircraft because you can't Mount the heavy engine directly on the tail next to the propeller that would make the airplane tail heavy the way around that is to mount the engine further forward closer to the wing and closer to the center of gravity and then connect the engine and propeller with a long drive shaft the problem there is it makes the engine a little bit harder to get at and harder to pull off the airframe if you want to modify anything or change anything with the engine installation I think there are ways around this you could do a Canard arrangement with the engine at the very back of the airplane but that's outside the scope of this video and we already have a pretty good solution here with the engine mounted at the front of the airplane there are a few more factors I think we should consider or at least bring into this discussion we'll head over to the marker board to talk about those and wrap this up we covered how prop placement affects performance in terms of the prop efficiency and airframe drag and also covered some of the potential impacts on engine Cooling and serviceability but there are other considerations that are important the prop clearance is inherently good in the tractor Arrangement when we're nose high at high angle of attack like during takeoff and Landing because the the nose being high moves the prop further away from the ground and decreases the risk for a prop strike it's the opposite situation for The Pusher arrangement with the prop at the tail because the tail being low decreases proper clearance to the ground it increases the risk for a prop strike this is a solvable problem for The Pusher configuration the path forward is to mount the prop higher up on the airframe or we can make the landing gear longer to further increase the prop clearance or the tail could be built inverted so that it serves as sort of a protective structure that would hit the ground before the prop hits whatever solution we pick this has to be considered in the design phase because we don't have inherently good prop clearance like we do with the tractor configuration I think it could be argued that you could still taxing the things and hit the prop on something with the tractor Arrangement and The Pusher Arrangement has the prop sort of protected at the back of the airplane so maybe the wing tip or nose would hit first but yeah maybe you could argue that the prop clearance or prop strike issue is sort of a wash between these two configurations something that definitely is better with The Pusher configuration though is pilot visibility with the prop and engine mounted at the back of the airplane we don't have to look over the cowling or look through the prop disk so that improves visibility and then on top of that with the heavy engine and prop being mounted aft we have to keep the airplane balanced so the solution there is to mount the cockpit and pilot forward to keep the CG within an acceptable window and that decreases the length of the nose that you have to look over and that further improves visibility and the improved visibility alone I think would make The Pusher Arrangement worth pursuing for certain applications the last consideration we'll discuss is the Aesthetics or the cool Factor no one's going to argue with the tractor Arrangement it's classic tried and true and this is pretty much what everyone expects an airplane to look like but I think there's more potential to make the airplane look cool with The Pusher configuration because the nose can be more streamlined and more pointy and whether it's better performance or not it looks faster sitting on the ground and that's worth something maybe just in terms of marketing there's still more to discuss but we can keep the conversation going in the comments section of this video we have more videos coming out talking about the progress we're making on the darker on Prototype so stay tuned for those otherwise thanks for watching we'll see in the next one [Music] foreign

Channel: DarkAero, Inc

Views: 218,884

Keywords: experimental aircraft, carbon fiber, experimental aircraft build, darkaero, darkaeroinc, carbon fiber plane, airplane build, experimental airplane, experimental airplane build, experimental airplane kits, homebuilt aircraft, homebuilt airplane, homebuilt aircraft construction, building an experimental aircraft, building an airplane, propeller, efficiency, engine, pusher propeller, tractor propeller, cirrus aircraft, vans aircraft, lancair, glasair, canard

Id: ZmIW4fTNVwg

Length: 11min 56sec (716 seconds)

Published: Fri Sep 22 2023