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  1. #1

    Question Difference between speed and thrust

    What is the difference between speed and thrust? And what exactly is thrust? From what i have read, a smaller, high pitched prop produces more speed and less thrust than a larger diameter, lower pitched prop. Now if i understand this correctly and in extremely basic terms....the larger diameter chews up more surface area in the air which gives the plane more "pull" or thrust at a stand still and the pitch must be lowered in order to maintain RPMs. And a smaller diameter prop can have a higher pitch which allows the plane to travel faster with higher RPMs but due to the lack of surface area, you sacrifice thrust. Someone correct me if i am wrong please. Now would a larger diameter prop with the same pitch of a smaller diameter prop produce more power or thrust than the smaller diameter prop (pretending engine performance and RPM is the same regardless of prop size)?

    Thank You all for help.

  2. #2
    combatpigg's Avatar
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    You have the right idea.
    Go find a free thrust calculator to download and to play with. It is fun to see the HP that is required to turn the various combinations
    WHO GUNNA FEED MAW KEEEIDS..???

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    Horsepower is basically Torque x RPM, Turning an 11 x 10 at, say, 17,000 RPM requires more horsepower than turning a 6 x 10 at 17,000 RPM, because the larger prop needs more torque than the smaller prop. When an airplane is flying along level at a steady speed, thrust is equal to drag. I'm sure folks more expert than me will elaborate.

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    Be careful what question you ask...Thrust is a force, mass x acceleration. Speed is a scalar measurement; distance divided by time. Thrust always has a vector, that is a direction, speed does not, it doesn't care where it's pointed. That's the difference between thrust and speed. Not the answer you wanted? Better rephrase the question...
    Evan, WB #12.

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    Hi Wikit,

    The analogy normally given is that low pitch is like low gear in your car, you can't go as fast in low gear but you sure need it to start off up hill while towing something heavy. High pitch gives you a higher speed but acceleration is less, same as high gear.

    So you choose a pitch that suits yor model/flying style (eg high for a pylon racer, low for low speed, good rate of climb models eg vintage or 3d hoverers) then adjust the diameter of the prop to put the engine in the rpm band that you want. Low pitch will result in a larger diameter and vice versa.

    There's a bit more to it, it's more efficient to accelerate a large amount of air by a small amount so the large diameter/low pitch prop is more efficient. Then again a large diameter prop will have the tips going faster at the same rpm, which can result in higher drag slowing the engine and can give more noise too.

    It's a compromise like everything else in flying. Try several different brands/sizes of prop (they aren't that expensive) and fly what you like the most.

    Dave H
    Ultra Sport Brotherhood #89

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    The second part of you question makes assumptions about 'more' and 'less' thrust. Be careful. The faster the model is flying the greater the thrust from the engine/propellor. Thrust, when the model is flying steady state, straight and level, is equal to drag. The faster the model is flying, the greater the drag, and the greater the thrust. So, in your case, the smaller, high pitch, higher revving prop is creating more thrust. It is that simple. What you are alluding to is how efficiently that thrust is created. And that is entirely dependant on what you want the model to do. Put very simply, if you have a high drag model, that can't fly very fast anyway, there is little point using a 'high speed' engine/prop combination, and vice-versa. And as always, the best prop will only be found by experimentation (fly the thing) and likely won't be the prop you thought would be best.

  7. #7
    Moderator BMatthews's Avatar
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    Higher pitch props are typically operating in a stalled condition when on the ground or at low flying speeds if full throttle is being used. That and the smaller diameter is why higher pitch props have less thrust or pulling power during takeoff and during slower speed climbing.

    But once up to where you can level out and the model can accelerate up to where the speed is closer to the pitchxRPM speed of the prop and engine the higher pitch can pull a CLEAN, AERODYNAMICALLY EFFICIENT model to pretty high speeds. But if it's a big dumpy and draggy trainer like model the model's drag may well hold the engine and prop back from getting to that speed where the prop can un-stall and the engine pick up extra in flight RPM.

    Taken to extremes it may be that flying level isn't enough to un-stall the prop. In such cases you may need to labour up to some height and then level out and push into a mild dive to get to where the model flies fast enough for the prop to un-stall and the engine to pick up the extra RPM that will occur from the drop in drag of the prop blades when they un-stall. From there with appropriate maneuvers and care you should be able to keep the model flying at the higher speed range. But pull some hard G's or hold a long steep climb for too long and the model can slow down to where the prop sees a big enough increase in the angle of attack on the blades that it again stalls the blades and the engine pulls back in RPM as a result.

    All of this takes some tuning as the prop that delivers the most gain in top end flying speed has to be tuned to the model design and the engine being used. And of course it also implies that you need a strong engine with good horsepower.

    For the bigger diameter and finer pitch you MOSTLY have it right. All you said is true. But a big part related to keeping the pitch lower is to avoid the blades seeing a high enough angle of attack that they operate in a stalled condition. For example it's been my experience from prop and electric motor testing that the sound of the props alters to a big degree at 5 inches pitch. Give or take depending on brand and model of prop. Almost all 4 inch pitch props have a singing sound even on the ground when spinning fast. But at around 5 or 6 inches pitch this changes and in the case of motors the amps needed rises quickly and the thrust per amp drops like a brick.

    So who gains from bigger and finer pitch props? Folks flying 3D models that spend a lot of time hanging in hover or flying around in Harrier mode for starters. Hover or static on the ground, the conditions are the same. The prop isn't going anywhere and the air is coming to the blades at the same angle of attack. Sailplanes that need to climb at high angles but at moderate to slower speeds due to power limitations gain by using a moderate pitch prop suitable to the speed encountered during the climb.
    Witty saying to be plagarized shortly.....

  8. #8
    good info though thanks everyone

  9. #9
    Oh I forgot to ask, what is a stall exactly? I always thought it ment if you fly upwards too steeply, it was going to cause the engine to stall or draw too many amps off of your electric but from what you all have said, thats not the case

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    Quick answer...Stall is when the airflow around a wing is no longer smooth. With the small airfoil of a prop this happens around 8~9 degrees angle of attack. So, most props, which is just a small rotating wing, are stalled while the airplane is standing still. Lift, (thrust) is still being generated, mostly just at the tips, but the rest of the prop is just 'stirring' the air, and there is a lot of drag. As the airplane starts to move, the 'apparent' AOA starts to fine off, more of the prop unstalls, and the engine will start to increase revs ('unloads'). This is why 'static thrust' is pretty meaningless, and why low pitch props seem to have more thrust while the airplane is on the ground, more of the prop is unstalled.
    Evan, WB #12.

  11. #11
    So when the plane is in a stall in the air, is it just kind of free falling?

  12. #12
    Moderator BMatthews's Avatar
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    Quote Originally Posted by wikitjuggla View Post
    So when the plane is in a stall in the air, is it just kind of free falling?
    Well, yes and no.

    When the airfoil is stalled it has a big turbulent bubble on the top side which trails behind the wing. You can see this sort of image on any basic description of lift and airfoils that includes pictures.

    But just because the air breaks away from the airfoil and creates the stalled condition it doesn't mean that the lift goes away. But why the airplane drops the nose and goes into a dive is because with that sudden break away of the upper surface airflow comes a HUGE increase in drag. And without lots of power the speed drops fast when the stall occurs and the wing no longer has enough lift to keep the airplane up. And when that happens the airplane does drop. But the tail has enough effect on all this to hold the tail back and the nose drops off into a dive. At that point the speed rises, the wing gets back to work and the pitch stability built into the trim of the plane raises the nose back so it flies away at the original trimmed airspeed..... unless the pilot is still holding a lot of back stick. In that case the nose continues to rise up to another stall.

    If you have enough power you can actually keep the airplane flying even when it's stalled. We can see this on videos of over powered aerobatic models that are flying in the "harrier" mode at a 45'ish degree angle of flight. Part of the lift comes from the prop and part from the wing in this case.
    Witty saying to be plagarized shortly.....

  13. #13
    isn't that a 90 degree angle? So why are trainers when teaching a new pilot to fly so concerned about stalls? Is it cause most new pilots will end up stalling too low to the ground with out enough time to recover from it?

  14. #14
    Moderator BMatthews's Avatar
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    Probably. And are you talking models or full size? Stalls on a model are pretty easy to recover as long as the model is more than about 30 feet off the ground.

    When those flat foamie things are flying at 90 degrees it's called "hovering". The "harrier" is where the model is flying at a nose high but still angled attitude and moving forward. Or in the case of knife edge or inverted "harriers" moving sideways or "back". But the key is that if it's angled at up to around 60 degrees we call it a "harrier" where if it's pointed up steeper than 60 it's a hover. At least that's how I call 'em....
    Witty saying to be plagarized shortly.....

  15. #15
    I mean models. The guy that trained me was always going a bit over the top about stalls which is what made me think stalls made the engine kill. I'd say my favorite maneuver is doing that 180 degree rudder turn when you drive the plane into a stall just before you kick over the rudder to spin the plane around. I can't remember what thats called. I've only been flying for a few months. Kinda ready to move up from the trainer. Ive got a Hobbico nexstar and my teacher sold me the engine with a 10x6 prop on it. The manual for the nexstar says use a 11x5 but my teacher said it shouldn't make a difference but it takes practically the whole field to get off the ground which is why I think i'm gonna throw an 11x5 on there to see if that helps except the 10x6 already mows the grass so that 11x5 is going to make a mess of my plane every time I use it

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    If the 10x6 is just clearing the grass, an 11x5 maybe slightly more efficient, but I doubt that it will solve the problem.
    I'd say that either the prop or the engine is not the right size for the model. An engine has a "preferred" RPM range, and generally develops the maximum
    power within the range.
    The basic rule of thumb is to first weigh the model.
    Then determine the required power.
    There are a lot of different ways of doing this.
    I prefer the watts/LB method
    75-100 watts / Lb for typical trainers, and so forth. "Gentle" flyers a bit less, maybe 50-75W/Lb
    100-150 (I prefer this range) for mild to moderate aerobatics.
    150+ aggressive aerobatics up to 3D
    746 W = 1HP
    A "good" .60 2cy will develop ~1400-1700W or around 2 HP.

    An example of what a favorable power/weight ratio can do.
    P-51 "40" Hanger 9 Mustang ARF, electric power, retracts, all up weight 10-11 LB (Don't believe the 7.5Lb spec)
    Motor Eflite Power 52, 1650W maximum continuous. 12x8x3 prop (grass clearance, etc.)
    ~HP 2.2 at full throttle
    Power/Weight ~1.2p:1.0w
    Max speed (GPS 88mph)
    Ground run is determined by thrust side effects and tail wheel configuration.
    Slowly increase power with elevator up. Once the rudder, elevator, and ailerons are effective, reduce up elevator and somewhat gently,
    Add more power and up elevator as appropriate. Too much power too soon will result in a ground loop or nose over. (Much like the full scale A/C)
    After it's cleared the ground, rotate and apply full throttle (if not already applied) for vertical climb out.
    Electric motors do have a major difference in comparison to engines. An electric motor develops maximum torque
    at a much lower RPM than the typical engine. A three blade prop can take advantage of this at the expense of efficiency at higher speed.
    Last edited by chuckk2; 12-20-2013 at 11:08 PM.

  17. #17
    Moderator BMatthews's Avatar
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    Try holding about 1/4 to 1/3 up elevator during the first part of the takeoff roll. And as speed builds reduce the elevator until you're holding just a small amount at liftoff. What this will do is unload the nose wheel so it has a little less drag. It also will rotate the model earlier in the takeoff roll so the wing begins taking the weight sooner.

    Just be sure you remember to ease off the elevator hold as the speed builds. If you lock up and hold that much the model is going to jump into the air prematurely and stall badly. Doing a lot of mental picturing in your mind and moving the sticks on the Tx alone will help a lot to train your mind into doing this correctly. The good news is that when you get it right this does aid in getting the wheels up out of the grass and shortens the takeoff. And when done right the model doesn't leap uncontrollably into the air. It just rises and flies away at a steady climb angle.

    If your manual suggests an 11x5 but the gear isn't long enough I'd suggest that past hard landings have bent the landing gear. Either that or somebody needs to cut the lawn more often at your flying site. They wouldn't suggest a size of prop that has no hope of clearing the ground. So some checking and fixing might be in order. For the main gear it may be just a case of bending things back into shape. On the nose gear if it uses a screw collar to hold the front leg in the nose bracket it may have jammed the leg up past where it is supposed to rest. Study the front gear and see if you can adjust it so it becomes somewhat longer.
    Witty saying to be plagarized shortly.....

  18. #18
    iron eagel's Avatar
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    When the Nextstar came out it came with a 11X7 prop and an OS .46 more than enough power to make it leap off of the ground. With a 11X5 prop it should literally leap into the air. With a 10 X 6 the motor is making lots of noise but little usable thrust, an inch in diameter does make a significant change in produced thrust. If your mowing the field with a 10 inch prop on take off, I am with Bruce, either you field is in serious need of mowing or you have bent the gear, or too small of a diameter wheels on it.
    The maneuver you mentioned is called a Hammerhead.
    Knowing how to recover from a stall is an important skill to have as a pilot, as well as recognizing the onset of one to prevent it. A stall in aviation unlike a car, has nothing to do with the motor stopping, but is rather the loss of lift. But as you may have gleaned it is just as possible to have a prop stall as well...
    Quote Originally Posted by wikitjuggla View Post
    I mean models. The guy that trained me was always going a bit over the top about stalls which is what made me think stalls made the engine kill. I'd say my favorite maneuver is doing that 180 degree rudder turn when you drive the plane into a stall just before you kick over the rudder to spin the plane around. I can't remember what thats called. I've only been flying for a few months. Kinda ready to move up from the trainer. Ive got a Hobbico nexstar and my teacher sold me the engine with a 10x6 prop on it. The manual for the nexstar says use a 11x5 but my teacher said it shouldn't make a difference but it takes practically the whole field to get off the ground which is why I think i'm gonna throw an 11x5 on there to see if that helps except the 10x6 already mows the grass so that 11x5 is going to make a mess of my plane every time I use it

  19. #19
    I seem to already do that elevator trick on take off. Kinda picked it up all on my own without even thinking about it. I have a OS FP .40 on it. Recommended sizes are .46 to .52 or something like that but I got the motor for $35 and it does the job for the most part but i'm sure the smaller engine coupled with the smaller prop wasn't a good idea. The landing gear on the Nexstar is made of either cast aluminum or plastic or something of the sort and isn't adjustable. I'm pretty sure it would break before it would bend but the front steer wheel is metal but its nice and straight and the plane sits level on all three wheels. It has 2 1/2 inch wheels so throwing some 3 inchers on there should compensate for the 11x5. The grass isnt super tall, probably could use a mow but the field hasn't been rolled in a while so its a little bumpy. doesn't bother the plane too much except when trying to turn and not tip the plane over
    Last edited by wikitjuggla; 12-21-2013 at 06:58 PM.

  20. #20
    iron eagel's Avatar
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    OS ,40 FP is rated 1 Hp the fx.46 is a bit better than 1.6 hp big difference in power so you don't have a lot of power to spare with your setup. The bigger wheels will help it roll easier as well as provide a bit more clearance for the prop. As far as the grass it is typical to get a bit on the plane unless you have a ton of ground clearance. The FP.40 should spin the 11X5 just fine even though it has a bit less power, between that and larger wheels you should see some improvement in the takeoff roll but don't expect miracles.
    Your right the gear are pretty robust I think the main gear are pressed steel that is painted or powder coated.
    Did you take the flaps and drooped leading edges off of it? If not that will shed a bit of weight and improve your performance a bit.
    Also did you add weight ot get the COG right? If you did try moving the components around so you don't have to have any added weight of you can.
    With the lower pitch larger diameter prop you will have more thrust and that should help you takeoffs with a bit shorter roll but you top speed will be diminished a bit.
    Last edited by iron eagel; 12-21-2013 at 08:11 PM.

  21. #21
    the center gravity is just barely tail heavy (prolly cause of the smaller engine) which my instructor said helps with take offs and I never have used the trainer flaps or air foils. Bought it used from another club member. Heaviest things i got in it are a 5s nimh battery stuffed behind/under the fuel tank and 1 metal gear servo for steer and rudder. Prolly don't need metal gear but its what I had lying around. The bottom of the wing is bubbling a little bit with the foil cover. Don't know if that creates excess drag or not.

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    Your instructor's aversion to stalls is somewhat understandable. That aside, stalls and recovery (At a reasonable, "safe" altitude) are a necessary part of training and useful in determining how a particular plane behaves. A "tail heavy" plane, coupled with low power, can make stall recovery difficult, and in an extreme, unlikely.

    Underpowered A/C, be they models or full size, can be problematic in recovery, since there may not be enough thrust to overcome the drag, and get the control surfaces to be effective again. An overpowered model may have enough thrust and propeller induced airflow to make the control surfaces effective, even though there is no forward motion.
    (3D models)

    Back in the 60's "dark ages" spin recovery training was still considered a part of basic pilot training, even for a private license. I still remember this and the club Cessna 140 that was used. It's eventual demise was the result of the local FBO's moving it from it's usual location, and failing to retie it down properly. There was quite a flap, since a windstorm repositioned it atop a PEPSICO De Havilland Dove. The 140 an older model and design, was not really designed to "self recover" from a spin. Later designs had an FAA imposed requirement that they self recover under certain conditions.
    Last edited by chuckk2; 12-22-2013 at 10:15 AM.

  23. #23
    iron eagel's Avatar
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    Quote Originally Posted by wikitjuggla View Post
    the center gravity is just barely tail heavy (prolly cause of the smaller engine) which my instructor said helps with take offs and I never have used the trainer flaps or air foils. Bought it used from another club member. Heaviest things i got in it are a 5s nimh battery stuffed behind/under the fuel tank and 1 metal gear servo for steer and rudder. Prolly don't need metal gear but its what I had lying around. The bottom of the wing is bubbling a little bit with the foil cover. Don't know if that creates excess drag or not.
    Tail heavy is not good, while it makes the elevator very responsive, as Chuck points out it is an issue. Move the motor forward a bit until you get it to balance properly, spacers off of the firewall, or drill new mounting holes on the mount and move the motor forward. It wont take much to get the COG set properly.
    The bubbles are not a big issue for a trainer unless the covering is getting very saggy then it be a good idea to hit it with a heat gun to get it tight.

  24. #24
    Moderator BMatthews's Avatar
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    Trainers are often spec'ed with overly conservative CG ranges. So I suspect that "slightly tail heavy" in this case is what the rest of us would call comfortably normal.

    I can see the instructor's point about not wanting you to stall coming out of turns and such or during a landing approach. But it's foolish on his part to avoid stalls altogether. You really do want to learn what they do and how they affect the model so that you know what to do in case you manage to get into a stalled situation at the wrong time.

    The trick is to learn all this stuff up at the classic "3 1/2 mistakes high" sort of altitude. Once up there raise the nose and ease off the power until the model just stops flying and the nose drops. Then try the same thing in a turn where you throttle back and hold the nose up until it falls off into the turn.

    The point is that you should learn this stuff instead of being afraid of it.

    If the field is overly lumpy then temporarily switching to bigger wheels will help the acceleration as long as you buy the lighter weight versions. They'll act like the bush pilots' "Tundra Tires" at rolling over the roughness instead of the lumps acting like wheel chocks with each strike.
    Witty saying to be plagarized shortly.....

  25. #25
    I Think BMatthews is probably pretty dead on with the comfortably normal. It really is just a hair tail heavy. I practice that hammerhead move a lot and push the model to practically a stand still in the air before applying rudder. I've never put it directly in a stall though as to where it starts falling. The bubbles are pretty slight and mostly reside on almost the very edge of the bottom of the back of the wing. Had a pretty close call with it the other day. It was real real cold out and the engine ran fine...till it was in the air. Then the rpms kept cutting in and out and I decided to bring it down immediately. Of course thats when the wind kicks up and causes me to miss the runway entirely. Ended up putting it down roughly 2 football fields away in some grass, give or take half of a football field. Scared the heck out of myself with that one but brought it down safely. Don't know if the engine stalled in the air or if the prop hit the ground but I pulled it off and it was my first dead landing


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