By Terry Dunn | [email protected]
As seen in the June 2025 issue of Model Aviation.
I RECENTLY ATTENDED A SWAP MEET, where I picked up a Multiplex Twin-Jet. This tailless model was popular approximately 25 years ago. I never had a Twin-Jet back then, so I was stoked to find one at a good price. This example is a bit rough around the edges, but I will have fun flying it. It still has the twin Speed 400 brushed motors from its original power system.
A funny thing I noticed is that the propellers were installed backward. This is a mistake that I often see on electric-powered models, especially with pusher aircraft such as the Twin-Jet. Let’s dig into why this is an error that you definitely want to avoid.
Propeller Overview
Propellers are essentially spinning wings. The cross-section of a propeller blade has an airfoil shape exactly like what you would see on the cross-section of a wing. Likewise, propellers have a leading edge (LE) and a trailing edge (TE), just as a wing does. As the terminology suggests, the LE should face the direction that the aircraft is heading, and the TE should face where the aircraft has been.
Notice that I said should. Many models will fly even if the wing is installed with the TE forward. In fact, I used to purposely install the wing backward on one of my models, just to prove to the doubters that it could be done. The model did indeed fly, but it was a handful. I always tried to play it cool and hide how much it was fighting me, lest I give the naysayers any shred of satisfaction. Those parlor-trick flights typically consisted of one trip around the pattern with a white-knuckle landing. The laws of physics were on my side, but they sure did protest!
When I say that the propellers on the Twin-Jet were installed backward, I mean that the TEs, rather than the LEs, were facing forward. Some people think that turning a propeller around will make the propwash blow in the opposite direction, but that is not the case. The air will still blow the same way whether the propeller is installed correctly or backward. Just like my backward-winged model, a backward propeller will still work to some degree. It will just not deliver the same level of performance as it would if it was correctly oriented.
Tractor Versus Pusher
To avoid clouding this discussion, I’m going to ignore specialty airscrews such as variable-pitch propellers and reversing propellers for 4D models. I will focus on the majority of model airplane propellers, which are intended to spin and blow air in only one direction. They are typically classified as tractor or pusher propellers. This terminology was easy to understand back when most models were powered with glow engines that were predominantly designed to spin only counterclockwise (as viewed from the prop nut). With modern brushless motors that can easily spin either direction, the legacy nomenclature can be confusing.
The reality is that either type of propeller can work equally well in a tractor or pusher configuration. Some propeller manufacturers have replaced "tractor" and "pusher" with "standard" and "reverse-rotation" to address this ambiguity. The only real difference is which direction you are supposed to spin the propeller. Tractor (standard) propellers are meant to spin counterclockwise (as viewed from the front of the propeller) and pusher (reverse-rotation) propellers are meant to spin clockwise. It is possible to install either type of propeller backward, with a subsequent loss in performance.
Pull Test
A correctly oriented propeller performing better than a backward propeller is something that I have always just accepted as fact, but I had never quantified the difference. I decided to take a scientific approach to determine exactly how much of a difference it makes by correctly orienting propellers.
My basis for comparison was static thrust. I used a digital fish scale to measure the full-throttle thrust of the Twin-Jet and three other models in my fleet. I recorded these measurements with the propellers backward and with them properly oriented. The results were rather surprising.
Let’s start by reviewing the Twin-Jet. I measured 12 ounces of thrust with the propellers in the incorrect, backward orientation. When I flipped the propellers around to the correct orientation, the thrust jumped to 25 ounces! With this model, the simple mistake of installing the propellers incorrectly results in a loss of more than half the thrust that should be generated. That would cause a huge decrease in flight performance for the airplane—especially during a hand launch. I wonder whether this is why the previous owner lost interest in the Twin-Jet?
I observed similar results while testing my other models. The loss of static thrust from backward propellers ranged from 46% to 59%. I did not expect the numbers to be so high, but the data doesn’t lie. Anecdotally, the propellers tended to make more noise and run less smoothly when installed backward.
If these results do not convince you to check your propellers for proper orientation, there is additional compelling data.
Using More, Getting Less
I also recorded wattmeter readings during my static thrust tests. Not only did all of the power systems produce significantly less static thrust when the propellers were backward, but they also pulled more electrical current (amps) while doing it! This means that the motors were working harder to produce less thrust. That’s a big double whammy!
In the case of the E-flite Carbon-Z Cub 2.1m, the power system pulled 52.5 amps when the propeller was installed correctly. That number jumped to 67.2 amps when the propeller was backward. That was a 28% increase in current while the system was making 46% less thrust. It should also be noted that this pushed the current above the ESC’s 60-amp rating. Simply put, there is no positive side to having a backward propeller.
Making It Right
Now that the effects of having a misoriented propeller are understood, the obvious question is, "How do you know which side of the propeller is the front?" For most molded propellers, the raised numbers denoting the size of the propeller are on the front side. Wooden and carbon-fiber propellers usually have printed numbers and/or a logo. Those will also be on the front side.
If you’re still not sure which side of the propeller is the front, try contacting the manufacturer for clarification. When all else fails, you can perform your own static thrust tests. There should be no question after that!
After you have identified the front of the propeller, make sure that you mount it facing forward on the model. Regardless of whether the propeller is being used in a tractor or pusher configuration, or whether it is standard or reverse rotation, the front of the propeller must face the direction that the airplane will fly. That is the only correct way.
A flying buddy told me that he sometimes intentionally reverses the propeller on his 1/2A Free Flight model. The point of doing this is to limit how high the model will climb and help prevent a flyaway on days with good lift. I think that is a pretty clever tuning trick. For the rest of us, however, we typically want all of the performance we can get, so check your fleet (especially the pushers) and make sure that all of your propellers are correctly oriented.
As always, I invite you to share your projects, tips, and opinions.
SOURCES:
E-flite
Multiplex
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