Engine Thrust: Is it the only factor?
 

I just ordered a Carden 30% Xtra. It ways between 19 to 21 pounds and calls for a 45cc to 70cc engine.

I like the ZDZ's and the 60cc claims to generate 38lbs of thrust with a 22x10 prop.

Is thrust the only factor to consider or is it misleading, assuming the 38lbs is good number.

Thanks

Rich Thorne
AMA: N5ZC
Amarillo, TX

Engine Thrust: Is it the only factor?
 

The thrust is helpful, but not the only factor. However, the ZDZ 60 should power that plane very well. Considering the light weight of the ZDZ's (I have the ZDZ 80 single) it should come out near the bottom of the recommended weight range.

Thrust
 

I agree with T-ster that the ZDZ-60 will be an excellent engine for this plane....but at those weights it is right on the edge of needing to go to the 80, but the 80 would also be on the side of overkill. T-ster is right that if it comes out on the bottom of that weight range with the 60, it will probably be all that you need. It all depends on what type of flying you are planning on doing.

T-ster is again right that a "real" thrust number would be helpful, but as far as the numbers ZDZ posts, forget them, they are virtually worthless, at least if you are using them as absolutes. They are based on calculations using a thrust calculator. You would find that actual thrust readings will be 20-25% lower. If you want to compare power to other motors, check the RPM readings from the various engines using the same brand, type and size of prop. As far as actual thrust numbers, there is very little data available on this, though it sure would be helpful.

As far as your original question "Is it the only factor?", no the other important factors are reliability, cost, smoothness, ease of starting and weight. The ZDZ60 and 80 both do well in all of these categories....except I've heard some posts about difficult starting from a couple of sources.

Engine Thrust: Is it the only factor?
 

It is scary that someone agrees with many of my comments , lol... Thanks bpryor!

Anyway, my ZDZ80 is somewhat difficult to start. I'm on my 3rd ignition system and it has become gradually more and more difficult to start. I'm expecting an updated version of the ignition in the mail from ZDZ. (I got one of the first ZDZ's over in the US. The ignitions out now from Falkon are supposed to be pretty good, or so I've heard. I will find out shortly.)

The best comparison, as bpryor states, is to compare rpm numbers between engines with the same type & size of prop. Try going to rcfaq.com, it will compare many different engines, but take it with a grain of salt. I'm not confident everyone gives true readings (kind of like asking women to tell you what they weigh!!!)

RPM accuracy
 

I agree with you again T-ster. ;)

>>but take it with a grain of salt.

I agree. What I do is gather a bunch of numbers from various sources and see if I can see a pattern. Then I throw out the high numbers and have an idea of what to expect.

Also, although rcfaq.com tries to get people to enter specifics, like altitude, temperature, and accessories(mufflers, etc), not everyone does, and these factors can make a big difference. As an example if you look at the ZDZ40 numbers up there, many look very impressive, but if you look in the notes section you'll see most of the numbers are from the European ZDZ distributor and they're using a tuned pipe in most cases(so you can bet this is the absolute best case scenario), and other numbers from other people are also using a tuned pipe.


...but with that said, I still believe they can be a valuable piece of data when used properly.

Engine Thrust: Is it the only factor?
 

T-ster,
I can't believe that you have trouble starting the ZDZ. I'm guessing that if you had the Falkon ignition you would be happier. I have the ZDZ 80B2 and it starts on the second or third pull through every time. I have found that if you try to flip it through too fast or hard it will stumble some and quit, (go figure). I just pull mine through real easy and it takes the prop right out of my hand. Idles at around 1000 ~ 1100 but I'm still in the break-in process. As far as the thrust claims go, I agree that the calculations are theoretical only. I use an electronic strain gauge on my test stand. I have incorperated a psudo-cowl and baffels on my stand as the ZDZ runs very hot. I keep the head temperatures at 250 ~275 deg. always. After three gallons of fuel I am getting 6000 RPM with a 26-10 RCS prop (I usually run a 24-10 for the first 3 ~ 4 gallons of fuel). According to the ThrstHp program, this equates to 47.95 pounds of thrust. My strain gauge reads 42.24 pounds. Still outstanding numbers for a 80 cc. engine that isn't broken in yet, don't ya think?

Walt

Engine Thrust: Is it the only factor?
 

The ignition I currently have is a Falkon, but an earlier version. I received a new ignition from ZDZ, a couple of days ago. When I have a chance, I'll install it and see how it runs.

The temp on your 80 is 250-275? That sounds quite high. I've heard to keep the temps below 200. My 3W100 runs around 165, from what I can remember.

Of course, if you are running it on a test stand, that would explain the higher temps.

Engine Thrust: Is it the only factor?
 

Yes it's high. But this is on the stand and static. I talked to the folks at RCshowcase and they recommended this as a limit on the stand. I shoot the head with a infrared gun. In the plane I look for 175 max.

Engine Thrust: Is it the only factor?
 

T-ster,
What are you using for fuel in your ZDZ. Have you checked the plug for fouling? I burn Aviation 100LL gas. Although this fuel does tend to foul the plugs quicker, I like the performance of the 100 octane. Additionally, during break-in I try to use AT LEAST 93 octane. The instructions say 91 ~ 93 octane for mine, but the oil fuel mix at 40:1 effectively reduces the octane of the gas and I have noticed some preignition when using 91 oct. for the first gallon. Is the engine popping or just nothing when trying to start. You may also want to check the timing since you have changed ignition systems, they may have different requirements.

Engine Thrust: Is it the only factor?
 

I run 100LL as well and I did check the plug. Looked ok, but I replaced it anyway. It runs better after replacing the plug, but still the starting can be somewhat difficult. (I run Avgas since there is MUCH less smell, considering I keep my planes in a bedroom.)

With the last ignition, I did adjust the timing and believe it is set where it should be.

It will take several flips after priming to pop. Then it will pop for 4-6 times, then generally start. Not terrible, but not great either.