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Help needed from engine and prop gurus!!

Mark33

Mark33

Well Known Member
I have a question for all of you engine and prop gurus. I have a newly completed RV-7 with an IO-360 and a whirlwind RV-200 C/S prop. The primary problem that I'm having is that I'm only getting 2650 RPM's out of it (both on the ground and in flight) and can only achieve, on average, about 145 MPH (not knots) indicated airspeed. The engine is making good power at around 28-29" MP. I have insured that my prop cable has about 3/8" cushion in it prior to it touching the panel and the arm on the governor is making good contact with its stop. I've even tried adjusting the stop on the governor to give me some additional through, but that hasn't help any at all. I've also had a prop shop check the angles on the prop and they have insured me that it's within specification. During ground runs, and at full throttle takeoff, it will momentary flash to 2700 RPM's but will immediately come right back down to 2650 RPM's. I really wouldn't be all that concerned about it because even at 2650 RPM's it still has plenty of power for takeoff. My main concern is my cruising speed. So, I guess my main question is, is this low RPM affecting my speed any or do I have another problem? So, in other words, with all things being equal, when I roll my prop back, to lets say 2400 RPM's for cruise flight, am I getting the same effect in performance regardless if I was starting out at 2650 or 2700 RPM's? Is 2400 RPM's 2400 RPM's regardless as to where I started from? I feel like I have a race car that's not geared properly and the performance is suffering because of it. My friend also has an RV-7 with a bone stock O-360 and a fixed-pitch prop and he can literally run off and leave me behind. Something is just not right!! I don't have the wheel pants or landing gear fairings on yet and I know I'll pick up some speed once I have these installed, but I can't imagine I'll pick up much more than 10-12 MPH. I once had an RV-4 with an O-360 and a C/S prop and at full throttle when I would come back on my prop to 2350-2400 RPM's it would literally feel like something kicked it in the tail....I could feel a surge in speed. With this -7, I don't feel any surge at all and I barley pick up any speed at all when I pull the prop back. This is really frustrating and so far I'm very disappointed in the performance of this airplane. So, with all that being said, I would appreciate any help or suggestions that anyone can provide.

Thanks,
Mark
 
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Put the gear leg fairings and pants on. Then measure TAS, not IAS. This question comes up often and the answer is always to install the pants and fairings.
 
Exactly what Kurt said. In the -10 we gain 17Kts with gear leg fairings and wheel pants. Not sure what you'll gain on the -7.

If your prop is reading correctly, then cruise RPM and MAP will determine your speed, not takeoff RPM and MAP. The latter will determine your takeoff roll and initial climb.

In response to your RPM, the only time I see a prop go to a certain RPM then immediately back is when the governor is pulling it back. I would adjust your full prop stop some more. It sounds like your prop can hit the 2,700RPM mark, but the governor is pulling it back. However, that will only affect, as I said, the takeoff roll and initial climb. I always takeoff full power and prop and then pull back to 25 squared at 500 feet. Once you hit that point, then your max RPM means nothing at all. For e ample, the IO-540-D4A5 is 260HP at 2700rpm, whereas the -C4B5 is derated to 2575rpm and 250Hp on the Aztec. Both have 8.5:1 compression. In this case, the -D will slightly outperform the -C model until my 500' mark, then they become the exact same engine as far as performance is concerned.
 
https://www.youtube.com/watch?v=ftq8jTQ8ANE

Like KRW said, at least install the leg fairings & intersection fairings.

To get an idea of whether the engine is really an issue or is making near full power, measure your climb rate. (Drag has much less influence at max climb rate speeds.) If you come close to Van's advertised climb rate for your HP & weight, then the engine is likely OK.

Charlie
 
Thank you gentlemen for your replies.

Mel, I haven't strobed the prop yet but I have checked it with two different tachometers...but I do plan on strobing the prop next.

Jesse, what you said was exactly what I was thinking with regards to RPM's and engine power/performance. I was planning on taking the governor off and sending it in to have it bench tested but if power is power at any selected RPM's once the prop has been rolled back "below" the maximum RPM's that the engine is capable of making with everything full forward, than I may wait until after I've installed the wheel pants and fairings, and as Charlie suggested, measure the climb rate, which will double check the actual engine performance.

After doing as you guys are suggesting, and If my performance doesn't improve "a lot", then I guess my next step will be to send the governor in and have it checked. But, if everything improves like I hope it will, than I probably won't be concerned about that 50 RPM's that I'm not seeing right now. Like I said before, even at 2650 RPM's the airplane has plenty of power during takeoff. This may be more of an issue at higher elevations, but down south, where I'll be doing the majority of my flying, the loss of 50 RPM's really isn't a big deal.

Charlie, that was a very interesting and educational YouTube video that you provided the link to!!

Thanks again guys for your replies and I'll post my results after I've done the things that y'all have suggested.
 
Keep in mind that people with a F/P prop will see more of an increase because it allows the engine to turn a higher RPM, thereby producing more hp.
 
Mel said:
Keep in mind that people with a F/P prop will see more of an increase because it allows the engine to turn a higher RPM, thereby producing more hp.
Click to expand...

Hmmmm.....So, with that in mind, what is the best/most efficient RPM to operate an RV-7 with a whirlwind RV-200 prop at to get the most speed out of the airplane? I know overall engine power is going to come in to play, but for arguments sake, lets say it's a 200HP engine. In my case, I even tried shifting it into really high gear and rolled the engine back to 2200 RPM's, but this still had no effect on my speed.
 
Posters and fairings. They know.

Just stop worrying and install your fairings and pants. Go test before and after and report back. You shall see a world of difference. Also might help if you google "horsepower" to understand the effects that torque (aka manifold pressure) and rpm have on one another.
 
rv7charlie said:
That's not what he said.
Click to expand...

My point is that with a fixed pitch prop airspeed will suffer more without fairings installed because the engine will achieve less rpm at slower speeds.
Lower rpm = lower hp.

With a F/P prop try pulling the aircraft into a higher AOA with WOT. As the aircraft slows, so will the rpms.
If this were not true, you would be able to achieve max rpm on the ground sitting still. Rpms come up as the aircraft accelerates, even at cruise.
 
Mark33 said:
After doing as you guys are suggesting, and If my performance doesn't improve "a lot", then I guess my next step will be to send the governor in and have it checked. But, if everything improves like I hope it will, than I probably won't be concerned about that 50 RPM's that I'm not seeing right now. Like I said before, even at 2650 RPM's the airplane has plenty of power during takeoff. This may be more of an issue at higher elevations, but down south, where I'll be doing the majority of my flying, the loss of 50 RPM's really isn't a big deal.
Click to expand...

Why send it in? It's actually pretty easy to adjust yourself - I'd imagine moving the arm one spline over and re-adjusting the RPM control accordingly would do it.
 
ChiefPilot said:
Why send it in? It's actually pretty easy to adjust yourself - I'd imagine moving the arm one spline over and re-adjusting the RPM control accordingly would do it.
Click to expand...

Good point...I may try that as well. However, when I spoke to the guys in the governor shop at American Prop, they said that this could be fairly tedious job due to the associated internal springs and that I could easily get everything out of whack if I wasn't very careful....but after I get it off, it may not be all that bad. All I know is that the governor is going to be fairly difficult to remove and I just want to do this one time and not have a lot of trial and error involved.
 
Mel said:
My point is that with a fixed pitch prop airspeed will suffer more without fairings installed because the engine will achieve less rpm at slower speeds.
Lower rpm = lower hp.

With a F/P prop try pulling the aircraft into a higher AOA with WOT. As the aircraft slows, so will the rpms.
If this were not true, you would be able to achieve max rpm on the ground sitting still. Rpms come up as the aircraft accelerates, even at cruise.
Click to expand...

I don't know this for a fact, but wouldn't the MAP increase somewhat proportionately to compensate? For example, if I set MAP in a C/S to 20" at 2500rpm, then pull the RPm back to 2400, the MAP goes up. If at WOT, the MAP would be higher at lower rpm, which would mean more hp, somewhat offset by the lower hp at lower rpm. Someone more knowledgeable than I am may be able to speak to this, but I don't think it's as simple as you say.
 
Mark33 said:
Good point...I may try that as well. However, when I spoke to the guys in the governor shop at American Prop, they said that this could be fairly tedious job due to the associated internal springs and that I could easily get everything out of whack if I wasn't very careful....but after I get it off, it may not be all that bad. All I know is that the governor is going to be fairly difficult to remove and I just want to do this one time and not have a lot of trial and error involved.
Click to expand...

Be careful with this so as not to increase it enough that the prop will overspeed. The low pitch stops may be set so that your HP would yield 2700rpm at sea level WOT, but you won't know until you try.
 
Mel said:
My point is that with a fixed pitch prop airspeed will suffer more without fairings installed because the engine will achieve less rpm at slower speeds.
Lower rpm = lower hp.

With a F/P prop try pulling the aircraft into a higher AOA with WOT. As the aircraft slows, so will the rpms.
If this were not true, you would be able to achieve max rpm on the ground sitting still. Rpms come up as the aircraft accelerates, even at cruise.
Click to expand...

Ah, so. It really is all about what the definition of it is....

:)

I see what you mean, now. When I first read it, it sounded like you were saying that a fixed pitch prop was, by definition, faster than a c/s.
 
Jesse said:
Be careful with this so as not to increase it enough that the prop will overspeed. The low pitch stops may be set so that your HP would yield 2700rpm at sea level WOT, but you won't know until you try.
Click to expand...

Roger that!!
 
Jesse said:
I don't know this for a fact, but wouldn't the MAP increase somewhat proportionately to compensate? For example, if I set MAP in a C/S to 20" at 2500rpm, then pull the RPm back to 2400, the MAP goes up. If at WOT, the MAP would be higher at lower rpm, which would mean more hp, somewhat offset by the lower hp at lower rpm. Someone more knowledgeable than I am may be able to speak to this, but I don't think it's as simple as you say.
Click to expand...

You're correct, but the effect is not large enough to completely compensate. The MAP goes up at lower rpm because even at WOT there is a small pressure drop across the throttle and induction system. But a typical example might be: 20"/2500 RPM, vs (pull back the prop but leave throttle untouched) 20.5"/2400 RPM. e.g., 2% gain in MP but a 4% loss in RPM. The MP gain might be even smaller. I think I see (IO-540) less than a 1" MP rise going from 2600 to 2300 rpm.
 
BobTurner said:
You're correct, but the effect is not large enough to completely compensate. The MAP goes up at lower rpm because even at WOT there is a small pressure drop across the throttle and induction system. But a typical example might be: 20"/2500 RPM, vs (pull back the prop but leave throttle untouched) 20.5"/2400 RPM. e.g., 2% gain in MP but a 4% loss in RPM. The MP gain might be even smaller. I think I see (IO-540) less than a 1" MP rise going from 2600 to 2300 rpm.
Click to expand...

So, with that being said, what is the ideal RPM (if there is one) to run the prop at to acheave the maxim speed for the airplane? Is there a ratio of MP to prop RPM that would acheave the best results?
 
Mark,
Even though you said you were getting 28-29", I would still check the rigging on your throttle to make sure the the throttle plate is opening fully.

Jerry Esquenazi
RV-8 N84JE
 
Mark33 said:
So, with that being said, what is the ideal RPM (if there is one) to run the prop at to acheave the maxim speed for the airplane? Is there a ratio of MP to prop RPM that would acheave the best results?
Click to expand...

Both levers full forward - maximum MP and maximum RPM
 
Mark33 said:
So, with that being said, what is the ideal RPM (if there is one) to run the prop at to acheave the maxim speed for the airplane? Is there a ratio of MP to prop RPM that would acheave the best results?
Click to expand...

Full throttle and max RPM is the obvious answer. The real answer depends on the prop efficiency not going down at redline, e.g., if the tips go supersonic there's a big decline in efficiency. I know some racers use 50 or 100 rpm less than redline.
 
Fixed pitch can be faster

The reason that a FP prop can be faster than a CS is that it will allow the engine to overspeed at top end. This is not necessarily a good thing, as most Lycoming O/IO 360 engines are designed to redline at 2700 rpm. However these engines "breath" very well, even at speeds over redline. The gain in hp is about 4% per 100 rpm at rpm in the range of 2700. Therefore, if the FP prop will let the engine run to, say, 2900rpm at terminal velocity, the engine will develop about 194 hp. HP determines the top speed of a given aircraft. Note that the speed is proportional to the HP cubed. IE It will take 8 times the HP to double the speed! It might argue that we should use an HIO 360 C engine which is rated at 2900 rpm to take advantage of the extra power. It would also argue that the CS prop governor should be set at the top of the engine and propellor manufacturers' acceptable range for best TAKEOFF performance.
 
gear1 said:
The reason that a FP prop can be faster than a CS is that it will allow the engine to overspeed at top end. This is not necessarily a good thing, as most Lycoming O/IO 360 engines are designed to redline at 2700 rpm. However these engines "breath" very well, even at speeds over redline. The gain in hp is about 4% per 100 rpm at rpm in the range of 2700. Therefore, if the FP prop will let the engine run to, say, 2900rpm at terminal velocity, the engine will develop about 194 hp. HP determines the top speed of a given aircraft. Note that the speed is proportional to the HP cubed. IE It will take 8 times the HP to double the speed! It might argue that we should use an HIO 360 C engine which is rated at 2900 rpm to take advantage of the extra power. It would also argue that the CS prop governor should be set at the top of the engine and propellor manufacturers' acceptable range for best TAKEOFF performance.
Click to expand...

I'm sure you meant "cube root of HP", not "HP cubed".
 
SMO said:
Both levers full forward - maximum MP and maximum RPM
Click to expand...

Well this throws off my understanding of how a C/S works and how it can improve both efficiency and speed. I've always thought of a C/S prop like a transmission on a car. True, if your car is in first or second gear at wide open throttle and full RPM's, you may be making full HP but your speed will be limited because you're in a low gear. If you shift into third or forth gear your actual HP may decrease because the RPM's have now decreased but your speed has actually increased because you're now in a higher gear. So in the case of a C/S prop as you roll the RPM's back and increasing the pitch, you are taking a bigger bite out of the air and pulling the aircraft through the air faster. I know that at some point, just like as in a car that's geared too high, you'd actually start losing efficiency and start lugging the motor down if the RPM's drop down too low. So what am I missing here? If I just leave everything full forward (throttle and prop) I'm sure the engine will be making full HP but it'll be in such a low gear that it could never achieve full speed. Please enlighten me.
 
Mark33 said:
Well this throws off my understanding of how a C/S works and how it can improve both efficiency and speed. I've always thought of a C/S prop like a transmission on a car. True, if your car is in first or second gear at wide open throttle and full RPM's, you may be making full HP but your speed will be limited because you're in a low gear. If you shift into third or forth gear your actual HP may decrease because the RPM's have now decreased but your speed has actually increased because you're now in a higher gear. So in the case of a C/S prop as you roll the RPM's back and increasing the pitch, you are taking a bigger bite out of the air and pulling the aircraft through the air faster. I know that at some point, just like as in a car that's geared too high, you'd actually start losing efficiency and start lugging the motor down if the RPM's drop down too low. So what am I missing here? If I just leave everything full forward (throttle and prop) I'm sure the engine will be making full HP but it'll be in such a low gear that it could never achieve full speed. Please enlighten me.
Click to expand...

Well, I'm not the right guy to provide the technical answer, however my RV-4, my Decathlon, and now my Rocket all had (have) CS props and that is how they worked.

Here is how my simple brain interprets the result:

The throttle lever full forward (MP) provides max power for any rpm.

The prop lever adjusts RPM, and more RPM = more power (look at the RPM/Power curve).

More power with CS prop (max MP and RPM) = more speed.

The prop pitches as coarse as it can for the RPM requested - 2700 in this case. If you could somehow provide even more power at that RPM (inject nitrous?) the prop would be able to pitch even coarser and still maintain 2700; you would go faster.

With a FP prop adding more power won't necessarily add more speed given the limitation of max rpm. If you are propped for 2700 rpm cruise and you add more power the rpm will rise, you will have to manually cut back the throttle to maintain your 2700 rpm redline and you won't be going any faster.

What the CS prop is doing for you is giving you a prop pitched for max climb, a prop pitched for max cruise, and pitched for everything in between all at the same time. With the FP prop by definition you have to choose a single pitch.
 
Mark33 said:
Well this throws off my understanding of how a C/S works and how it can improve both efficiency and speed. I've always thought of a C/S prop like a transmission on a car. True, if your car is in first or second gear at wide open throttle and full RPM's, you may be making full HP but your speed will be limited because you're in a low gear. If you shift into third or forth gear your actual HP may decrease because the RPM's have now decreased but your speed has actually increased because you're now in a higher gear. So in the case of a C/S prop as you roll the RPM's back and increasing the pitch, you are taking a bigger bite out of the air and pulling the aircraft through the air faster. I know that at some point, just like as in a car that's geared too high, you'd actually start losing efficiency and start lugging the motor down if the RPM's drop down too low. So what am I missing here? If I just leave everything full forward (throttle and prop) I'm sure the engine will be making full HP but it'll be in such a low gear that it could never achieve full speed. Please enlighten me.
Click to expand...

Imagine a car with infinitely variable gears (such cars exist, using belts which slide back and forth on shafts with smoothly varying diameters). You push the gas to the floor (WOT), the car starts in a low gear that allows the engine to turn at redline. The car accelerates rapidly. As speed builds up, the belts shift gears to always keep the engine at redline. The car accelerates more slowly, since that takes force, and force scales as power divided by velocity. Eventually the force delivered is matched by the air resistance, and that's as fast as you can go. On the airplane instead of shifting belts you have a CS prop which is changing pitch to keep RPM at redline.
While it is a good compromise over a large range of speeds, it is "best" only at one speed. That's because the twist in the blades is still fixed.
 
Mark and Bob,....great explanations; thank you!! Bob, the way you described it as with a car with CVT really turned the light on for me. If you keep your foot on the accelerator the RPM's come up to achieve maximum HP but unlike a standard transmission, or even a conventional automatic transmission for that matter, the CVT maintains that set RPM but the gear ratio is constantly changing depending on the needs of the vehicle at any given moment weather it's for power to pull it up a hill or speed on a flat road. Even though I knew that the prop had an infinite number of pitch angles that it could achieve and those pitch angles are constantly changing according to the needs of the airplane to maintain that set RPM, I guess I always imagined the prop staying very flat if it were pushed full forward. But, I guess that as the aircraft gains speed and the prop can move through the air easier, it will actually start putting more and more pitch into itself even if it's pushed full forward. The prop is trying to maintain those RPM's (and HP) weather those RPM's are being used for climb out where the pitch is flatter, or for speed where there's more pitch being applied. So looking at it like that I can see how a fixed pitch prop, that's designed for speed, could actually be faster than a C/S prop that's designed for cruise and efficiency. I never really ever thought of a C/S prop as having a "built in pitch" (for lack of better words) that it works best at.

So, this actually brings me back full circle to part of my problem and my engine only achieving 2650 RPM's. With the wheel pants and gear leg fairings aside, It sounds like I'm potentially giving up some speed if my engine can't make its full 2700 RPM's.
 
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I would spend my time on the fairings and quit worrying about TOP speed at this time. Mine CS is set at 2670 and it will go over 2700 if you accelerate to fast on the take off roll. Sounds like your prop is set up pretty close to where you need it. If it aint broke, don't fix it!
 
Mark33 said:
So, this actually brings me back full circle to part of my problem and my engine only achieving 2650 RPM's. With the wheel pants and gear leg fairings aside, It sounds like I'm potentially giving up some speed if my engine can't make its full 2700 RPM's.
Click to expand...

I have a similar problem with my Rocket, cannot get it to spin up past 2670 even with the stop backed all the way out. Turns out there are a number of "governor to crank" gear ratios and my governor was set up for the wrong gear ratio. Engine is currently disassembled for a prop strike inspection and while having the governor inspected for damage I had the shop adjust it for the correct ratio. As has been mentioned this is a simple procedure involving moving the lever on the shaft by one spline. Not saying this is your problem necessarily, but it may be.
 
SMO said:
I have a similar problem with my Rocket, cannot get it to spin up past 2670 even with the stop backed all the way out. Turns out there are a number of "governor to crank" gear ratios and my governor was set up for the wrong gear ratio. Engine is currently disassembled for a prop strike inspection and while having the governor inspected for damage I had the shop adjust it for the correct ratio. As has been mentioned this is a simple procedure involving moving the lever on the shaft by one spline. Not saying this is your problem necessarily, but it may be.
Click to expand...

Mark,
I actually had this exact conversation with American Propeller, which is where I bought the prop and governor from. They went back in their records and looked everything up and they insured me that I have the proper gear ratio. Like I said in an earlier post I may, just to be on the safe side, send the governor in and have it bench checked and along with that verify that I do indeed have the proper gear for it.
 
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